The night sky in May 2021

posted 29 Apr 2021, 02:04 by Pete Collins   [ updated 29 Apr 2021, 05:41 ]
by Anne Holt

Sunrise        1st:   05.34          31st:   04.47

Sunset         1st:   20.39          31st:   21.26

Astronomical darkness

1st:   23.27 to 02.43    31st:   none

Astro darkness ends at 01.25 on 13th and that’s it until 30th July.  Even astronomical twilight is getting shorter, by month end it only lasts from 23.38 to 02.36.

Day length   1st:   15:04:57     31st:   16:38:48

New Moon:  11th at 19.59       

Full Moon:    26th at 12.15  (angular diameter 33’ 24”)

Lunar apogee:    11th at 22.55   (406511 Km,   angular diameter 29’ 22”)

Lunar perigee:    26th at 02.53   (357309 Km,  angular diameter 33’ 25”)  

There will be a total lunar eclipse on 26th, visible from Australia, New Zealand, Antarctica, parts of New Guinea and most of the Pacific Ocean.  Observers in Eastern Asia and Western Canada & USA will be able to see part of it.

This month’s full Moon occurs 9 hours 24’ after perigee so will appear slightly larger and brighter than average - a Supermoon. 

The Old Farmers Almanac name for the May full Moon is the Flower Moon - for obvious reasons.  Other names are the Corn Planting Moon, the Frog Moon and the Moon of Shedding Ponies. The Colonial American name was the Milk Moon and it was the Celtic Bright Moon or  Dyan Moon (though some sources give that one for June).  For the Chinese it was the Dragon Moon, to the Medieval English, the Hare Moon and the Neo Pagan name was the Grass Moon.  Among the Indigenous American names are the Cherokee’s Planting Moon, the Choctaw’s Panther Moon and the Ojibwa tribe’s Budding Moon. The Dakota Sioux called it the Moon when Leaves are Green - quite mundane compared with some of their other names!


Still nothing spectacular predicted. Jupiter and Saturn’s positions in the morning sky are improving but, as they are in the most southerly part of the ecliptic, remain quite low.  We’re losing Mars, by the end of May it is only 12 degrees above the horizon at dusk.  Venus is also very low in the evening sky, Mercury is higher for most of the month, early to mid May is the best time this year to see it in the evening.  The downside is that as it gets higher, it also gets fainter.  There is one reasonable meteor shower but the radiant is so low that it is better seen from further south.  However a few may be visible just before dawn.

From mid month we lose astronomical darkness but, as the Sun doesn’t get more than 18 degrees below the horizon, we may see some Noctilucent Clouds.  These wispy silvery or bluish white clouds are formed when the Sun shines on ice crystals in the mesosphere, about 50 miles above the Earth, so high that they are still in daylight when the Sun has set for observers on the ground.  They are most likely to be seen at latitudes between 50 and 70 degrees north.  They also occur in the southern hemisphere but are not reported as often because there isn’t much land mass at the right latitudes.


As the sky darkens at the start of the month Lyra and Cygnus are rising in the north east, followed a couple of hours later by Aquila.  In the later part of the night the Summer Triangle formed by Vega, Deneb and Altair, the brightest star in each of these three constellations, should be easily visible. By the end of the month Aquila will be above the horizon by around 11pm. The brightest part of the Milky Way visible to us in the UK runs through the Summer Triangle and down through Scutum and Sagittarius.

The Plough is still very high in the sky for most of the night, standing on its handle, so Cassiopeia, the W shaped 'Lady in the Chair',  on the opposite side of the Pole Star is very low down in the north.

Bootes, the herdsman, is now riding high although only Arcturus, the brightest star in the celestial northern hemisphere, is above magnitude 2, so its kite asterism may not be easily visible in our light polluted skies. Arcturus is easy to find though - just follow the arc of the Plough's handle down to the south until you come to Arcturus.  Carry on the arc a bit further and you come to the star Spica, the brightest star in Virgo.

At this time of year when you look up to the south you are looking out of the plane of our Milky Way galaxy instead of along it like you do in winter and summer, so there aren't many bright stars, open star clusters and nebulae. However, if you've got a telescope this is a good time of year to hunt down globular clusters like M13, the Great Globular Cluster in Hercules, and faint galaxies like the many galaxies lying in the bowl of Virgo and into Coma Berenices.


Mercury:  in Aries, mag -1.1

Early to mid May is the best time this year to see it in the evening twilight.  On 1st it is 6 degrees above the horizon as the sky darkens, setting at 22.02.  It moves into Taurus on 2nd and the following day should be visible for a few minutes, soon after 9pm, when it is 8 degrees in the NW.  On 13th the thin crescent Moon passes 2 degrees 08’ to the south at 18.58, the pair are separated by 2 degrees 40’ at 21.40 when Mercury is 9 degrees above the horizon.  It reaches its highest point in the evening sky on 15th, when it will be at 15 degrees at sunset but down to 9 degrees by the time the sky darkens, and considerably fainter at mag 0.1.  It is at greatest eastern elongation on 17th, when it appears 22 degrees from the Sun but still only 9 degrees above the horizon at dusk.  On 29th, now faded to mag 2.3, it is just 25’ south of Venus at 06.34.  The pair are quite close on the evenings of 28th, about 30’, and 29th, about 1 degree.  However they will be very difficult to spot, almost on the horizon in twilight.  On 31st Mercury appears only 14 degrees from the Sun, setting at 22.35.

Venus:  in Aries, mag -3.9

Not visible at the start of May.  On 1st it is only 2 degrees above the horizon at dusk, setting at 21.25.  It moves into Taurus on 4th and on 12th the very thin, relatively small (29’ 24” - only a day past apogee) crescent Moon passes 42’ to the south at 23.03.  The separation at 21.00 is about 2 degrees but Venus is only 3 degrees above the NW horizon as the sky darkens, setting at 22.04, ten minutes after the Moon.  On 28th Venus is close to Mercury and might be visible, 5 degrees above the horizon, in twilight.  Mercury, much fainter, is unlikely to be seen. 

REMEMBER:  if trying to see the pair using binoculars, wait until the Sun has fully set.  

On 31st Venus is still only 5 degrees above the horizon at dusk, setting at 22.52.

Mars:  in Gemini, mag 1.6

An early evening object in the first part of the month.  On 1st it should become visible around 21.45, when it is 29 degrees above the western horizon. By midnight it will be too low to be seen easily, setting at 01.31.  On the nights of 15th and 16th the crescent Moon passes north of the planet, closest at 05.15 on 16th.  As the sky darkens on 15th, the separation is around 5 degrees, slightly more the following day when the Moon is to the east.  By the end of May, Mars will be difficult to spot, on 31st it is only 11 degrees above the horizon at dusk, setting at 00.35.

Jupiter:  in Aquarius, mag -2.2

Now becoming more prominent in the pre-dawn sky. On 1st it rises at 03.41 and reaches 10 degrees in the SE by 04.45.  On the morning of 5th the 35% Moon is just under 6 degrees SSE of the planet in the morning twilight.  They are closest, 4 degrees 21’, at 00.55 while still below the horizon.  Jupiter now reaches 11 degrees before the sky brightens. It gets higher as the month progresses, on 31st it is visible from 3am and reaches 17 degrees in the SE by dawn.

Saturn:  in Capricorn, mag 0.7

Another pre dawn object but much fainter and harder to see than big brother Jupiter. On 1st it rises soon after 3am and reaches 9 degrees by dawn.  On 3rd the 45% Moon passes 4 degrees 09’ to the south at 17.58.  On the morning of 3rd they are separated by 8 degrees at 5am, with Saturn reaching 10 degrees in relative darkness. The pair are slightly closer, 6 degrees, on the morning of 4th with Jupiter about 15 degrees to the left, so the three form a triangle.   On 23rd Saturn appears to stand still for a short while before changing direction and moving from east to west against the background stars - retrograde motion.  On 31st it rises at 01.16 and is again close to the Moon, which passes 4 degrees 10’ south at 02.18 while the planet is still too low to be seen, becoming visible around 3am, when it will be 15 degrees in the east.  They are closest, 4 degrees 03’, soon after 4am as the sky begins to brighten.  

Uranus:  in Aries, mag 5.9

Not visible this month following solar conjunction at the end of April.  On 1st it is only 1 degree from the Sun.  By month end it is still 10 degrees below the horizon by dawn, rising about an hour before the Sun.

Neptune:  in Aquarius, mag 7.9

Also not visible in May.  It is below the horizon at dawn throughout the month. 

Dwarf Planets

Ceres:  in PIsces, mag 9.2

Another one which appears very close to the Sun this month.  On 1st the separation is 15 degrees.  It moves into Cetus on 13th and on 31st is still below the horizon at dawn, rising only half an hour before the Sun.

Pluto:  in Sagittarius, mag 15.4

Still much too low in the morning sky for imaging or telescopic observation.

Haumea:  in Bootes, mag 17.3.  Makemake:  in Corona Borealis, mag 17.2.

Both high in the sky for most of the night.  If any experienced astrophotographers fancy having a try at imaging these, exact positions can be found in in-the-sky - details at the end of these notes.


Eris:  in Cetus, mg 18.3

It is too close to the Sun for imaging, following April’s solar conjunction.


No bright comets again this month.

C/2020 R4 (ATLAS) in Canes Venatici

Predicted to end April at around mag 8.4, but failed to live up to expectations.  Now expected to start May at around mag 10, but is still above the horizon for most of the night.  On 1st it reaches its highest point, 69 degrees, at 00.35.  It fades quite rapidly and gets lower in the sky as the month progresses, on 7th, down to mag 10.9, it moves into Coma Berenices and culminates at 22.48, soon after the sky darkens enough for it to be seen.  It spends 12th & 13th in Ursa Major then goes into Leo for the rest of May.  On 31st, predicted mag now 13.8, it is at 32 degrees in the west at dusk, high for just over an hour and setting soon after qam.

Meteor Showers

Eta Aquarids:  active April 19th to May 28th, peak around 3am on 6th, but with good rates for about a week centred on this date. ZHR is given as 50 but the radiant is so low that the shower is much better seen from the southern hemisphere.  From the Manchester area the best that we can expect is about 10.  They are fast moving meteors, often leaving persistent trails, and are best seen between 02.30 and dawn.  The waxing crescent Moon doesn’t rise until 04.25 on 6th, so won’t interfere.  It is one of 2 showers associated with Comet 1P/Halley.

Eta Lyrids:  active May 3rd to 14th, peak 8th, ZHR 3

These are medium speed meteors, parent comet C/1983 H1 (IRAS-Araki-Alcock) 

The Antihelion Source (ANT) has a ZHR of 2 - 4 in May. The radiant moves through northern Scorpio and into Ophiuchus during the month.

May, especially the later part of the month, is good for daytime showers, which have a radiant so close to the Sun that they can’t be observed visually.  They are detectable only with radio or radar equipment.

Epsilon Arietids:  active April 24th to May 7th, peak May 9th.   Rates given as low (numbers not given for any of these)

May Arietids:  May 4th to June 6th, peak May 16th.  Again rates are low.

Omicron Cetids:  active May 5th to June 2nd, peak May 20th.  This one is slightly better - rates given as medium.

More active are the daytime Arietids, beginning May 14th but not peaking until mid June.  Rates for this shower are expected to be high.


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

Other information is from various internet sources, including NASA, Britannica, Space Facts, Universe Today and, when all else fails,Wikipedia.