The night sky in September 2021
Post date: Aug 30, 2021 2:51:3 PM
by Anne Holt
Sunrise 1st: 06.18 30th: 07.09
Sunset 1st: 19.58 30th: 18.47
Astronomical darkness 1st: 22.10 to 04.07 30th: 20.44 to 05.13
Day length 1st: 13.59.50 30th: 11.38.39
The autumnal equinox in the northern hemisphere is on 22nd at 20.21, when the centre of the Sun crosses the celestial equator moving southwards, marking the start of Autumn. This day is 12hrs 12 mins and 15 seconds long, despite the name equinox, which means equal night. There are 2 reasons for this, firstly it is the day when the centre of the Sun is above the horizon for exactly 12 hours, whereas sunrise and sunset times are when the top edge appears and disappears. Also, refraction of the light by our atmosphere means that we can see the sun for a few minutes before it rises and after it sets. The closest day to 12 hours is 25th at 11.59.39.
New Moon: 7th at 01.51, when it passes 4 degrees 56’ north of the Sun.
Full Moon: 21st at 00.54, angular diameter 30' 37".
Lunar perigee: 11th at 11.07, distance 368463 km, angular diameter 32' 24"
Lunar apogee: 26th at 22.45, distance 404639 km, angular diameter 29' 30"
This month’s full Moon is the closest to the autumnal equinox, making it the Harvest Moon. Other names given in the Old Farmer’s Almanac are the Corn Moon and the Barley Moon. Many more of this month’s names relate to the harvest and various crops, among those which don’t, we have the Celtic Singing Moon or Wine Moon - could there be a connection there? It’s the Chinese Chrysanthemum Moon, the Inuit Harpoon Moon and the Dakota Sioux Moon when calves grow hair.
Highlights
The best sight in the sky this month is the planet Jupiter, shining brightly for much of the night, reaching 22 degrees in the south as it culminates. Saturn, though lower and much fainter, is still reasonably placed and the 2 ice giants, Uranus and Neptune, are high in the sky. We have the autumnal equinox, when the Sun rises due east and sets due west, and a reasonable amount of astro darkness now beginning well before midnight - a couple of minutes short of 6 hours on 1st and 8 and a half hours by the end of the month.
Unfortunately there are still no bright comets and not much in the way of meteor showers - though the fireball season begins in mid September with the Southern Taurids.
And we have just confirmed the date of our Open Day, in the classroom behind the Stables Cafe, as Sunday Sept 26th.
Constellations
The Milky Way is still prominent overhead, albeit not in these parts! Find a dark sky site though, and it's spectacular.
The Summer Triangle is high in the southern sky for much of the night in early September. By month end Aquila is setting in the west at about 2am, with Lyra and Cygnus following just before dawn.
However, on the opposite side of the sky, the Pleiades are climbing above the horizon in the east by 10.30pm at the start of September, and as darkness falls at month end. Capella, in Auriga, and the V shaped Hyades cluster at the head of Taurus the Bull are not far behind.
If you stay up until about 4am (or get up very early) you might see Orion making a welcome return to the night sky. By the end of September, it should be above the horizon by 2am.
The ecliptic is now slightly higher across the Eastern sky, passing through Capricorn, Aquarius and Aries - though none of these are particularly bright or memorable.
Pegasus, Perseus and Andromeda are still well placed, rising in the east to north east from mid evening, as is the bright W asterism of Cassiopeia higher in the north east.
Planets
Mercury: in Virgo, mag -0.1
Hardly visible this month, an evening object but extremely low in the sky. On 1st it is 2 degrees above the horizon 15 minutes after sunset and sets at 20.30, before the sky is dark enough for it to be seen easily. It is at aphelion, the furthest point in its orbit from the Sun, on 6th at a distance of 0.47AU. It reaches greatest eastern elongation on 13th, when the apparent separation is 26 degrees 48’. However the angle of the ecliptic to the horizon at this time is very small, the planet sets only 20 minutes after the Sun and is 3 degrees below the horizon at dusk. On 30th, now down to mag 1.8, it sets around sunset.
Venus: in Virgo, mag -4.0
Also very low in the evening sky. Throughout September it is about 4 degrees above the horizon as the sky darkens and sets an hour after the Sun. However, because it is so bright, it should be easily visible in the evening twilight to observers with a low, clear western horizon. On 1st it sets at 20.59 and on the evening of 5th it is only 1 degree 30’ north of Spica, Alpha Virginis, though it will be very difficult to see the first magnitude star, just above the horizon. On 10th, the 3 day Moon passes 4 degrees 04’ north of the planet at 03.08, separation at dusk on 9th is around 7 degrees, with the Moon to the right, a degree more and to the left after sunset on 10th, when Venus sets at 20.35. On 30th, slightly brighter at mag -4.2 it sets at 19.50.
Mars: in Leo, mag 1.8
Not visible this month as it appears very close to the Sun, only 12 degrees separation on 1st, down to 2 degrees on 30th. It is at apogee at 12.33 on 20th, at a distance of 2.64AU from Earth.
Jupiter: in Capricorn, mag -2.8
Shining brightly in the evening sky. On 1st it should become visible shortly before 9pm, in nautical twilight. It culminates at 00.22 reaching 22 degrees in the south and remains visible until around 4am, setting at 05.06. On 18th the gibbous Moon passes 3 degrees 57’ to the south at 07.54, separation is 6 degrees at midnight of 17th/18th down to around 5 degrees at 02.40 as the planet gets too low in the SW to be seen easily. On 30th it should be seen soon after 7pm, 11 degrees in the SE, culminating at 22.12 when it is 21 degrees in the south. It sinks to 8 degrees in the SW by 01.45, setting a little over an hour later.
Saturn: in Capricorn, mag 0.3
Now around 17 degrees SW of Jupiter. On 1st it is at 10 degrees in the SE when it becomes visible around 9pm, culminating, 17 degrees in the south, at 23.05 and sinking to 10 degrees in the SW by 01.40, setting at 03.18. The 89% Moon is 3 degrees 45’ to the south at 03.33 on the night of 16th/17th, the separation is around 6 degrees when Saturn culminates at 22.07. By 2am it is just over 4 degrees. On 30th Saturn becomes visible, 14 degrees in the SE, at 19.30 and culminates, 3 degrees higher, at 21.08. By 23.30 it is down to 10 degrees in the SW, and sets at 01.19.
Uranus: in Aries, mag 5.7
Well placed this month, a good time to try to spot the distant ice giant with the naked eye - given good eyesight, a very dark sky and knowing exactly where to look. Otherwise it should be visible in good binoculars as a star like object - a scope is needed to show the small blue/green disc. On 1st it rises at 21.44 and should be visible, 16 degrees WSW of the Pleiades, from soon after midnight until dawn, when it will be at 51 degrees in the south. After the first week in September it culminates in darkness, on 8th it rises at 21.16 and culminates at 52 degrees, a few minutes before it is lost in the morning twilight around 5am. On 24th the 83% Moon passes 1 degree 14’ to the south at 18.08 while the planet is still below the horizon. The separation at 22.45, as it becomes visible 22 degrees in the east, is around 2 degrees. When it culminates at 03.43 the pair are almost 5 degrees apart. On 30th Uranus is 21 degrees in the east soon after 22.15, reaches 52 degrees in the south at 03.21 and is still high, 42 degrees in the SE, when the sky begins to brighten a little before 6am.
Neptune: in Aquarius, mag 7.8
High in the sky for most of the night, could possibly be seen in decent binoculars (by those who can see Uranus without optical aid!), a scope will show its rich blue colour. On 1st it rises at 20.21 and is high enough for observing soon after 23.00, when it is at 21 degrees in the SE. It reaches 32 degrees in the south at 02.01 and is down to 22 degrees in the SW as the sky begins to brighten. It is at opposition on 14th, when it is at its highest point at 01.07 and high enough for observing for most of the night. On 30th it rises at 18.25 and gets to 21 degrees in the SE by 21.15, culminating soon after midnight and down to 21 degrees in the SW by dawn.
Dwarf Planets
Ceres: in Taurus, mag 8.9
On 1st it rises at 23.27 and should be observable from soon after 2am, reaching 44 degrees in the SE by dawn. The 15th is a good time to try to find it as it is just under 1 degree SE of Alpha Tauri - Aldebaran, the eye of the Bull. It culminates in darkness in the last few days of September, on 27th it reaches 52 degrees in the south at 05.32, a few minutes before it is lost in the brightening sky. On 30th it is at 22 degrees at 00.20, culminates at 05.18 and is still at 52 degrees when the sky begins to brighten half an hour later.
Pluto: In Sagittarius, mag 15.0
Too low for imaging or telescopic observing, max 13 degrees above the horizon.
Haumea: in Bootes, mag 17.4
Only high enough for imaging for a very short time at the beginning of September. On 1st it is at 23 degrees in the west as the sky darkens around 21.30, but is too low after only 15 minutes, setting at 00.20. By 30th it is only 18 degrees at dusk, setting at 22.26.
Makemake: in Coma Berenices, Mag 17.2
Too low in the evening sky for imaging. On 1st it is only 18 degrees above the western horizon at dusk, setting at 23.58. It gets even lower during the month and on 30th is at solar conjunction, on the opposite side of the Sun to the Earth. Because its orbit is inclined by 29 degrees to the plane of the ecliptic it will not appear close to the Sun at this time, it passes 27 degrees to the north at 15.37 and is 13 degrees above the horizon at dusk, setting at 22.00 - more than 3 hours after sunset.
Eris: in Cetus, mag18.8
High enough in the morning sky for imaging, but so faint that it is only a target for the most experienced astrophotographers. On 1st it is at 21 degrees in the SE at 1am, culminating at 04.15, when it reaches 35 degrees in the south. It is only a degree lower when it is lost in the brightening sky around 04.45. By 30th it is higher than 21 degrees from 23.00 until dawn, culminating at 02.20.
Asteroids at Opposition
2 Pallas: in Pisces, mag 8.8
The second asteroid to be discovered and the third largest - one of the so-called Big 4. On 1st it is at 21 degrees in the SE soon after 10pm, reaches 38 degrees in the south at 01.46 and is down to 26 degrees in the SW by dawn. It is at opposition on 14th, slightly brighter at mag 8.6, higher than 21 degrees between 21.35 and 04.25 and highest, 38 degrees in the south, at 00.59. On 30th it is high from 20.35 to 02.19 and culminates, 32 degrees in the south, at 23.08.
Comets
Again, nothing spectacular. C/2021 03 (PanSTARRS) is predicted to reach binocular, or maybe even naked eye, brightness early next year. It’s currently in Pegasus, high in the sky but too faint, at mag 20, for amateurs to attempt.
67P/Churyumov-Gerasimenko: in Aries, mag 10
On 1st it is high in the sky from around 1am, reaching 51 degrees in the south by dawn. Towards the end of the month it culminates before fading from view as the sky brightens. On 30th, now at mag 9, it rises at 21.16, reaches 21 degrees in the east by midnight and is at its highest point, 57 degrees in the south, at 05.26 - about 20 minutes before it is lost in the morning twilight.
15P/Finlay: in Gemini, mag 10.3
Getting higher in the morning sky, but fading. On 1st it rises at 00.21, becoming visible around 03.30 and reaching 34 degrees in the east by dawn. It moves into Cancer on 28th and on 30th rises at 23.44, high enough to be observed from 02.45 and getting to 47 degrees in darkness. It will have faded to mag 11.5.
4P/Faye: in Taurus, mag 11.0
Brightens very slightly during September. On 1st it rises at 23.24 and should be high enough for imaging from 2am until dawn, when it will be at 43 degrees in the SE. It is at perihelion, 1.62 AU from the Sun, on 10th and moves into Orion on 27th. On 30th, now at mag 10.7, it rises at 22.56 and is observable from 01.30, getting to 51 degrees in the south before the sky begins to brighten.
And: if you’ve read about the discovery of the largest comet ever seen, don’t get too excited. C/2014 UN271 was found by Bernadinelli and Bernstein when they studied data from the Dark Energy Survey between 2013 to 2019, and announced on June 19th this year. It’s nucleus is over 100 km in diameter, maybe much larger. It is currently at a distance of around 21AU and won’t get closer than 10.9AU at perihelion in January 2031. Calculations of its orbit show that it has been travelling towards the Sun, from the Oort Cloud, for about 1.5 million years, and won’t get back there for another 4.5 million years.
Meteor Showers
After the anticipation, and disappointment, of last month’s Perseids we are back to having only a couple of minor showers.
Aurigids: active Aug 25th to Sept 5th, peak on the night of 31st/1st. ZHR 6, probable maximum 5 from Manchester. Peak activity is predicted for 4am on the morning of 1st. The radiant is highest after dawn, at 9am, so is reasonable in the early hours, however the 28% Moon rises half an hour before midnight so will interfere. There could be an outburst earlier in the night, around 22.30, when the Moon is out of the way but the radiant is very low - only 10 degrees in the NE. These are fast moving meteors, parent comet C/1911 (Kiess).
Epsilon Perseids: active 5th to 21st, peak 9th, ZHR 5 (4 from Manchester)
The peak of this shower is around midday, so it is best observed before dawn and after dusk on 9th, though the circumpolar radiant is highest in the early hours. The 10% Moon sets shortly before 9pm, so won’t interfere. Also fast moving meteors, only marginally slower than Aurigids.
Southern Taurids are active from Sept 10th, but don’t peak until October. Rates are low, even at the maximum, but it’s worth keeping an eye out for these fairly slow moving meteors in the early hours, as the shower often includes bright fireballs.
Daytime Sextantids: active early Sept to early Oct, peak around Sept 29th, ZHR 5. The dates for this shower are said to be uncertain. The radiant is only 30 degrees west of the Sun, so most activity is in the daytime. However, a few may be spotted visually, just before dawn, around the time of the peak.
Credits
Most of the solar system information given here is from:
More information, exact co-ordinates and finder charts of all solar system objects can be found on this site.
Also quite useful is https://theskylive.com
https://www.imo.net/resources/calendar/
https://www.timeanddate.com
Other information is from various internet sources, including NASA, Britannica, Space Facts, Universe Today and, when all else fails,Wikipedia.