What’s Up! May 2021

A monthly look at astronomical events in the sky and on Earth 

Compiled by John Rowland

First, a Thank You.
Many thanks to all the people who commented on following What’s Up! April 2021. Those reactions have given us the reassurance that these articles are worth continuing.


Astronomy from the UK has always been a challenge. When we have long dark nights and a wealth of fascinating objects to observe, that means it’s winter, and only dedicated stalwarts are prepared to forsake the comfort of a warm house and brave the cold to set up and use the telescope. But we do it because we love our chosen hobby. So do we rejoice when the weather warms up in the spring? Of course we do; it’s just that by then, it doesn’t get dark enough for us to see anything!

OK, I exaggerate, but the fact remains that by the time we enter May, nighttime observing opportunities are limited to the Moon and planets and a very small number of deep sky objects. May 2021 is no exception, and apart from a brief appearance by Mercury, even the planets are not available to observe. May however is a very good month for observing the Moon because hopefully it’s warmer in the evenings and the Moon is at an altitude that makes for comfortable viewing with all telescope types.

So this month, we’re providing a Special Supplement on Observing the Moon. Here is a quote from an excellent piece by Charles Wood in Sky & Telescope. He states,

The night sky offers an object that is larger, brighter, and more visually captivating than anything on Messier’s list: the Moon. Yet many backyard astronomers never go beyond the astro-tourist stage to acquire the knowledge and understanding necessary to really appreciate what they’re looking at, and how magnificent and amazing it truly is. Perhaps this is because after they identify a few of the Moon’s most conspicuous features, many amateurs don’t know where to look next.”

Is that you? I suspect that in many cases it is. It certainly applies to me. Charles Wood addressed this issue by compiling a list of lunar features – The Lunar 100 – that not only provides a lunar equivalent to Messier’s Catalogue but an annotated map and a list of the features in order of ease of visibility. In this month’s special supplement, I have used a modified version of Charles Wood’s list sorted by the date that each feature becomes visible as the terminator moves across the lunar surface. Not only will this make it much easier to find the feature but bring it into sharp relief due to the grazing angle of the sunlight on it. You’ll find a link to this special supplement at the end of this edition, following the usual rundown of items of interest this month.

Please note that all 4-figure times given in this article are in BST.

May Objects

The Sun

As was the case in April, the Sun remains relatively inactive. It is only just emerging from solar minimum so sunspots may be few and far between but it’s worth checking spaceweatherlive.com to see whether more activity may happen.

The Moon

Do please read the special article on the Moon at the end of this What’s Up edition. But in brief for now, the best evenings to observe the Moon are from Friday 14th (slender crescent) through to Monday 24th (late gibbous). For those wishing to spot the earliest crescent moon, the best day will be Thursday 13th after the Sun sets at 2058 (But see Crescent Moon Watch page for more details).

The Planets (and one Minor Planet)

Mercury and Venus

Orange-coloured Mercury reaches the greatest eastern elongation on the 17th and at magnitude 0.4 may be visible for a few days on either side of that, low (11° altitude) in the WNW around 2140 hrs.

Venus is just emerging from the far side of the Sun and that too may be glimpsed in the same area of sky but much lower and at magnitude -3.9 much brighter too. See chart below.

Fig.1 Western Evening Sky (Skyviewcafe.com)


Mars is now a diminutive object only 4 arc seconds in diameter and of magnitude 1.7. It can be seen 4 degrees to the left of the Moon on the 15th and halfway between Mercury and the crescent Moon on the 17th.

Jupiter, Saturn, Uranus and Neptune

All these planets are early morning objects and too low or too near the Sun to bother with this month.

Minor Planet Vesta

Vesta, one of the first four minor planets discovered, is the only one that can be seen with the naked eye (just). It is currently moving through the “body” of Leo, and at magnitude 6.0 can be seen with just binoculars. Its position is shown on the sky chart below. To positively identify it, make a note of the brightest stars in that area – there are very few of them – then look again on another night. The “star” that’s moved is Vesta.


Deep Sky Objects (DSOs)

The first week of May marks the last time until August that there is any period of astronomical darkness. On the 1st, the truly dark window is between 2330 and 0230 and by the 6th has shrunk to midnight to 0150. After that, there is only nautical twilight until 6th June. After that,  through to 8th July, there is only civil twilight. The chart below shows innumerable DSOs but as indicated, only two of them are brighter than 6th magnitude (the naked eye limit). Decent binoculars will reveal many of the fainter objects but a 4-inch or larger telescope is needed to show detail.

So given that we are somewhat limited on what to observe, these are my recommendations. (All best seen after 2300 hrs.)


Fig.2 SkyChart 2 (Skyviewcafe.com)

Looking south, horizon-to-zenith. As at 2330 on 5th May. All the DSOs shown except M13 (top left) and M5 (centre left) are fainter than 6th magnitude. From SkyViewCafe.com

Globular Clusters

M3 is a truly fine globular about 18 arc minutes in diameter and of magnitude 6.2. It lies north of Arcturus and about halfway between it and the 3rd magnitude star Cor Caroli in Canes Venatici. As with all globulars, whack up the magnification for a chance to see individual stars (of which there are 500,000!).

M5 is another stunning globular, magnitude 6.0 and 23 arc minutes in diameter. Find it by following the line of 4th magnitude stars running along the top edge of Virgo.

M13 is the daddy of all globulars and deserves the name “The Great Globular Cluster”. Its magnitude is 5.8, 2/3rds the apparent diameter of the moon and easy to find on the edge of the central “square” of Hercules. A small telescope will see it as a fuzzy patch but 6-inch telescope is needed to resolve its stars. Do give it a go.

Double Stars

Mizar and Alcor

This famous naked eye double star one away from the tail end of the Great Bear is directly overhead in May and offers a quick opportunity to check your eyesight. The separation is about 12 arc minutes. Mizar itself is a double with a separation of 14 arc seconds and is easily seen with even a small telescope.  This Sky and Telescope article describes them in more detail.

Cor Caroli (Alpha Canum Venaticorum) which we mentioned above is also a beautiful double star, in fact one of the finest in the sky for small telescopes. The components are of magnitude 2.9 and 5.4 with a separation of 19.6 arc seconds. The brighter star has a spectral type of A0 and looks white; its companion has a spectral type of F2 (not unlike the Sun) and looks yellow. There’s quite an interesting Wikipedia entry about Cor Caroli.

Man’s Space Activities

Spotting the International Space Station

The ISS is visible 65 times (yes 65!) this month. The Heavens Above web site gives all the details.  The link is preset to return the data for sightings from York. Use the left and right chevron buttons to change the date range. All times are BST.

General Space News

In case you didn’t see NASA’s Ingenuity Mars Helicopter’s first flight, here it is. Scroll down to the video below the three people clapping, go to full screen, look just above the centre of the bottom edge then hit play..

There’s always something of interest going on in space. See the space.com web site for the latest news. And for a detailed timetable of projected launches and other space-related activities, the Wikipedia web site is an excellent place to start as it has links to each project/mission mentioned.

I wish you clear skies and good viewing. Stay safe.

John Rowland

Special Supplement – Observing the Moon

The supplement – a link to it provided below – is based on a modified version of Charles Wood’s excellent and useful list of lunar features. His original list of 100 telescopic sights on the Moon is sorted into order of ease of viewing. No.1 is the Moon itself; No.2 is earthshine, and No.3 is to note the difference between the mare and the highlands. These three can of course be observed without any optical aid. From No.4 onwards, you increasingly need binoculars then small and finally medium telescopes. The features are shown on the map below.


Fig.3 Moon Map (Charles Wood’s list of lunar features)

For nearly all of these features, the best and easiest time to see them is when they are just on the sunlit side of the terminator. Then, the Sun is shining on them from low in the lunar sky, and creating extended shadows that bring the details of the feature into sharp relief. Indeed, many features can only be seen under such conditions. So I thought the best way to help us spot these sights was to list them by the date the terminator reaches them. This day-by-day approach coupled with only looking along the terminator means one isn’t overloaded with choice on any given night.

The PDF file supplement includes this list, a copy of the map above, photographs of each feature and comprehensive directions on how and when to view them.

“Lunar Observers’ Feature Finder” (PDF)

And good luck. I hope this has made lunar observing a bit more interesting.

8 thoughts on “What’s Up! May 2021

  1. I have been quite disappointed to realise, as a newbie to astronomy, that my telescope is finally arriving next week just as the night sky gets too bright for observing most DSO. The great news is that YAS have put together a great list for lunar observations, which is going to give me something meaningful to look at this I learn how to use the mount. Thank you very much for pulling the newsletter together.

  2. Many thanks for compiling this informative article.
    I enjoyed the link to Mizar and Alcor . The moon chart is lovely. Good to be able to use my binoculars thanks

  3. Many thanks for this fascinating article. I enjoyed the moon chart and it will be good to use my binoculars . Link to Mizar and Alcor info very interesting
    thanks for putting it together

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