The Hanoi Astronomical Society (HAS) has announced that this September will feature some of the most exciting astronomical phenomena of the year, such as the Harvest Moon Supermoon on the 15th day of the 8th lunar month, the autumn equinox marking the start of fall in the Northern Hemisphere, and the comet C/2023 A3 reaching perihelion…
September 3: New Moon of the 8th Lunar Month
The Moon’s orbital motion causes it to revolve around the Earth approximately every four weeks, resulting in its phases changing cyclically from New Moon to First Quarter, Full Moon, Last Quarter, and back to New Moon every 29.5 days. This motion also means that the Moon shifts over 12 degrees eastward in the sky from night to night, causing it to rise and set nearly an hour later each day.
The New Moon of the 8th Lunar Month is an astronomical event worth watching.
On September 3 at 8:56 AM, the Moon will pass in front of the Sun at a distance of 2 degrees 16 minutes, becoming obscured by its brilliant light for a few days. At the New Moon phase (young Moon), the Moon is positioned between the Earth and the Sun, making nearly half of its dark hemisphere visible, rendering it nearly invisible in the sky.
In the following days after the New Moon, the Moon will appear in the sky as a thin crescent in the late afternoon and will set just a few hours after the Sun. About a week later, the Moon will remain visible in the night sky until midnight.
September 5: Mercury at Greatest Western Elongation
Mercury will reach its greatest western elongation during this morning appearance at 9:24 AM on September 5. From Hanoi, this event is not particularly prominent and can be difficult to observe as Mercury will only reach a maximum altitude of about 17 degrees above the horizon at sunrise.
Saturn at opposition in 2017 taken from Esfahan, Iran. (Photo credit: Kamran Janamian).
If you have binoculars or a telescope and point towards this planet in the morning sky in early September, Mercury will appear as a gradually thickening crescent. It will rise in the east around 4:30 AM, giving you about an hour to observe it before the Sun rises at 5:40 AM.
Warning: Never attempt to point binoculars or a telescope at an object too close to the Sun. This can severely damage your eyes and may even cause permanent blindness.
September 8: Saturn at Opposition
Saturn will reach opposition—position directly opposite the Sun in the sky as seen from Earth—at 11:27 PM on September 8. From Hanoi, Saturn can start to be observed from 6:55 PM when it will be approximately 11 degrees above the eastern horizon.
It will reach its highest position in the sky at 11:54 PM, at an altitude of 61 degrees above the southern horizon, and will set in the west at 5:44 AM the following morning. At the same time, as Saturn reaches opposition, it will also come closest to Earth, making this planet appear brighter and larger than any other time of the year.
September 18: Full Moon of the 8th Lunar Month – Harvest Supermoon
The Moon will reach its Full Moon phase at 9:34 AM on September 18. At the same time, this satellite will also pass through its closest position to Earth in its geocentric orbit. Such Full Moons are known as “Supermoons,” a term that originated among astrologers in the late 1970s. According to Richard Nolle, the creator of the Supermoon concept, there will be four Supermoons in 2024 on August 19, September 18, October 17, and November 15.
The Harvest Moon rises over Yokkaichi City, Japan in 2020.
During Supermoon nights, you will see the Moon in the sky with a disk that appears about 8% larger and 15% brighter than on regular Full Moon nights. Therefore, even with a normal observation, you can notice this difference.
The Full Moon of the 8th Lunar Month occurring on September 18 is also known as the Harvest Moon. This name is given to the Full Moon that occurs closest to the autumn equinox (around September 23). Depending on the year, the Harvest Moon can occur anytime from two weeks before to two weeks after the equinox, so it can happen in either September or October.
One unique aspect of the Harvest Moon is that close to the equinox, the time difference of the Moon rising between consecutive nights shortens. Instead of the usual 40 minutes to an hour, this interval reduces to about 20 minutes to under half an hour at this time, giving the impression that the Moon rises at the same hour for several consecutive nights.
September 21: Neptune at Opposition
Neptune will reach its position directly opposite the Sun in the sky as seen from Earth at 7:08 AM on September 21. Located in the constellation Pisces, this planet can be observed for most of the night and will reach its highest position in the sky at midnight local time.
From Hanoi, the planet will appear in the sky at 7:25 PM at an altitude of about 21 degrees above the eastern horizon. Neptune will reach its highest point in the sky at 11:47 PM, at an altitude of 67 degrees above the southern horizon, and will gradually fade after around 4 AM when it is lower than 21 degrees above the western horizon.
At the time of its opposition, Neptune will also come closest to Earth, making this planet appear brighter and larger than any other time of the year.
September 22: Autumn Equinox
The Autumn Equinox in 2024 will occur at 7:42 PM on September 22. This marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. On the day of the equinox, everywhere on Earth experiences nearly 12 hours of daylight and 12 hours of night as the Sun’s annual journey across the sky crosses the celestial equator at this time. No matter where you live, on the day of the equinox, the Sun will rise directly in the east and set directly in the west. The term “equinox” derives from the Latin words aequus (equal) and nox (night).
September 27: Comet C/2023 A3 (Tsuchinshan – ATLAS) at Perihelion
Comet C/2023 A3 (Tsuchinshan – ATLAS) will reach its closest point to the Sun in its orbit on September 27, 2024, at a distance of 0.39 AU. From Hanoi, it will rise in the eastern sky after 4:30 AM in the area of the constellation Sextans. You will have about an hour to observe this comet before the Sun rises at 5:50 AM.
Comet C/2023 A3 (Tsuchinshan – ATLAS) captured on July 28, 2024 with a long, thin ion tail. It continues to brighten as it moves inward toward the Sun and will reach perihelion on September 27. (Photo credit: Gerald Rhemann).
Comets are very unpredictable objects since their brightness depends on the scattering of sunlight from the dust particles in their head and tail. This dust is continuously released from the nucleus, and the density of dust at any given time depends on the sublimation rate of the ice in the comet’s nucleus as it is heated by sunlight. This is very difficult to predict and can change even between consecutive appearances of the same comet. Thus, predictions about a comet’s future brightness often lack reliability.
