The night sky in August 2021
Post date: Jul 28, 2021 9:4:38 PM
by Anne Holt
Sunrise 1st: 05.24 31st: 06.16
Sunset 1st: 21.04 31st: 20.00
Astronomical Darkness
1st: 00.35 to 01.55 31st: 22.13 to 04.04
Day Length:
1st: 15.39.37 31st: 13.43.57
New Moon: 8th at 14.51, when the Moon passes 4 degrees 40’ north of the Sun.
Full Moon: 22nd at 13.01. Angular diameter 31’ 29”
Lunar perigee: 17th at 10.15. (distance 369126 Km, angular diameter 32’ 21”, 74% waxing)
Lunar apogee: 2nd at 08.35 (distance 404410 Km, angular diameter 29’ 31”, 29% waning)
30th at 03.22 (distance 404098 Km, angular diameter 29’ 33”, 47% waning)
The August full Moon is a Blue Moon using the original definition of the third full Moon in a season which has 4. This is much less common than the more widely used definition of the second full Moon in a calendar month, the next one won’t be until August 2024.
Because August has only one full Moon, the other names for it also apply.
The most common name is the Old Farmer’s Almanac’s Sturgeon Moon, so called because the fish were said to be caught easily at this time. Other names are the Colonial American Dog Days Moon, the Medieval English Corn Moon and the Neo Pagan Lightning or Lightening Moon (mis-spelling or different meaning?). The Chinese have their Harvest Moon at this time and the Inuit call it the Swan Flying Moon. It’s the Celtic and Medieval English Dispute Moon - did the warm weather make them more argumentative?
Among the dozens of Indigenous American names given in various sources are the Hopi Joyful Moon, the Choctaw Women’s Moon, the Cherokee Fruit Moon or Drying Moon, the Sioux Black Cherries Moon, the Ojibwa Rice Making Moon and the Dakota Sioux Moon When all Things Ripen.
Highlights
Jupiter is very bright, reaching opposition on 20th, about 9 degrees higher than last year. The full Moon passes close by on the morning of 22nd. In early August Saturn is at opposition, much fainter and a few degrees lower but well worth observing through a scope as the rings are much brighter around this time. For those with a very dark sky site and good binoculars the distant ice giants are a good target, especially towards the end of the month. We have one of the best meteor showers of the year, moonlight free, and another, usually minor, shower may produce an outburst right at the end of the month. For a change, this month’s showers favour northern observers.
And, astronomical darkness is increasing - 80 minutes on 1st, up to almost 6 hours on the night of 31st/Sept 1st.
Constellations
When it finally gets dark enough, the Milky Way is now at its best. From a dark sky site it can be seen stretching right across the sky and down to the southern horizon, passing almost overhead around midnight.
The Milky Way passes through the Summer Triangle, which is now high in the sky, with Deneb and Vega particularly prominent. Alberio, a beautiful yellow and blue double star at the head of Cygnus the swan, is very well placed for telescopic observation.
The Plough and its host constellation Ursa Major are now very low in the Northern sky which means that the W asterism of Cassiopeia is riding high in the south east and very easy to spot.
Pegasus and Andromeda are now well above the horizon for most of the night and Perseus, followed by Auriga, are rising soon after midnight.
Planets
Mercury: in Cancer, mag -2.1
Very bright at the start of the month but, unfortunately, not visible as it is at superior conjunction on 1st, when it passes 1 degree 41’ north of the Sun at 15.14. It then becomes an evening object but remains very low. It moves into Leo on 6th, when it sets at 21.17 and appears only 5 degrees from the Sun. On 19th, now down to mag -0.5, it is only 4’ south of Mars at 04.00, however on the evenings of 18th and 19th it is on the horizon at dusk, setting around 21.00. It reaches its (very low) highest point in the evening sky on 21st, still only 4 degrees above the horizon at sunset, but one degree below by the time the sky darkens. It is in Virgo from 27th and on 31st sets at 20.32, only half an hour after the Sun.
Venus: in Leo, mag -3.9
Also very low in the evening sky, setting not long after Mercury - about an hour on 1st, down to 30 minutes by month end - but much easier to see as it is so bright. On 1st it is 4 degrees above the horizon at dusk, setting at 22.12, visible for a short time to observers with a low, clear western horizon. On 11th, when it moves into Virgo, the Moon passes 4 degrees 19’ north at 07.59. They are separated by about 9 degrees after sunset on 10th,when the Moon is 5% lit, and 6 degrees on 11th, with the Moon now 11%. On 31st Venus is still 4 degrees above the horizon in twilight and sets at 21.02.
Mars: in Leo, mag 1.8
Not visible this month as it is so low in the evening sky, setting in twilight. On 1st it sets at 21.53 and is 5 degrees below the horizon by the time the sky darkens. On 19th it is just 4’ north of Mercury at 5am, a few degrees apart on the evenings on 18th and 19th, but much too low to be seen in the evening twilight and too close to the Sun for safe binocular viewing. On 31st Mars sets at 20.25, only 40 minutes after the Sun, apparent separation 12 degrees.
Jupiter: in Aquarius, mag -2.8
Bright enough to be seen before midnight, despite being quite low in the sky. On 1st it rises at 21.47 and reaches 7 degrees in the SE by 23.00, culminating, 23 degrees in the south, at 02.40 and remaining visible until dawn. On 19th, when it crosses into Capricorn, its moons Io and Ganymede are only 2” apart as the planet is lost to the morning twilight. If looking at these through a scope or binoculars take great care to stop before sunrise. The following day it is at opposition, visible from dusk till dawn and reaching its highest point, 22 degrees in the south, at 01.14. On the morning of 22nd the full Moon passes just under 4 degrees to the south at 05.56, a few arcseconds closer at 08.11. The separation is around 6 degrees at midnight of the 21st/22nd. On the evening of 22nd Ganymede, Europa and their shadows cross the planet as it is rising for UK observers. On 31st it rises at 19.42 and gets to 8 degrees in the SE by 9pm, culminating at 00.27 and visible until around 5am, as dawn breaks.
Saturn: in Capricorn, mag 0.2
Much fainter than Jupiter and quite low in the sky but well worth observing through a scope. On 1st it rises at 21.05 and is at 10 degrees by 22.45, culminating, 18 degrees in the south, at 01.23. It will remain visible until the end of astro darkness at 4am, when it is down to 10 degrees in the SW. It is at opposition on 2nd, culminating at 01.18. For a few days around this time the rings appear noticeably brighter, this is known as the Seeliger effect - see July’s notes for an explanation. On 20th the 98% Moon is 3 degrees 42’ south of the planet at 23.15, about 45 minutes before it culminates at 17 degrees in the south. The two are marginally closer at 00.39. On 31st Saturn rises at 18.56, and gets to 10 degrees above the SE horizon around 20.45. It culminates, 7 degrees higher, at 23.10 and sinks to 10 degrees in the SW by 2am.
Uranus: in Aries, mag 5.8
Its position improves during the month, on 1st it rises a few minutes before midnight and should be visible from 02.30, with the 35% Moon 2 degrees 30’ to the south, until dawn, when it reaches 31 degrees. On 20th it begins retrograde motion, appearing to move from east to west across the sky. On this day it is visible from 1am and gets to 47 degrees by dawn. On 28th, the Moon, now at 60%, again passes close by - 5 degrees to the SW at 2am. On 31st, now very slightly brighter at mag 5.7, Uranus rises at 21.48 and is high enough for observing soon after midnight, getting to 47 degrees by 04.15, when the sky begins to brighten. It could possibly be seen with the naked eye from a very dark sky site, especially in the later part of the month when it gets quite high in darkness. It should be an easy binocular target but a scope is needed to show the small blue/green disc.
Neptune: in Aquarius, mag 7.9
Also improving its position during August, though it doesn’t get as high as Uranus. On 1st it rises at 22.24 and should be observable soon after 1am, reaching 31 degrees before it gets lost in the brightening sky around 03.30. After the first week in the month it culminates in darkness - on 7th it becomes visible before 1am and reaches its highest point, 33 degrees, at 03.42, a couple of minutes before fading from view as the sky brightens. On 24th the waning gibbous Moon passes 4 degrees to the south at 3am, half an hour before the planet culminates. On 31st Neptune rises at 20.25, observable soon after 23.00, when it is at 23 degrees in the SE. Highest point, 32 degrees, is at 02.05 down to 22 degrees in the SW by dawn. If conditions and eyesight are good enough to see Uranus without optical aid, it should also be possible to observe Neptune using binoculars, but the rich blue coloured disc can only be seen through a scope.
Dwarf Planets
Ceres: in Taurus, mag 9.2
The closest dwarf planet, the only one orbiting in the (relatively) nearby Asteroid Belt, is not at its best this month. On 1st it rises at 01.03 and is still only 19 degrees by dawn - not quite high enough for telescopic observing. After the first few days it is briefly high before fading from view as the sky brightens, on 4th it reaches 22 degrees at 03.30. On 31st, now at mag 8.9 it rises at 22.20 and is observable from soon after 2am, reaching 23 degrees in darkness.
Pluto: in Sagittarius, mag 14.9
Still too low for imaging, maximum 13 degrees above the horizon. Anyone wanting to have a try at imaging it any time in the next few years will have to travel much further south.
Haumea: in Bootes, mag 17.4
Now too low in the west after sunset for successful imaging.
Makemake: in Coma Berenices, mag 17.3
Slightly higher, but only for a very short time in the first few days in August.
Eris: in Cetus, mag 18.8
The most distant, and faintest, of the five officially designated dwarf planets is a difficult target for even the very best astrophotographers. It is high enough for a short time at the start of the month, half an hour on 1st, up to about 4 hours by 31st.
Asteroids at opposition
43 Ariadne: in Aquarius, mag 9.6
Opposition on 19th at 20.10. Observable from 22.37 when it reaches 21 degrees in the SE. It culminates at 01.10, when it is at 30 degrees in the south, and is down to 21 degrees in the SW by 03.45.
89 Julia: in Aquarius, mag 9.0
Opposition on 24th at 15.22. Observable from 10pm, when it is at 23 degrees in the SE, reaching 36 degrees in the south at 01.07. By 04.30 it has sunk to 21 degrees in the SW.
Comets
67P/Churyumov-Gerasimenko: in Pisces, mag 11.1
Still very faint but reaching a reasonable altitude by dawn. On 1st it rises at 23.34 and should be high enough for imaging from 2am, reaching 31 degrees in the SE before the sky gets too bright around 03.30. It moves into Cetus on 11th, now at mag 10.7, observable from 01.36 and reaching 39 degrees in the SE by dawn. It is in Aries from 23rd, when it is at 44 degrees in the SE before fading from view and on 31st, now at mag 10, rises at 22.15, becoming observable just before 1am and reaching 48 degrees in the SE in darkness.
It is predicted to peak at mag 8.3 in early November, when it will be visible for most of the night, culminating at 63 degrees.
15P/Finlay: in Taurus, mag 9.0
Very low in the pre dawn sky in early August. On 1st it only reaches 18 degrees by the time the sky brightens. A week later it gets to 22 degrees a few minutes before dawn. It is in Gemini from 17th, now at mag 9.6 and high enough for imaging for about half an hour, getting to 26 degrees in the east before fading from view. On 31st it is observable from 03.30 and reaches 34 degrees in the east in darkness but has faded to mag 10.2.
Meteor Showers
We have one of the best showers of the year this month - and it won’t be spoiled by moonlight. Fingers crossed for clear skies!
Perseids: active 17th July to 24th August, peak on the night of 12th/13th, ZHR could be as high as 150 - though some sources say around 110 or even as low as 50 to 70. We could see almost peak rates from the darkest areas around Manchester. Rates are said to be quite low for most of the period of activity but quite high for a day or so either side of the peak.
The radiant is circumpolar, highest at 07.00, so the shower is best seen in the hours before dawn. However the peak time is given as 20.00 on 12th, so it’s also worth looking after dusk on that day, though the radiant at that time is low in the north.
Perseids are fast moving meteors, around 60 Km/sec, often leaving trails. Many are extremely bright so visible even from light polluted areas. There may also be some fireballs. The peak is close to the new Moon, so there won’t be any interference. Parent comet is 109P/Swift-Tuttle.
Kappa Cygnids: active 3rd to 25th August, peak on the night of 17th/18th, ZHR 3 (2 from Manchester).
The radiant, in Draco, is circumpolar, highest around 22.00. This shower is said to be unpredictable because the dust cloud responsible is very diffuse. The exact time of the peak is often regarded as uncertain, though one source does give it as 01.00 on 18th, not long after the Moon has set. The shower showed higher activity in 2007 and 2014 but nothing is forecast for this year.
These are very slow moving meteors, 25 Km/sec, parent body not known for sure, could be minor planet 2008 ED9.
Aurigids: active 28th August to 5th September, peak on the night of 31st/1st, ZHR usually around 6 but occasionally 50 or more. An outburst in 2007 had a ZHR of 130 - but only for 20 minutes. There could be good activity this year around 22.30 on 31st. The bad news is that, at this time, the radiant is only 10 degrees above the NE horizon. The best time to see this shower is usually in the pre dawn hours, when the radiant is high, however the gibbous Moon rises soon after 11pm on 31st so will interfere at this time. They are fast moving meteors, 66 Km/sec, parent comet C/1911/Kiess.
There could be some activity on 12th at 05.22, unfortunately only 20 minutes before sunrise, from the dust trail of comet C/1852 (Chacornac), The radiant is in Cetus, much lower than that of the Perseids which also peak around this time, so they should be easily distinguishable.
There could be one or two meteors from the Antihelion Source, but it isn't at its best in August.
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.