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235 Years Ago: Herschel Discovers Saturn’s Moon Enceladus


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On Aug. 29, 1789, German-born British astronomer William Herschel observed a tiny bright dot orbiting around Saturn. His son later named the object Enceladus. Because of its distance from Earth and proximity to bright Saturn, for the next two centuries little remained known about Enceladus other than its size, orbital parameters, and that it held the honor as the most reflective body in the solar system. It took the Voyager flybys through the Saturn system in the early 1980s and especially the detailed observations between 2005 and 2015 by the Saturn orbiter Cassini to reveal Enceladus as a truly remarkable world, interacting with Saturn and its rings. Harboring a subsurface ocean of salty water, Enceladus may possibly be hospitable to some forms of life.

Portrait (1785) of William Herschel by Lemuel Francis Abbott Drawing of Herschel’s 40-foot telescope Portrait (1867) of John Herschel by Julia Margaret Cameron
Left: Portrait (1785) of William Herschel by Lemuel Francis Abbott. Image credit: courtesy National Portrait Gallery, London. Middle: Drawing of Herschel’s 40-foot telescope. Right: Portrait (1867) of John Herschel by Julia Margaret Cameron. Image credit: Metropolitan Museum of Art.

Herschel’s previous astronomical accomplishments include the discovery of Uranus in 1781 and two of its moons, Oberon and Titania, in 1787. He also catalogued numerous objects he termed nebulae, but remained frustrated by the limitations of telescopes of his age. He began to build ever larger instruments, finally building the world’s largest reflecting telescope of its time. At 40 feet long, and with a 49-inch diameter primary mirror weighing a ton, it looked impressive although its optical characteristics did not advance the field as much as he had hoped. Nevertheless, Herschel used this telescope to observe Saturn and its five known moons, looking for others. On Aug. 28, 1789, he observed a bright point orbiting the planet and believed he had discovered a sixth moon. On Sept. 17, he discovered a seventh moon orbiting the ringed planet. He did not name these moons, that task fell to his son John who believed Saturn’s moons should be named after the Titans of Greek mythology. He named the first moon Enceladus and the second Mimas.

Relative sizes of Earth, Earth’s Moon, and Enceladus Best Voyager 2 image of Enceladus
Left: Relative sizes of Earth, Earth’s Moon, and Enceladus. Right: Best Voyager 2 image of Enceladus.

For nearly two centuries, Enceladus remained not much more than a point of light orbiting Saturn, just another icy moon in the outer solar system. Astronomers estimated its diameter at around 310 miles and its orbital period around Saturn at 1.4 days, with a mean distance from the planet’s center of 148,000 miles. Enceladus has the distinction as one of the brightest objects in the solar system, reflecting almost 100 percent of the Sun’s light. Unusual telescope observations during the 20th century showed an increase in brightness on its trailing side, with no known explanation at the time. In 1966, astronomers discovered a diffuse ring around Saturn, the E-ring, and found in 1980 that its density peaked near Enceladus. The Voyager 1 spacecraft flew within 125,570 miles of Enceladus during its passage through the Saturn system on Nov. 12, 1980. Its twin Voyager 2 came within 54,000 miles on Aug. 26, 1981, during its flyby. These close encounters enabled the spacecraft to return the first detailed images of the moon, showing various terrains, including heavily cratered areas as well as smooth crater-free areas, indicating a very young surface.

False color image of Enceladus from Cassini showing the tiger stripes at bottom Limb view of Enceladus showing plumes of material emanating from its surface Cassini image of Enceladus backlit by the Sun showing the fountain-like plumes of material
Left: False color image of Enceladus from Cassini showing the tiger stripes at bottom. Middle: Limb view of Enceladus showing plumes of material emanating from its surface. Right: Cassini image of Enceladus backlit by the Sun showing the fountain-like plumes of material.

After the Cassini spacecraft entered orbit around Saturn in July 2004, our understanding of Enceladus increased tremendously, and of course raised new questions. Between 2005 and 2015, Cassini encountered Enceladus 22 times, turning its various instruments on the moon to unravel its secrets. It noted early on that the moon emitted gas and dust or ice particles and that they interacted with the E-ring. Images of the moon’s south polar region revealed cracks on the surface and other instruments detected a huge cloud of water vapor over the area. The moon likely had a liquid subsurface and some of this material reached the outside through these cracks. Scientists named the most prominent of these areas “tiger stripes” and later observations confirmed them as the source of the most prominent jets. During the most daring encounter in October 2015, Cassini came within 30 miles of the Enceladus’ surface, flying through the plume of material emanating from the moon. Analysis of the plumes revealed an organic brew of volatile gases, water vapor, ammonia, sodium salts, carbon dioxide, and carbon monoxide. These plumes replenish Saturn’s E-ring, and some of this material enters Saturn’s upper atmosphere, an interaction unique in the solar system. More recently, the James Webb Space Telescope imaged the water vapor plume emanating from Enceladus’ south pole, extending out 40 times the size of the moon itself. The confirmation of a subsurface ocean of salty water has led some scientists to postulate that Enceladus may be hospitable to some forms of life, making it a potential target for future exploration. Enceladus may yet have more surprises, even as scientists continue to pore over the data returned by Cassini.

James Webb Space Telescope image of a water vapor plume emanating from Enceladus Illustration of the interaction of Enceladus and Saturn’s E-ring
Left: James Webb Space Telescope image of a water vapor plume emanating from Enceladus. Right: Illustration of the interaction of Enceladus and Saturn’s E-ring.

Map of Enceladus based on imagery from Cassini, turning our view of Enceladus from a small point of light into a unique world with its own topography
Map of Enceladus based on imagery from Cassini, turning our view of Enceladus from a small point of light into a unique world with its own topography.

Events in world history in 1789:

January 29 – Vietnamese emperor Quang Trung defeats Chinese Qing forces at Ngọc Hồi-Đống Đa in one of the greatest military victories in Vietnamese history.

March 10 – In Japan, the Menashi-Kunashir rebellion begins between the Ainu people and the Japanese.

April 7 – Selim III succeeds Abdul Hamid I as Sultan of the Ottoman Empire.

April 28 – Aboard the HMS Bounty in the Pacific Ocean, Fletcher Christian leads the mutiny against Captain William Bligh.

April 30 – Inauguration of George Washington as the first President of the United States of America.

July 14 – Citizens storm The Bastille fortress in Paris during the French Revolution.

September 15 – Birth of American writer James Fenimore Cooper in Burlington, New Jersey.

December 11 – Founding of the University of North Carolina, the oldest public university in the United States.

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      Acknowledgements
      The author wishes to acknowledge the decades of hard work of the many hundreds of people who have contributed to the success of the international Aura mission. There are too many to acknowledge here and I’m sure that many names from the early days are lost to time. I would like to offer special thanks to those scientists who, back in the 1980s, first dreamed of the mission that would become Aura.
      Bryan Duncan
      NASA’s Goddard Space Flight Center (GSFC)
      bryan.n.duncan@nasa.gov
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      Last Updated Sep 16, 2024 Related Terms
      Earth Science View the full article
    • By NASA
      The Apollo 11 mission in July 1969 completed the goal set by President John F. Kennedy in 1961 to land a man on the Moon and return him safely to the Earth before the end of the decade. At the time, NASA planned nine more Apollo Moon landing missions of increasing complexity and an Earth orbiting experimental space station. No firm human space flight plans existed once these missions ended in the mid-1970s. After taking office in 1969, President Richard M. Nixon chartered a Space Task Group (STG) to formulate plans for the nation’s space program for the coming decades. The STG’s proposals proved overly ambitious and costly to the fiscally conservative President who chose to take no action on them.

      Left: President John F. Kennedy addresses a Joint Session of Congress in May 1961. Middle: President Kennedy addresses a crowd at Rice University in Houston in September 1962. Right: President Lyndon B. Johnson addresses a crowd during a March 1968 visit to the Manned Spacecraft Center, now NASA’s Johnson Space Center, in Houston.
      On May 25, 1961, before a Joint Session of Congress, President John F. Kennedy committed the United States to the goal, before the decade was out, of landing a man on the Moon and returning him safely to the Earth. President Kennedy reaffirmed the commitment during an address at Rice University in Houston in September 1962. Vice President Lyndon B. Johnson, who played a leading role in establishing NASA in 1958, under Kennedy served as the Chair of the National Aeronautics and Space Council. Johnson worked with his colleagues in Congress to ensure adequate funding for the next several years to provide NASA with the needed resources to meet that goal.
      Following Kennedy’s assassination in November 1963, now President Johnson continued his strong support to ensure that his predecessor’s goal of a Moon landing could be achieved by the stipulated deadline. But with increasing competition for scarce federal resources from the conflict in southeast Asia and from domestic programs, Johnson showed less interest in any space endeavors to follow the Apollo Moon landings. NASA’s annual budget peaked in 1966 and began a steady decline three years before the agency met Kennedy’s goal. From a budgetary standpoint, the prospects of a vibrant, post-Apollo space program didn’t look all that rosy, the triumphs of the Apollo missions of 1968 and 1969 notwithstanding.

      Left: On March 5, 1969, President Richard M. Nixon, left, introduces Thomas O. Paine as the NASA Administrator nominee, as Vice President Spiro T. Agnew looks on. Middle: Proposed lunar landing sites through Apollo 20, per August 1969 NASA planning. Right: An illustration of the Apollo Applications Program experimental space station that later evolved into Skylab.
      Less than a month after assuming the Presidency in January 1969, Richard M. Nixon appointed a Space Task Group (STG), led by Vice President Spiro T. Agnew as the Chair of the National Aeronautics and Space Council, to report back to him on options for the American space program in the post-Apollo years. Members of the STG included NASA Acting Administrator Thomas O. Paine (confirmed by the Senate as administrator on March 20), the Secretary of Defense, and the Director of the Office of Science and Technology. At the time, the only approved human space flight programs included lunar landing missions through Apollo 20 and three long-duration missions to an experimental space station based on Apollo technology that evolved into Skylab.
      Beyond a general vague consensus that the United States human space flight program should continue, no approved projects existed once these missions ended by about 1975. With NASA’s intense focus on achieving the Moon landing within President Kennedy’s time frame, long-term planning for what might follow the Apollo Program garnered little attention. During a Jan. 27, 1969, meeting at NASA chaired by Acting Administrator Paine, a general consensus emerged that the next step after the Moon landing should involve the development of a 12-person earth-orbiting space station by 1975, followed by an even larger outpost capable of housing up to 100 people “with a multiplicity of capabilities.” In June, with the goal of the Moon landing almost at hand, NASA’s internal planning added the development of a space shuttle by 1977 to support the space station, the development of a lunar base by 1976, and the highly ambitious idea that the U.S. should prepare for a human mission to Mars as early as the 1980s. NASA presented these proposals to the STG for consideration in early July in a report titled “America’s Next Decades in Space.”

      Left: President Richard M. Nixon, right, greets the Apollo 11 astronauts aboard the U.S.S. Hornet after their return from the Moon. Middle: The cover page of the Space Task Group (STG) Report to President Nixon. Right: Meeting in the White House to present the STG Report to President Nixon. Image credit: courtesy Richard Nixon Presidential Library and Museum.
      Still bathing in the afterglow of the successful Moon landing, the STG presented its 29-page report “The Post-Apollo Space Program:  Directions for the Future” to President Nixon on Sep. 15, 1969, during a meeting at the White House. In its Conclusions and Recommendations section, the report noted that the United States should pursue a balanced robotic and human space program but emphasized the importance of the latter, with a long-term goal of a human mission to Mars before the end of the 20th century. The report proposed that NASA develop new systems and technologies that emphasized commonality, reusability, and economy in its future programs. To accomplish these overall objectives, the report presented three options:

      Option I – this option required more than a doubling of NASA’s budget by 1980 to enable a human Mars mission in the 1980s, establishment of a lunar orbiting space station, a 50-person Earth orbiting space station, and a lunar base. The option required a decision by 1971 on development of an Earth-to-orbit transportation system to support the space station. The option maintained a strong robotic scientific and exploration program.

      Option II – this option maintained NASA’s budget at then current levels for a few years, then anticipated a gradual increase to support the parallel development of both an earth orbiting space station and an Earth-to-orbit transportation system, but deferred a Mars mission to about 1986. The option maintained a strong robotic scientific and exploration program, but smaller than in Option I.

      Option III – essentially the same as Option II but deferred indefinitely the human Mars mission.
      In separate letters, both Agnew and Paine recommended to President Nixon to choose Option II. 

      Left: Illustration of a possible space shuttle, circa 1969. Middle: Illustration of a possible 12-person space station, circa 1969. Right: An August 1969 proposed mission scenario for a human mission to Mars.
      The White House released the report to the public at a press conference on Sep. 17 with Vice President Agnew and Administrator Paine in attendance. Although he publicly supported a strong human spaceflight program, enjoyed the positive press he received when photographed with Apollo astronauts, and initially sounded positive about the STG options, President Nixon ultimately chose not to act on the report’s recommendations.  Nixon considered these plans too grandiose and far too expensive and relegated NASA to one America’s domestic programs without the special status it enjoyed during the 1960s. Even some of the already planned remaining Moon landing missions fell victim to the budgetary axe.
      On Jan. 4, 1970, NASA had to cancel Apollo 20 since the Skylab program needed its Saturn V rocket to launch the orbital workshop. In 1968, then NASA Administrator James E. Webb had turned off the Saturn V assembly line and none remained beyond the original 15 built under contract. In September 1970, reductions in NASA’s budget forced the cancellation of two more Apollo missions, and  in 1971 President Nixon considered cancelling two more. He reversed himself and they flew as Apollo 16 and Apollo 17 in 1972, the final Apollo Moon landing missions.

      Left: NASA Administrator James C. Fletcher, left, and President Richard M. Nixon announce the approval to proceed with space shuttle development in 1972. Middle: First launch of the space shuttle in 1981. Right: In 1984, President Ronald W. Reagan directs NASA to build a space station.
      More than two years after the STG submitted its report, in January 1972 President Nixon directed NASA Administrator James C. Fletcher to develop the Space Transportation System, the formal name for the space shuttle, the only element of the recommendations to survive the budgetary challenges.  NASA anticipated the first orbital flight of the program in 1979, with the actual first flight occurring two years later. Twelve years elapsed after Nixon’s shuttle decision when President Ronald W. Reagan approved the development of a space station, the second major component of the STG recommendation.  14 years later, the first element of that program reached orbit. In those intervening years, NASA had redesigned the original American space station, leading to the development of a multinational orbiting laboratory called the International Space Station. Humans have inhabited the space station continuously for the past quarter century, conducting world class and cutting edge scientific and engineering research. Work on the space station helps enable future programs, returning humans to the Moon and later sending them on to Mars and other destinations.

      The International Space Station as it appeared in 2021.
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      Curiosity Navigation Curiosity Home Mission Overview Where is Curiosity? Mission Updates Science Overview Instruments Highlights Exploration Goals News and Features Multimedia Curiosity Raw Images Images Videos Audio More Resources Mars Missions Mars Sample Return Mars Perseverance Rover Mars Curiosity Rover MAVEN Mars Reconnaissance Orbiter Mars Odyssey More Mars Missions The Solar System The Sun Mercury Venus Earth The Moon Mars Jupiter Saturn Uranus Neptune Pluto & Dwarf Planets Asteroids, Comets & Meteors The Kuiper Belt The Oort Cloud 5 min read
      Sols 4304-4006: 12 Years, 42 Drill Holes, and Now… 1 Million ChemCam Shots!
      In celebration of ChemCam’s milestone, here is a stunning image from its remote micro imager, showing details in the landscape far away. This image was taken by Chemistry & Camera (ChemCam) onboard NASA’s Mars rover Curiosity on Sol 4302 — Martian day 4,302 of the Mars Science Laboratory mission — on Sept. 12, 2024, at 09:20:51 UTC. NASA/JPL-Caltech Earth planning date: Friday, Sept. 13, 2024
      Today, I need to talk about ChemCam, our laser and imaging instrument on the top of Curiosity’s mast. It one of the instruments in the “head” that gives Curiosity that cute look as if it were looking around tilting its head down to the rocks at the rover’s wheels. On Monday, 19th August the ChemCam team at CNES in France planned the 1 millionth shot and Curiosity executed it on the target Royce Lake on sol 4281 on Mars. Even as an Earth scientist used to really big numbers, this is a huge number that took me a while to fully comprehend. 1 000 000 shots! Congratulations, ChemCam, our champion for getting chemistry from a distance – and high-resolution images, too. If you are now curious how Curiosity’s ChemCam instrument works, here is the NASA fact sheet. And, of course, the team is celebrating, which is expressed by those two press releases, one from CNES in France and one from Los Alamos National Laboratory, the two institutions who collaborated to develop and build ChemCam and are now running the instrument for over 12 years! And the PI, Dr Nina Lanza from Los Alamos informs me that the first milestone – 10000 shots was reached as early as Sol 42, which was the sol the DAN instrument used its active mode for the first time. But before I am getting melancholic, let’s talk about today’s plan!
      The drive ended fairly high up in the terrain, and that means we see a lot of the interesting features in the channel and generally around us. So, we are on a spot a human hiker would probably put the backpack down, take the water bottle out and sit down with a snack to enjoy the view from a nice high point in the landscape. Well, no such pleasures for Curiosity – and I am pretty sure sugar, which we humans love so much, wouldn’t be appreciated by rover gears anyway. So, let’s just take in the views! And that keeps Mastcam busy taking full advantage of our current vantage point. We have a terrain with lots of variety in front of us, blocks, boulders, flatter areas and the walls are layered, beautiful geology. Overall there are 11 Mastcam observations in the plan adding up to just about 100 individual frames, not counting those taken in the context of atmospheric observations, which are of course also in the plan. The biggest mosaics are on the targets “Western Deposit,” “Balloon Dome,” and “Coral Meadow.” Some smaller documentation images are on the targets “Wales Lake,” “Gnat Meadow,” and “Pig Chute.”
      ChemCam didn’t have long to dwell on its milestone, as it’s busy again today. Of course, it will join Mastcam in taking advantage of our vantage point, taking three remote micro imager images on the landscape around us. LIBS chemistry investigations are targeting “Wales Lake,” “Gnat Meadow,” and “Pig Chute.” APXS is investigating two targets, “College Rock” and “Wales Lake,” which will also come with MAHLI documentation. With all those investigations together, we’ll be able to document the chemistry of many targets around us. There is such a rich variety of dark and light toned rocks, and with so much variety everywhere, it’s hard to choose and the team is excited about the three targeted sols … and planning over 4 hours of science over the weekend!
      The next drive is planned to go to an area where there is a step in the landscape. Geologists love those steps as they give insights into the layers below the immediate surface. If you have read the word ‘outcrop’ here, then that’s what that means: access to below the surface. But there are also other interesting features in the area, hence we will certainly have an interesting workspace to look at! But getting there will not be easy as the terrain is very complex, so we cannot do it in just one drive. I think there is a rule of thumb here: the more excited the geo-team gets, the more skills our drivers need. Geologists just love rocks, but of course, no one likes driving offroad in a really rocky terrain – no roads on Mars. And right now, our excellent engineers have an extra complication to think about: they need to take extra care where and how to park so Curiosity can actually communicate with Earth. Why? Well, we are in a canyon, and those of you liking to hike, know what canyons mean for cell phone signals… yes, there isn’t much coverage, and that’s the same for Curiosity’s antenna. This new NASA video has more information and insights into the planning room, too! So, we’ll drive halfway to where we want to be but I am sure there will be interesting targets in the new workspace, the area is just so, so complex, fascinating and rich!
      And that’s after Mars for you, after 12 years, 42 drill holes, and now 1 Million ChemCam shots. Go Curiosity go!!!
      Written by Susanne Schwenzer, Planetary Geologist at The Open University
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    • By NASA
      NASA, ESA/Matthias Maurer An astronaut aboard the International Space Station snapped this picture of the Moon as the station orbited 265 miles above the U.S. state of Minnesota on Dec. 17, 2021.
      Astronauts aboard the orbital lab take images using handheld digital cameras, usually through windows in the station’s cupola, for Crew Earth Observations. Crew members have produced hundreds of thousands of images of the Moon and Earth’s land, oceans, and atmosphere.
      On Saturday, Sept. 14, 2024, International Observe the Moon Night, everyone on Earth is invited to learn about lunar science, participate in celestial observations, and honor cultural and personal connection to the Moon. Find an event to join in the celebration.
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    • By NASA
      Skywatching Skywatching Home Eclipses What’s Up Explore the Night Sky Night Sky Network More Tips and Guides FAQ 23 Min Read The Next Full Moon is a Partial Lunar Eclipse; a Supermoon; the Corn Moon; and the Harvest Moon
      The Next Full Moon is a Partial Lunar Eclipse; a SuperMoon; the Corn Moon; the Harvest Moon; the Fruit or Barley Moon; the end of Ganesh Chaturthi and the start of Pitru Paksha; Madhu Purnima; the Mid-Autumn, Mooncake, or Reunion Festival Moon; Chuseok; and Imomeigetsu or the Potato Harvest Moon.
      The full Moon will be Tuesday night, September 17, 2024, at 10:35 PM EDT. This will be on Wednesday from Newfoundland and Greenland Time eastward across Eurasia, Africa, and Australia to the International Date Line. Most commercial calendars will show this full Moon on Wednesday based on Greenwich or Universal Time. The Moon will appear full for about three days, from Monday evening through Thursday morning.
      This will be a partial lunar eclipse. The Moon will start entering the Earth’s partial shadow at 8:41 PM EDT. The slight dimming of the Moon will be difficult to notice until the top edge of the Moon starts entering the full shadow at 10:13 PM. The peak of the eclipse will be at 10:44 PM with only the top 8 percent of the Moon in full shadow. The Moon will finish exiting the full shadow at 11:16 PM and the partial shadow on Wednesday morning at 12:47 AM.
      The phases of the Moon for September 2024. NASA/JPL-Caltech This will be a supermoon. The term “supermoon” was coined by astrologer Richard Nolle in 1979 as either a new or full Moon that occurs when the Moon is within 90% of its closest to Earth. Since we can’t see new Moons, what has the public’s attention are full supermoons, the biggest and brightest Moons of the year. Although different publications use different thresholds for deciding which full Moons qualify, most agree this will be the second of four consecutive supermoons (effectively tied with the full Moon in October for the closest of the year).
      The Maine Farmer’s Almanac first published “Indian” names for the full Moons in the 1930s and these names have become widely known and used. According to this almanac, as the full Moon in September the Algonquin tribes in what is now the northeastern USA called this the Corn Moon, as this was the time for gathering their main staple crops of corn, pumpkins, squash, beans, and wild rice.
      As the full Moon closest to the autumnal equinox, this is the Harvest Moon. The first known written use of this name in the English language (per the Oxford English Dictionary) was in 1706. During the fall harvest season farmers sometimes need to work late into the night by moonlight. On average moonrise is about 50 minutes later each night. Around the Harvest Moon this time is shorter, about 25 minutes for the latitude of Washington, DC, and only 10 to 20 minutes farther north in Canada and Europe.
      Other European names for this full Moon are the Fruit Moon, as a number of fruits ripen as the end of summer approaches, and the Barley Moon, from the harvesting and threshing of barley.
      For Hindus, this full Moon marks the end of Ganesh Chaturthi and the start of Pitru Paksha. Ganesh Chaturthi (also called Vinayaka Chaturthi or Vinayaka Chavithi) is a 10 or 11 day festival honoring the god Ganesha that ends with this full Moon. Ganesha is easily recognized by his elephant head and is worshiped as the god of beginnings, wisdom, arts and sciences, and as the remover of obstacles. Throughout the festival celebrants offer food, sweets, and prayers to clay statues of Ganesha at home and on public stages. Traditions include chanting of Vedic hymns and Hindu texts, prayers, and fasting. On the last day (near the full Moon), people carry the statues to a nearby river or ocean and immerse them. As the clay dissolves, Ganesha is believed to return to his parents, the god Shiva and goddess Parvati, on Mount Kailash.
      Pitru Paksha (fortnight of the ancestors) is a 15 days long festival that ends with the new Moon. During this time, Hindus honor their ancestors (pitrs) with rituals, food offerings, and scripture reading. Pitru Paksha is also known by a number of other names.
      For some Buddhists in Bangladesh and Thailand this full Moon is Madhu Purnima, the Honey Full Moon Festival or the Honey-offering Festival. The legend is that when the Buddha was trying to bring peace between two factions in a forest, an elephant and a monkey fed him, with the elephant offering fruit and the monkey offering a honeycomb.
      In China, Vietnam, and some other Asian countries, this full Moon corresponds with the Mid-Autumn Festival, a traditional harvest festival. In China, other names for this festival include the Moon Festival, the Mooncake Festival, and the Reunion Festival (with wives visiting their parents then returning to celebrate with their husbands and his parents). Part of the festival includes offerings to the Moon Goddess Chang’e (the name the China National Space Agency gives their lunar missions).
      In Korea, this full Moon corresponds with the harvest festival Chuseok, during which Koreans return to their traditional hometowns to pay respect to the spirits of their ancestors.
      This full Moon corresponds with the first of two Japanese Tsukimi or “Moon-Viewing” festivals, also called Imomeigetsu (which translates as “potato harvest Moon”) because of the tradition of offering sweet potatoes to the Moon. These festivities have become so popular that they are often extended for several days after the full Moon.
      In many traditional Moon-based calendars the full Moons fall on or near the middle of each month. This full Moon is near the middle of the eighth month of the Chinese year of the Dragon and Rabi’ al-Awwal in the Islamic calendar, the month in which many Muslims celebrate Mawlid, the birth of the Prophet Muhammad. This full Moon is near the middle of Elul in the Hebrew calendar. Elul is a time of preparation for the High Holy Days of Rosh Hashanah and Yom Kippur. Customs include granting and asking others for forgiveness as well as beginning or ending all letters with the wish that the recipient will have a good year.
      As usual, the wearing of suitably celebratory celestial attire is encouraged in honor of the full Moon. Go out and observe the Moon, enjoy this harvest season (including corn, fruit, and sweet potatoes, and honey), remember your ancestors, stay in touch with your parents, and forgive and ask forgiveness. Here’s wishing you a good year!
      Comet C/2023 A3 (Tsuchinshan-ATLAS)
      Pay attention to the news about Comet C/2023 A3 (Tsuchinshan-ATLAS)! There are a number of “ifs” so we don’t like to raise expectations. Similar visitors from the Oort Cloud have broken apart and fizzled out as they passed close to the Sun. If this comet survives its passage by the Sun (closest approach on September 27, 2024) and if the amount of gas and dust it gives off does not decrease significantly, this might be one of the best comets in a long time. If it strongly scatters sunlight towards the Earth it might even be visible in the glow of dusk just after its closest approach to Earth on October 12.
      From the Washington, DC area and similar latitudes, this comet will be above the horizon before morning twilight begins from September 22 through October 4, with the current brightness curve predicting a steady increase in brightness from about visual magnitude 4 to near 3 (the smaller the number, the brighter the object). As it brightens it may be visible under dark sky conditions and even more impressive through binoculars or a telescope, although towards the start and end of this period it may be too low on the horizon to see when the sky is completely dark.  
      Between about October 4 and October 11 the Sun’s glare will mask visibility from the Northern Hemisphere. Check your local news or web sites for viewing information for your latitude. For example, Sky and Telescope reports that Southern Hemisphere skywatchers should fare better.
      Comet C/2023 A3 (Tsuchinshan-ATLAS) will be at its closest to Earth on October 12 at 11:10 AM EDT. Around closest approach the comet’s brightness is predicted to peak at about visual magnitude 3 (similar to many stars). Forward scattering might increase the brightness significantly, possibly as high as -1 (brighter than every star except Sirius). How bright the comet actually appears will depend upon how much gas and dust it is giving off, which can change quickly. Also, brightness comparisons between comets and stars can be misleading as the light of the comet is spread out making it less distinct than a star with the same brightness.
      The best time to look should be the evenings on and shortly after October 12 with the comet above the western horizon after sunset. The evening of October 12 the comet will be 4 degrees above the western horizon as evening twilight ends, similar in altitude and to the right of Venus. The comet is expected to dim as it moves away from the Earth, but will appear higher in a darker sky and set later each evening, which could make it easier to see. As evening twilight ends on October 13 it will be 10 degrees above the western horizon, 12 degrees on October 14, 16 degrees on October 15, etc. The brightness will decrease to about magnitude 6 by the end of October.
      Meteor Showers
      During this lunar cycle four minor meteors showers are predicted to peak at 5 or fewer visible meteors per hour (under ideal viewing conditions), making them basically not visible from our light-polluted urban areas.
      Evening Sky Highlights
      On the evening of Tuesday, September 17 (the evening of the full Moon), as twilight ends (at 8:10 PM EDT), the rising Moon will be 11 degrees above the east-southeastern horizon with Saturn to the upper right at 14 degrees above the horizon. Later in the evening the partial shadow of the Earth will cover a small upper part of the Moon. Bright Venus will be 2 degrees above the west-southwestern horizon with the star Spica on the horizon to the lower left. The bright star closest to overhead will be Vega, the brightest star in the constellation Lyra the lyre, at 87 degrees above the western horizon. Vega is part of the Summer Triangle along with Deneb and Altair. It is the 5th brightest star in our night sky, about 25 light-years from Earth, has twice the mass of our Sun, and shines 40 times brighter than our Sun.
      As this lunar cycle progresses, Saturn and the background of stars will appear to shift westward each evening (as the Earth moves around the Sun). Bright Venus will shift to the left along the west-southwestern horizon, appearing slightly higher each evening. The waxing Moon will pass by Venus on October 5, Antares on October 7, and Saturn on October 14. Comet C/2023 A3 (Tsuchinshan-ATLAS) will be at its closest to Earth on October 12 at 11:10 AM. Assuming it survives its pass by the Sun on September 27 and depending upon how much gas and dust it gives off, it could be a good show in the evenings on and after October 12. See the comet summary above and keep an eye on the news for updates on this comet.
      By the evening of Thursday, October 17 (the evening of the full Moon after next), as twilight ends (at 7:24 PM EDT), the rising Moon will be 9 degrees above the eastern horizon. Saturn will be 27 degrees above the southeastern horizon. Bright Venus will be 6 degrees above the west-southwestern horizon. Comet C/2023 A3 (Tsuchinshan-ATLAS) will be 22 degrees above the western horizon. The bright star closest to overhead will be Deneb at 80 degrees above the northeastern horizon. Deneb is the 19th brightest star in our night sky and is the brightest star in the constellation Cygnus the swan. Deneb is one of the three bright stars of the “Summer Triangle” (along with Vega and Altair). Deneb is about 20 times more massive than our Sun but has used up its hydrogen, becoming a blue-white supergiant about 200 times the diameter of the Sun. If Deneb were where our Sun is, it would extend to about the orbit of the Earth. Deneb is about 2,600 light years from us.
      Morning Sky Highlights
      On the morning of Wednesday, September 18 (the morning of the night of the full Moon), as twilight begins (at 5:55 AM EDT), the setting full Moon will be 15 degrees above the west-southwestern horizon. The brightest planet in the sky will be Jupiter at 71 degrees above the south-south eastern horizon. Near Jupiter will be Mars at 61 degrees above the east-southeastern horizon. Saturn will be below the Moon at 1 degree above the western horizon. The bright star appearing closest to overhead will be Capella, the brightest star in the constellation Auriga the charioteer, at 80 degrees above the northeastern horizon. Although we see Capella as a single star (the 6th brightest in our night sky), it is actually four stars (two pairs of stars orbiting each other). Capella is about 43 lightyears from us.
      As this lunar cycle progresses, Jupiter, Mars, Saturn, and the background of stars will appear to shift westward each evening. After September 19 Saturn set before morning twilight begins. The waning Moon will pass by the Pleiades star cluster on September 22, Mars on September 25, Pollux on September 26, and Regulus on September 29. Comet C/2023 A3 (Tsuchinshan-ATLAS) will be above the horizon before morning twilight begins from September 22 through October 4. Comets are notoriously difficult to predict, but if the amount of gas and dust it gives off remains constant it should increase in brightness each morning. See the comet summary above and keep an eye on the news for updates on this comet.
      By the morning of Thursday, October 17 (the morning of the full Moon after next), as twilight begins (at 6:22 AM EDT), the setting full Moon will be 11 degrees above the western horizon. The brightest planet in the sky will be Jupiter at 63 degrees above the west-southwestern horizon. Mars will be at 72 degrees above the south-southeastern horizon. The bright star appearing closest to overhead will be Pollux, the 17th brightest star in our night sky and the brighter of the twin stars in the constellation Gemini, at 75 degrees above the southeastern horizon. Pollux is an orange tinted star about 34 lightyears from Earth. It is not quite twice the mass of our Sun but about 9 times the diameter and 33 times the brightness.
      Detailed Daily Guide
      Here for your reference is a day-by-day listing of celestial events between now and the full Moon on October 17, 2024. The times and angles are based on the location of NASA Headquarters in Washington, DC, and some of these details may differ for where you are (I use parentheses to indicate times specific to the DC area). If your latitude is significantly different than 39 degrees north (and especially for my Southern Hemisphere readers), I recommend using an astronomy app or a star-watching guide from a local observatory, news outlet, or astronomy club.
      Saturday night, September 14, is International Observe the Moon Night! See https://moon.nasa.gov/observe-the-moon-night/about/overview/ for more information.
      Our 24 hour clock is based on the average length of the solar day. Solar noon on Sunday, September 15 to solar noon on Monday, September 16, will be the shortest solar day of the year, 23 hours, 59 minutes, and 38.6 seconds long.
      Monday night into Tuesday morning, September 16 to 17, Saturn will appear near the full Moon. As evening twilight ends (at 8:12 PM EDT) Saturn will be 6 degrees to the left of the Moon. When the Moon reaches its highest for the night (at 12:17 AM) Saturn will be 4 degrees to the upper left. By the time morning twilight begins (at 5:54 AM) the Moon will be 1 degree above the west-southwestern horizon with Saturn 1 degree above the Moon. For parts of western North America and across the Pacific Ocean towards Australia the Moon will pass in front of Saturn. See http://lunar-occultations.com/iota/planets/0917saturn.htm for a map and information on the areas that will see this occultation.
      Tuesday morning, September 17, will be the last morning that Mercury will be above the horizon as morning twilight begins (at 5:54 AM EDT).
      As mentioned above, the full Moon will be Tuesday night, September 17, at 10:35 PM EDT. This will be on Wednesday from Newfoundland and Greenland Time eastward across Eurasia, Africa, and Australia to the International Date Line. Most commercial calendars are based on Greenwich or Universal Time and will show this full Moon on Wednesday. The Moon will appear full for about three days from Monday evening through Thursday morning.
      This will be a partial lunar eclipse. The Moon will start entering the partial shadow of the Earth at 8:41 PM EDT. The slight dimming of the Moon will be difficult to notice until the top edge of the Moon starts entering the full shadow at 10:13 PM. The peak of the eclipse will be at 10:44 PM with just the top 8.4% of the Moon in full shadow. The Moon will finish exiting the full shadow at 11:16 PM and the partial shadow on Wednesday morning at 12:47 AM.
      This will be the second of four consecutive supermoons, appearing larger than last month’s supermoon and effectively tied with the full Moon in October for the closest full Moon of the year.
      Tuesday and Wednesday evenings, September 17 and 18, the star Spica will appear a little over 2 degrees from the bright planet Venus. On Tuesday evening as evening twilight ends (at 8:10 PM EDT) Spica will be to the lower left of Venus and on the verge of setting on the west-southwestern horizon. Wednesday evening Spica will be a few hundredths of a degree closer and will appear below Venus, but will set about 2 minutes before evening twilight ends.
      Wednesday morning September 18, at 9:29 AM EDT, the Moon will be at perigee, its closest to the Earth for this orbit.
      Thursday morning, September 19, will be the last morning the planet Saturn will be above the western horizon as morning twilight begins.
      If you are interested in spotting the planet Neptune through a telescope, Friday evening, September 20, will be when it will be at its closest and brightest for the year. Neptune will reach its highest in the sky early Saturday morning (at 1:02 AM EDT).
      Saturday night into Sunday morning, September 21 to 22, the Pleiades star cluster will appear near the waning gibbous Moon. The Pleiades will be 5 degrees to the lower left as they rise on the east-northeastern horizon (at 9:23 PM EDT), 1.5 degrees to the upper left by the time the Moon reaches its highest for the night (at 4:44 AM), and less than 1 degree to the upper left as morning twilight begins (at 5:59 AM). The Moon will actually pass through the Pleiades (at about 8 AM) when daylight will mask these stars from view.
      Sunday morning, September 22, will be the first morning Comet C/2023 A3 (Tsuchinshan-ATLAS) will be above the horizon before morning twilight begins, with the current brightness curve predicting it at visual magnitude 4. Unless it breaks apart, this comet is likely to brighten each morning until October 4 (after which it will no longer be above the horizon before twilight begins).
      Sunday morning, September 22, at 8:44 AM EDT, will be the autumnal equinox, the astronomical end of summer and start of fall.
      Monday night into Tuesday morning, September 23 to 24, the bright planet Jupiter will appear to the lower right of the waning half-full Moon. Jupiter will be 6 degrees to the lower right as it rises on the east-northeastern horizon (at 10:54 PM EDT). Jupiter will shift slightly clockwise as it moves away from the Moon.
      Thursday afternoon, September 24, the waning Moon will appear half-full as it reaches its last quarter at 2:50 PM EDT (when we can’t see it).
      Wednesday morning, September 25, the planet Mars will appear below the waning crescent Moon. Mars will be 6 degrees below the Moon as it rises on the east-northeastern horizon (at 12:16 AM EDT). Mars will be 5 degrees to the lower right as morning twilight begins (at 6:01 AM).
      Thursday morning, September 26, the star Pollux (the brighter of the twin stars in the constellation Gemini the twins) will appear near the waning crescent Moon. Pollux will be 3 degrees to the lower left as it rises on the northeastern horizon (at 12:47 AM EDT) and will be 2 degrees to the upper left by the time morning twilight begins (at 6:02 AM).
      Friday afternoon, September 27, at around 2 PM EDT, Comet C/2023 A3 (Tsuchinshan-ATLAS) will be at its closest to the Sun. This comet has an inbound orbital period of millions of years and may gain enough energy from this flyby of the Sun to leave the solar system forever.
      Sunday morning, September 29, the star Regulus will appear near the waning crescent Moon. As Regulus rises on the east-northeastern horizon (at 4:01 AM EDT) it will be 2.5 degrees to the lower right of the Moon. Morning twilight will begin 2 hours later (at 6:05 AM) with Regulus 3 degrees to the right.
      Monday afternoon, September 30, the planet Mercury will be passing on the far side of the Sun as seen from the Earth, called superior conjunction. Because Mercury orbits inside of the orbit of Earth, it will be shifting from the morning sky to the evening sky and will begin emerging from the glow of twilight on the west-southwestern horizon towards the end of October (depending upon viewing conditions).
      Wednesday, October 2, at 2:46 PM EDT, will be the new Moon, when the Moon passes between the Earth and the Sun and is usually not visible. For much of the Pacific Ocean as well as the southern part of South America, part of Antarctica, and a thin slice of the southwestern Atlantic, the Moon will block some of the Sun in a partial eclipse. For a narrow strip from the Pacific south of the Hawaiian Islands across the Pacific, part of Chile and Argentina, and into the southwestern Atlantic Ocean, the Moon will actually pass in front of the Sun, blocking most of it from view in an annular solar eclipse. Because the Moon will be at apogee (its farthest from the Earth) just 70 minutes later (at 3:56 PM) it will not block the entire Sun from view and this will not be a total solar eclipse.
      The day of or the day after the New Moon marks the start of the new month for most lunisolar calendars. Sundown on Wednesday, October 2, will be the start of Rosh Hashanah (the Head of the Year), the two-day Jewish New Year celebration that will end at sundown on Friday, October 4. Rosh Hashanah is the first of a series of holidays in Tishrei, the first month of the Hebrew calendar. The tenth day of Tishrei is Yom Kippur, the Day of Atonement. The 10 days from Rosh Hashanah to Yom Kippur, called the Days of Awe, are a time to reflect on the mistakes of the past year and make resolutions for the new year. The fifteenth day of Tishrei (close to the full Moon after next) is the start of the 7-day Sukkot holiday.
      The ninth month of the Chinese year of the Dragon starts on Thursday, October 3.
      In the Islamic calendar the months traditionally start with the first sighting of the waxing crescent Moon. Many Muslim communities now follow the Umm al-Qura Calendar of Saudi Arabia, which uses astronomical calculations to start months in a more predictable way. Using this calendar, sundown on Thursday evening, October 3, will probably mark the beginning of Rabiʽ al-Thani, also known as Rabi’ al-Akhirah.
      Friday, October 4, will be the last morning Comet C/2023 A3 (Tsuchinshan-ATLAS) will be above the horizon before morning twilight begins, with the current brightness curve predicting a visual magnitude near 3, similar in brightness to many visible stars. It may be visible to the naked eye under dark sky conditions and even more impressive through binoculars or a telescope.
      Saturday evening, October 5, you may be able to see the thin waxing crescent Moon 4.5 degrees to the lower left of the bright planet Venus. As evening twilight ends (at 7:41 PM EDT) the Moon will be a degree above the west-southwestern horizon. The Moon will set first 14 minutes later (at 7:55 PM).
      Monday evening, October 7, the bright star Antares will appear 2 degrees to the right of the waxing crescent Moon. As evening twilight ends (at 7:38 PM EDT) the Moon will be 11 degrees above the southwestern horizon. Antares will set first about 20 minutes later (at 9 PM).
      Thursday afternoon, October 10, the Moon will appear half-full as it reaches its first quarter at 2:55 PM EDT.
      Saturday morning, October 12, at 11:10 AM, Comet C/2023 A3 (Tsuchinshan-ATLAS) will be at its closest to Earth. If it survives its pass by the Sun this will likely be when it will be near its brightest. Although it will be on the horizon as evening twilight ends on Friday, our first chance to see it above the horizon as it emerges from the glow of dusk likely will be Saturday evening, when the comet will be 4 degrees above the western horizon as evening twilight ends (at 7:31 PM EDT), similar in altitude and to the right of Venus. Over the next few nights the comet will likely dim as it moves away from the Earth, but also appear higher in the sky and set later each evening, giving us more time and darker skies to look for this comet. As evening twilight ends on October 13 it will be 10 degrees above the western horizon, 12 degrees on October 14, 16 degrees on October 15, etc. Current brightness curves predict it will dim quickly and will be below magnitude 6 by the end of October. How bright the comet will be and how quickly it actually dims will depend upon the gas and dust it is giving off, which can vary quickly and unpredictably, but it could be a good show in the evenings after October 12.
      Monday evening, October 14, the planet Saturn will appear near the waxing gibbous Moon. As evening twilight ends (at 7:28 PM EDT) Saturn will be 4 degrees to the upper right. The Moon will reach its highest for the night about 3.5 hours later (at 10:53 PM) with Saturn 5 degrees to the lower right. The pair will continue to separate, with Saturn setting first 5 hours after that (at 4:09 AM). For parts of Southern Asia and Africa the Moon will block Saturn from view, see http://lunar-occultations.com/iota/planets/1014saturn.htm for a map and information on the areas that will acually see this occultation.
      Wednesday evening, October 16, at 8:57 PM EDT, the Moon will be at perigee, its closest to the Earth for this orbit.
      The full Moon after next will be Thursday morning, October 17, 2024, at 7:26 AM EDT. This will be late Wednesday night in the International Date Line West time zone and early Friday morning from New Zealand Time eastwards to the International Date Line. This will be the third of four consecutive supermoons (and the brightest by a tiny margin). The Moon will appear full for about 3 days around this time, from Tuesday evening through Friday morning.
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