What’s Up! April 2021

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

Compiled by John Rowland

Please see the “Leave a Reply” appeal at the end of the article – Editor.


Wow! All of a sudden it’s light in the early evening, with the Sun not setting until 7:40 pm on the 1st of the month. Summer skies and reduced viewing opportunities are ahead, so make the most of what darkness remains before you have to wait until after 10 pm for nautical twilight to end – which is when it ends on the 30th.

And the situation is exacerbated by the fact that all six planets – that are normally observable when skies are not particularly dark – are too distant (e.g. Mars) or impossible to see because they are too near the Sun, or only visible briefly at around 6 a.m. and very close to the horizon.

So this month, the emphasis will be on deep-sky and other dark-related subjects during the first half of the month (when the Moon isn’t there to brighten the sky) and the Moon itself when the phase allows us to get good views of the terminator.

Each month, we aim to highlight the objects that are conveniently placed for viewing in the four-hour evening slot of 1900 – 2300 hrs unless there’s a good reason to view them earlier or later.

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

April Objects

The Sun

As was the case in March, 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

The best evenings to observe the Moon are from Thursday 15th (slender crescent) through to Friday 23rd (mid-gibbous). For those wishing to spot the earliest crescent moon, the best day will be the 13th, after the Sun sets at 2003 (But see Crescent Moon Watch page for more details).


April is probably the best month to see earthshine on the Moon. This is when light from the Sun illuminates the Earth, which in turn illuminates the Moon. For a few days following the New Moon, the entire lunar disc can be clearly seen from Earth, not just the slender, directly illuminated crescent. This is often referred to as “the old Moon in the young Moon’s arms”. The image below is a stunning example of this phenomenon.

NASA Astronomy Picture of the Day 2018

Fig.1 NASA Astronomy Picture of the Day 2018 November 10 taken by Stan Honda

Note how clearly you can see the lunar maria. To explain this, imagine you were standing on the Moon looking up at the Earth. It’s the lunar night but there, high in the sky is a spectacular sight. It’s the Earth, almost fully illuminated (“full earth” as opposed to full moon) and four times the diameter – or 16 times the area – of how the Moon looks from the Earth. You can clearly see the details of continents, cloud formations and vividly blue oceans. Not just that, but it’s so bright. It’s actually bright enough to see your way easily as you avoid stumbling over the lunar rocks scattered about the surface under your feet. In fact, it’s over 40 times brighter than moonlight. The image below illustrates how very different the Earth looks from the Moon compared to the other way round.


Fig.2 Earth image: NASA Apollo 17

It’s hardly surprising then, that we can see the lunar surface so clearly under these circumstances. Look out for earthshine on the Moon if you can. The best nights will be the 15th and 16th. And how about grabbing a few shots and sending them into the Society. Your images could be displayed on our website gallery page. Please contact our IT Officer <it.officer@yorkastro.org.uk> for further instructions.


Conjunction with Mars on the 17th

From south-east Asia, the Moon occults Mars on this day. But from York all we’ll see is, not particularly close conjunction. At 2100, Mars will be 3½° to the right and below the Moon.


Full Moon near Perigee on the 27th

On this day, the full moon is at 0432 and the perigee (the Moon’s closest approach to Earth) is at 1625. This means that – just, and only just – the two events occur on the same day (i.e. within 12 hours of each other). This is technically a perigee syzygy. Unfortunately, the astrologer – note astrologer, not astronomer – Richard Nolle coined a word for this in 1979. I can’t bear to say it but it begins with “super” and ends with “moon”. To my great irritation, the popular press have used this ridiculous word every perigee syzygy since then. There was even an occasion (14th Nov. 2016) when perigee and full moon were the closest they had been since 1948, when the BBC sent a reporter out into a field and asked him on live TV and radio if the Moon looked big. Of course, he tried to hype it but had to admit in the end that without anything to compare it with, the Moon looked perfectly normal!

So what are the facts? How super is super? Let’s see. The distance to the Moon at perigee is 356,500 Km; the distance at apogee is 406,700 Km. Is the average distance therefore these two added together then divided by two, i.e. 381,600 Km? Actually no. Because the Moon slows down so spends longer when it’s further away than when it’s nearer, one has to use the “timed average”, which is 385,001 Km. Using that figure, the apparent average diameter of the Moon is 31.3 arc minutes. At perigee, the figure rises to 33.5. The difference is only 7.0%. The following image demonstrates that difference.


Fig.3 Image from Wikipedia by Marco Langbroek

Average compared to perigean

So what do you think? Could you tell the difference if you didn’t have both images? If you didn’t know whether the Moon was near to perigee or not and looked at the Moon in the sky on a randomly selected evening, could you honestly say you could judge how comparatively big it was?


So how super is super?

What is more likely to make a far greater difference to the apparent size of the Moon is its position in the sky. When it’s near the horizon it really does look much larger than when it’s high in the sky. This is a phenomenon known as the Moon Illusion, and will be explained in a separate article as I think we’ve dwelt for long enough on the Moon for now.

So let’s leave the “very slightly larger than average Moon” on the 27th to lovers and lunatics and get on with some real astronomy!


Lyrids Meteor Shower

This shower peaks on the 21/22nd but forget it. Not only will the gibbous Moon spoil things but the zenith hourly rate of 15-20 is no big deal, and that rate isn’t reached until 4:30 in the morning!


The Planets

And what about real astronomy on the planets? Easier said than done this month. As mentioned at the beginning of this article, all the planets are either unfavourably placed or impossible to observe because they’re too near the Sun. Forget ‘em; they’re a dead loss this month. Get over it and move on, to  …


Deep Sky Objects (DSOs) and Other Dark Sky Phenomena

April is your last chance to observe under truly dark skies. By “dark” I mean as dark or darker than the end of astronomical twilight and with the Moon completely out of the way. The table below gives the Saturday night dark sky time ranges for York. The “Begins” column refers to the morning of the following day.




Astro Twilight Description Observing Window
Ends Begins
03-Apr 2154 0421 Last quarter moon rises at 0110 2154 – 0110
10-Apr 2213 0357 No moon interference whatsoever 2213 – 0357
17-Apr 2235 0332 5-day-old crescent moon sets at 0213 0213 – 0332
24-Apr 2259 0305 Nearly full moon interferes all night No window
01-May 2330 0233 Waning gibbous moon rises at 0250 2330 – 0233
06-May 2356 0150 The last night with any dark time. No lunar interference. 2356 – 0150

It’s clear from the above table that the final series of truly dark nights will be the two weeks beginning on the 4th and ending on the 17th. And apart from an extra few days in May, that’s it until August. How much you take advantage of this ultra-dark observing opportunity is up to you. The following recommendations home in on Sat/Sun 10/11th April and it depends on how willing or not you are to stay up late – and ultimately very late.


1. Late evening objects – for those who must abed by midnight

If you can only see yourself staying up to observe until midnight, then this is what I’d go for.


Fig.4 From SkyViewCafe, 100° span zenith view for midnight 10/11th April

Please note, the chart above is NOT FULL SKY but rather encompasses a 100° span centred on the ZENITH.

Top Priority: Globular cluster M3. A fine globular, second only to M13. At magnitude 6.2 and diameter 18 arc minutes, it is an excellent telescopic or binocular object.

Next: Globular cluster M13. The finest globular in northern skies. At magnitude 5.8 and diameter 20 arc minutes, it truly deserves its name of the Great Globular Cluster in Hercules.

Then: Once your eyes are truly dark-adapted, have a go at M51, the Whirlpool Galaxy. At 11 x 7 arc minutes and of magnitude 8.4 it requires some skill, concentration and averted vision to do it justice.

Also: M80 and M81. These were described in last month’s What’s Up.

And finally, for those with larger aperture telescopes, there are all the galaxies in the Virgo cluster and the other objects on the chart above.


2. For those late-nighters – The Galactic Centre and The Scutum Star Cloud

This really is the best chance in the entire year to see the wealth of DSOs in the direction of the galactic centre. This is at a declination of -29° so only climbs to 7-8° above the southern horizon. If you don’t catch it when it’s near to due south at a time when the sky is really dark, you’ll miss it. You’ll need a really clear sky with no light pollution and views down to near the SSE horizon. If you’re keen enough you can do it, but be prepared to stay up until way past midnight. In fact it might be better to have an early night then get up at about 0230 and set up the telescope, because the absolutely best date and time is Sunday 11th April between 3 a.m. and 4.a.m. A few days either side should be ok but that Sunday morning is the tops. There will be a second chance to see the north-eastern end of this region in early September – in the SW and at a much more sociable hour – but the objects will be nearer the horizon, the sky won’t be so dark, and some objects will not be visible at all. The sky chart below shows all the DSOs of magnitude 6.0 or brighter that can be seen in that rich area of sky.



Fig.5 From SkyViewCafe, looking SSE on Sunday 11th April at 0345

Many of these deep sky objects are famous and well worth the effort. Even if you don’t try to find them all, by panning around with your telescope or even just binoculars, many of them will just suddenly pop into view. If you want to see an image and/or a finder chart for any of these objects, the links will take you to in-the-sky.org where there is more information. On that site, you can change the date (then click “Update chart”) to also see rising/setting and other visibility data.


GC – Globular Cluster  OC – Open Cluster  EN – Emission Nebula

ID Name Type Mag. Size

(arc mins)

M4 GC 5.4 26 V. easy to find 1.3° W of Antares.
M5 GC 5.8 23 A fine globular but if pushed for time leave until May.
M8 Lagoon Nebula EN 5.9 90 x 40 Hidden by NGC6530 on the chart above.
M11 Wild Duck Cluster OC 5.8 23 One of the richest and most compact OCs. Fan-shaped. Superb.
Scutum Star Cloud A 5° diameter region of the Milky Way, bright and richly packed with stars. Located where “Scutum” is on the chart above.
M16 Eagle Nebula OC/EN 6.0 70 x 50 Contains the “Pillars of Creation” formation.
M17 Omega Nebula EN 6.0 11 Similar to the Orion Nebula (M42).
M20 Triffid Nebula EN/OC 6.3 28 40’ SW of M21. Dramatic. A must see.
M21 OC 5.9 12 Not that impressive.
M22 Sagittarius Cluster GC 5.2 32 One of the brightest globulars but the one with this list’s lowest declination.
M23 OC 5.5 35
M25 OC 4.6 36 Rather loose.
NGC6530 OC 4.8 10 Is within the Lagoon Nebula (M8).
NGC6605 OC 6.0 Very loose. Leave it.
NGC6633 OC 4.6 27 Both large and bright. Great in binocs. aka Tweedledum & Tweedledee.
IC4756 Graff’s Cluster OC 4.6 24
IC4665 OC 4.2 45 Bright but loose and coarse.


3. Zodiacal Light

The zodiacal light is a faint, diffuse and roughly triangular white glow that can be seen extending from the Sun’s direction (below the horizon) up into the night sky following the line of the ecliptic. It’s caused by dust particles in the plane of the solar system and – from our latitude – is best seen after sunset in spring. Moonlight or light pollution will render it invisible but if the western sky is clear and you wait until about 2130 or 2200 on any day from the 1st to the 12th, you may see it extending from the NNW horizon up towards Mars. The Wikipedia entry has a good description and nice images.


4. Gegenschein

If you are thinking of observing late into the night on our key weekend of 10/11th you might like to see if you can see the gegenschein. It’s a ghostly bright spot at the antisolar point (the point on the celestial sphere directly opposite to the Sun) caused by the backscatter of light from interplanetary dust. On that weekend, it will be very near Spica, the brightest star in Virgo, due south at 0100 at an altitude of 26°. See this for a little more information.


Man’s Space Activities

Perseverance Rover on Mars

There is no question that the Perseverance Rover mission has been an astounding success. Since arriving on Mars on February 18th, everything has worked and Perseverance is sending us images and sounds galore. The main NASA web site for this mission is https://mars.nasa.gov/mars2020/. There you will find so many images and other information that there’s almost too much. To my mind, the single most absorbing image is a 360° panorama that you can download and/or use interactively. It’s high resolution and you feel as if you are really there, standing on the surface of Mars and scanning round with binoculars. Do please have a go. You won’t be disappointed. It’s at Perseverance’s Mastcam-Z First High-Resolution Panorama


The UAE’s Hope Mission to Mars

I was hoping (no pun intended) that by now, following the Hope Mission’s successful orbital insertion in February, that we would have some images, but none have been forthcoming. The UAE Space Agency’s web site has very few images or data results; their best page is at Images and scientific results from hope probe. Perhaps they’ll release more in due course.


China’s Tianwen-1 Mission to Mars

Same here I’m afraid. Lots about what it’s going to do and virtually nothing on what it’s done.


Spotting the International Space Station

The Heavens Above web site gives details for all dates the ISS can be seen (which for April are 1st, 2nd, 4th & 28th). 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

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


Finally – We need your help
It’s difficult to know how interesting or useful our What’s Ups are, and therefore whether it’s worth continuing with them. To inform our decision, it would help if you posted a quick comment using the “Leave a Reply” form below (with your full name and town). A one-liner is all that’s needed. If you’d like them to continue, please say so. Thank you.

17 thoughts on “What’s Up! April 2021

  1. Very good post. Shame some of the more ‘spectacular’ DSO’s mentioned turned out to be impossible from the northern hemisphere. I like the post. Makes me feel connected to the society even though I can’t meet up.

    • Thanks for your feedback, Alistair. Actually, all the DSOs listed in the article can be seen from York (but you do need a clear view down to low in the south and you do need to observe them at the right date/time range. I wouldn’t have included them if they were impossible to see. The one with the lowest declination is M22, which at -24 degrees, rises to over 12 degrees above the southern horizon when it culminates (is due south).

  2. A very informative read and very useful for anyone with the stamina and equipment, unfortunately I have neither the stamina nor much equipment since donating the telescope to the YAS… but I still like to know what is available for my perusal should I have the inclination and opportunity to do so. Thank you!

  3. Absolutely Brilliant and I love it keep up the amazing work! I do not zoom but this blog has reconnected me with YAS. Thanks for that.

  4. Excellent wide ranging article. Concise, to the point and flavoured with humour. I’m a fan so please keep ’em coming John.

  5. Hi John, thank you for all the work you have put into producing ‘what’s up’. There is a lot of information here that has been compiled and available to all at the touch of a button. I would like it to carry on but I appreciate it must take up a lot of your time.

  6. Excellent summary, thanks. I managed to get a photo of the Eagle Nebula earlier this week, despite it being a hard one to see from my property, this article has encouraged me to look further at that region this month.


    • Wow Paul, that’s brilliant, and with the full moon against you. I’m glad my suggestions have fired you up.

  7. I think this will be really useful along with the Sky at Night bits in the magazine for when I finally get a scope in a few weeks…..only been waiting nearly 3 months!!

  8. This is good and worthwhile continuing. We should flag it up more so visitors to the site can see how good a group we are.

    Just one question can it be downloaded?

    • Good question. Don’t know, and if it could be, don’t know whether to allow that. Will discuss with Qamar (IT Officer) and get back to you.

  9. This must take a lot of effort on your part John and it really shows.
    Very informative and professional. It prompted me to observe the objects in Ursa Major last night.

  10. I really enjoyed the What’s up article. Very informative and full of useful pointers. Thank you for putting in such effort and sharing your knowledge. Please keep writing!

  11. I really enjoyed the What’s Up article – varied and informative. Keep them going even though they must be time consuming!

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