Skip to main content
western sky late April showing fleeting Mercury

The Sky This Month Apr-May 2021

Submitted by hotair@compute… on 6 April 2021

Mercury is at its best for 2021 in late April, early May. Jupiter and Saturn are in the pre-dawn morning sky. For Apr-May 2021, we'll follow the meridian from south to north for an excellent sampler of deep sky objects. This will take us through the constellations Sextans, Leo, Leo Minor, and Ursa Major. We'll also peek at a bright nova in Cassiopeia.



Blake Nancarrow delivered an online presentation of The Sky This Month on 7 April 2021. It covers the dates 7 April through 5 May 2021. He prepared his usual month-at-a-glance calendar covering the period. You may view or download and print the colour calendar PDF file.

All these materials accompany the live stream and recorded video available on the RASC Toronto Centre YouTube channel. All photos, sketches, and graphics copyright the respective owners.

Edited video is available on our YouTube channel.


observing lists

We'll follow the meridian from south to north to sample deep sky objects. This will take us through Sextans, Leo, Leo Minor, and Ursa Major. We'll highlight a number of galaxies (including some Arps), one planetary nebula, a few double stars, a variable in its bright phase, an intriguing exoplanet system, and some fast-moving stars. You might also check out the bright nova in Cassiopeia.

The suggested targets for this TSTM were collated into observing lists for a couple astronomy software apps with planning capabilities. You may download a file and install it in your device or computer so to have a list of suggestions.

Note: Files stored on Blake's web site under the Computer Ease domain.

Note: Your browser may initially block or halt the download of binary files.

Many of the suggestions in this month's campaign are targets included in various RASC observing programs. We include objects from Explore The Universe, Explore the Moon, Messier, the RASC Finest NGCs, and the new Double Stars certificate program.

Many details of objects and events and phenomena are noted in the RASC Observer's Handbook.



While there are many anniversaries in April, here are a few of the noteworthy dates regarding space flight and telescopes.



Marks the first spacewalk by a Canuck. That was accomplished, unsurprisingly, by Chris Hadfield at the International Space Station. Later he went on to command the outpost in Low Earth Orbit. If you ever get a chance to see Hadfield, do try. He gives very inspirational talks.

Speaking of the ISS, there are a number of activities planned. Actually, the SpaceX Dragon was recently relocated to make way for new craft going uphill. On 9 April, a Soyuz launch will take place. That will allow Expedition 64 to end on 16 Apr and that crew to depart the ISS in their Soyuz. SpaceX Crew 2, the second official crewed mission for a Dragon, is due 22 Apr. Crew 1 will depart late Apr or early May. Medical research and cognition studies continue. Plasma physics research and microbial analysis continues. New solar arrays will be installed over the summer.

Visible ISS flyovers resume in the pre-dawn morning sky starting 25 Apr. Use a web site or mobile app to predict sightings.



The Hubble Space Telescope was deployed in orbit, from the cargo hold of the space shuttle Discovery. Of course, it had a major fault but that was corrected during a servicing mission. A number of servicing missions were completed over the years but they stopped with the retirement of NASA's shuttle.

See the mission page at NASA.

Controllers have been struggling of late with the HST. A major issue is that while the space telescope has 6 gyroscopic systems for orientation, only 3 are working. If one more fails, the HST will not function correctly. Recently, the primary motor for the lens cap or flap failed but fortunately the backup is working. NASA expects the HST to function another 4 years. It has delivered so many amazing images and science. Hang in there, Hubble!



As mentioned, the HST was deployed from a space shuttle. The first shuttle launch was of Columbia in 1981.

launch of the first Space Shuttle

The photograph is telling. Early missions showed the large fuel tank coloured white. But later, they stopped painting the disposable tank, saving a lot of additional weight. STS-1 lifted off from pad LC-39A and went on to complete 27 missions. Sadly, it did not make it home in 2003, when it disintegrated in the atmosphere, killing all the crew.

NASA's "space truck" was extraordinarily useful while not perfect. It was instrumental in helping build the International Space Station.



Amazingly, ten years before the first shuttle launch, the Soviet Union launched their own orbital outpost, the Salyut 1. The crew of Soyuz 10 were to be the first to work in the small station but when they could not dock they aborted the mission. The Soyuz 11 was successful in docking and the 3-man crew remained on board for 23 days, completing a major milestone. Tragically, they were killed during re-entry when a valve was opened too soon leading to asphyxiation. They are the only humans to have died "in space." These setbacks forced the Russian space program to de-orbit Salyut after a mere six months.



And 10 years before that was the amazing moment for CCCP and effectively all of humanity when we left the Earth for the first time. What started it all. This fuelled the Space Race.

Yuri Gagarin, just 27 years old, blasted off to Earth briefly around our home planet. Politics aside, this is an astonishing milestone symbolising the marked technological and scientific accomplishments.



The first binary star was discovered. Many astronomers of the time suspected stars appearing close together in space were related. It was William Herschel's that supported this. He measured the separation and orientations of over 700 pairs that appeared to be binary systems, finding some 50 doubles that changed orientation over two decades of observation.

William was aided in his astronomical work and telescope making by his sister Caroline Herschel. She went on to make her own significant contributions to astronomy, discovering several comets and performing work that would be the basis of the New General Catalogue of deep sky objects.



Perhaps the origin of our fascinating hobby... While dates and details are not terribly clear, the author notes April as when the telescope was "introduced."

In 1608, a patent was submitted to the Netherlands government by Hans Lippershey. But it seems it was Galileo Galilei who began to use instruments based on these designs.

Purportedly, the "far-seeing" device was presented by Galileo at a banquet with academics and learned people. Refractor telescopes today remain a popular instrument for visual astronomy and astrophotography.


the solar system

Let's have a look at the planets for the next month. We'll also talk about some asteroids, meteor showers, and a comet. And, because we have to, the Moon.

event   date
in line with Jupiter and Saturn 8 Apr (morning)
new Moon (dark) phase 11 Apr
Mi'kmaw Penatmuiku's period 12 Apr-10 May
very near Mars! 16-17 Apr
occults double star STT 164 18 Apr
first quarter phase 20 Apr
Lyrid meteor shower peak 21-22 Apr
full Moon (yuk) phase 26 Apr
favourable libration for crater Pingré 26 Apr
favourable libration for crater Bailly 27 Apr
favourable libration for crater Boguslawski 28 Apr
third (not last!) quarter 3 May
near Saturn then Jupiter 3-4 May (morning)
η (eta) Aquariid meteor shower peak 4-5 May

We note the main phases of the Moon in the table with additional key dates including appulses, when the Moon is near a planet. Early risers on the morning of 8 Apr will find the Moon in a line with Jupiter and Saturn in the east. More on this later.

In the Mi'kmaw language, the day after the new Moon begins the Penatmuiku's period which centres on the full Moon and runs to the next new Moon phase. It is the Birds Laying Eggs time.

The Moon will occult or block a double star STT (Otto Struve) 164. Sometimes additional stars in a multi-star system are revealed during a lunar occultation.

We also note two meteor showers in this list. More info later but, briefly, the moonlight will adversely affect the Lyrids while there is less impact for the η Aqr show.

For advanced Moon watchers, we note favourable libration dates for observing the craters Pingré, Bailly, and Boguslawski.

line graph showing magnitudes of solar system objects for 2021

The line graph for the solar system bodies shows the magnitude over the year. Notably, Mercury is peaking in brightness in mid-April. Mars meanwhile is slowly fading. But, happily, Jupiter and Saturn are getting a little bit brighter day by day. The green dashed horizontal line is the limit of human vision. Adjust for light pollution....



Mercury is favourably positioned at the end of April. In fact, this will be the best evening apparition for the entire year. This is due to a number of factors. Because the ecliptic (the path the Sun, Moon, and planets appear to follow) is canted steeply in evening, it raises the planets higher above our horizon in North America. Also, Mercury ascends through ecliptic on 21 Apr. When it's on the north or right side of this imaginary line in the evening, it makes for better viewing opportunities.

On 23 Apr, Mercury will be very near Venus, less than 2° away. But we will probably not be able to see that. At the end of the month of April however, Mercury will appear directly above Venus.

At 8:45 PM on 29 Apr, look in the WWN direction. The speedy planet will be a few degrees above horizon and a little above bright Venus. When the Sun is down, you can safely use your binoculars. Find Venus then slowly pan up. Take away the binoculars, now that you know where to look, and spot the tiny inner-most planet with the unaided eye. It's remarkably dim compared to Venus. Clouded out? Try the next night.

As the sky darkens, you will see that Mercury is near the Pleiades, Messier 45.

If you're working on your Explore the Universe (ETU) certificate, you can check a few boxes on 29 and 30 Apr. Planets, bright stars, an open cluster, and some constellations. 



As the western sky continues to darken, Mars will become apparent. It is moving eastward through Taurus and later into Gemini. In the telescope, the Red Planet will likely be too small to see any surface details. It drops below 5 arc-seconds in size.

The scene on 16 Apr at 9 PM will be very pleasing with a young crescent Moon to the right. Mars has just gone through the Horns of the Bull. One night later, the Moon will be the same distance from Mars but to the left, higher in the sky. Both nights if clear will make for excellent photo ops!

Moon and Mars in the horns of the bull 16 April

Also, notice how Mars is making a big triangle with orange alpha Orionis (aka Betelgeuse) and orange alpha Tauri (aka Aldebaran). Nice!

On 26 Apr, a pretty view is offered with Mars coasting past the open cluster Messier 35. Use binoculars or a telescope to see the sprinkling of stars white and blue stars in contrast to the ruddy planet. By the way, M35 aka NGC 2168, is another ETU target. Stellarium calls this the Shoe-Buckle Cluster.

Many eyes (and radio telescopes) are on Mars right now with all the space missions operating there. NASA might make history by flying a small drone-like helicopter. Ingenuity is currently on its own, having been released from the belly of the Perseverance rover, charging its batteries. The small aircraft is to go through some pre-programmed tasks. No direct control can be accomplished as the light travel time to Mars is currently 15 to 16 minutes. Space is big.

Keep tabs on Percy's Twitter feed...

Update! "Ginny's" blades are to be unlocked on 7 Apr! First flight no earlier than 11 Apr.

Update to the update. No earlier than 14 Apr.


gas giants

Did anyone see the Moon below Saturn on the morning of 5 Apr? Or the next morning, also at 6:00 AM, the Moon below Jupiter? If you are a morning person and you missed it, you can try again in about a month's time.

Good views of Jupiter and Saturn might be hampered though as the ecliptic is shallow in morning for people in the Northern hemisphere. Additionally, the gas giants are slightly below the ecliptic. That's not great news. But all this signifies that Jupiter and Saturn are returning to the night sky and will be putting on good shows later in the summer.

Jupiter is moving eastward through Aquarius. Saturn is slowing down to begin retrograde motion soon. It is in the middle of The Bat aka Capricornus.

Beware of errors in 2021 RASC Observer's Handbook. It shows 5 double shadows on Jupiter in April. Those are wrong: cross ‘em all out. There are 2 missing events in late May… See the Updates page for errata.

The free Stellarium software for Windows, Mac, and Linux, version 0.20.4 had an error affecting the positioning of the Great Red Spot (GRS). The new version of Stellarium 0.21.0 corrected this.

At the Sky and Telescope magazine web site, there is an excellent article on interesting events at Jupiter, such as moons occulting moons and moons falling into the shadows of moons. Neat stuff!



The asteroid Apophis missed the Earth. And it won't hit the Earth any time soon, despite what the media says. And that means we have to keep wearing our masks if we intend to survive a while on this planet.

On 11 Apr, asteroid (88705) Potato will be close to Earth. Yummy. Then on 3 May, asteroid (1896) Beer close to Earth, mmm. Forgive the pun but that's a match made in heaven.

There are no notable PHAs or Potentially Hazardous Asteroids of note. These objects might be what destroys all life on Earth. Fortunately, agencies around the world are working this problem and conferences continue.


a comet

Comet C/2020 R4 is moving rapidly from Aquila though Hercules and Boötes and rising higher and higher into the sky.

Discovered with the ATLAS system, it went through perihelion back in March and passed within 75 million km of the Earth.

Some reports show it sporting a tail and shining at magnitude 8. Predictions say it might reach a maximum brightness of magnitude 6 in mid-April. This is a late-night object but worth a shot if you've got the telescope or imaging rig up and running.

C/2020 R4 will pass near NGC 6709 on 12 Apr.

Keep tabs on comets with Aerith or Comet Chasing.



As the Earth speeds through space, two meteor showers peak in April and early May. Meteor sightings are best done after midnight in a dark sky location.

Lyrids. This is normally a medium-strength shower that produces good rates over three nights centred on maximum. While they lack persistent trains, the meteors can produce fireballs. The peak should occur the night of 21 Apr through early morning 22. The average rate is 18 meteors per hour. In 1982, a rate of 90/h was reported! The bad news is that there is a First Quarter Moon in the sky, actually 68% full, that doesn't set until 3:45 AM for Torontonians. If you're keen, you can observe between moonset and dawn, when the sky is fully dark, and by then the radiant will be high in the sky. But that window is only about one hour long, when astronomical twilight begins. It's just not good timing for this year.

η (eta) Aquariids. This is normally a strong shower when viewed from southern tropics. Ontario? Less so. Usually, we can enjoy medium rates of 10 to 30/hour just before dawn. The shower typically is good for a week centred the night of maximum. These are swift meteors with a high percentage leaving persistent trains but very few fireballs. The peak is scheduled for the evening of 4 May through 5. On this night, the Moon will be 38% illuminated and won't rise until 4:00 AM. So that's somewhat good news. This will likely be the better of the two showers.

Watch out for media outlets touting "the best show ever" or that meteors will "light up the sky." They like to do that but you have to take what they say with a grain of salt. See the American or International meteor groups for good information.


the deep sky

Let's check out some deep sky targets for April and early May. We're going to do our popular "follow the meridian" approach again.

Celestial objects on or near the meridian during your planned observing or imaging session are high in the sky. The meridian in an imaginary line running from north to south and passing directly overhead through the zenith. With the exception of objects below the North Celestial Pole (NCP) or north-star Polaris, again objects at the meridian are as high as they can be for your viewing location. We'll be touring through the constellations Ursa Major, Leo Minor, Leo, and Sextans.

We have targets that have a difficulty rating from easy through challenging. We will feature a number of galaxies (including some from Halton Arp's catalogue), one planetary nebula, a few double stars, a variable, an intriguing exoplanet system, and some "fast" stars. We're going to break our own rule of "avoid south of NCP" to have a quick look at a bright nova in Cassiopeia.

Again, many of these suggestions can be found in RASC observing programs.



Sextans is curiously portrayed in many charts as a simple line stick figure which is not very evocative of a sextant navigation tool with various arms, curved base, and sighting scope. Nevertheless, this small constellation harbours a number of interesting objects.

A fine subject is the Spindle galaxy also known as NGC 3115. It looks a bit like a flying saucer from a science-fiction B-movie. This distant UFO-shape is in the Caldwell catalogue, position 53. The edge-on galaxy with bright core is a RASC Finest target and on the Herschel 400 list.

Gamma Sextantis, west of the Spindle, is a nice double. Actually, it's a triple. The author has tried many times to split the A and B stars of the very tight binary without success. But this fast-rotating pair regularly changes orientation and distance over the 78 period so it bears repeat viewing. The low altitude doesn’t help but hopefully good seeing conditions at some point will reveal the second star. On the other hand, the C companion star is easy to spot.

A difficult target in Sextans in the trio of galaxies, NGCs 3169, 3166, and 3165. The first two are also Herschel 400 and the Astronomical League has them noted in it's fun "two in the view" observing list. They are dimmer than the Spindle so may pose a challenge for visual astronomers but might be a fine photographic subject. Your quarry is located north of alpha and beta.



Leo the lion is chock-a-block with interesting objects. It is one of the most “picturesque” or evocative constellations.

We have Regulus aka alpha Leonis at the bottom of the Sickle, a target on the ETU program. While working on your Explore the Universe, visit 35-36 Leo aka zeta, an easy double star, a fine target in binoculars.

There are three Messiers objects, 95, 96, and 105, below the fuzzy belly of the big cat. Boom, three targets from the Messier observing campaign.

Now look closely beside M105. You'll see NGC 3384 to the north-east. The RASC Finest and Herschel 400 object is equally bright and does not appear to be a spiral. Research lists it as a lenticular galaxy.

Not far away is NGC 3389! It is a mottled, small shape. An astrophotograph will reveal the different morphologies. Warning: NGC 3389 is also known as NGC 3373.

The famous Leo Triplet is to the east. M 65 and M 66 are here along with and Hamburger Galaxy aka NGC 3628. Together, group is listed in Halton Arp's catalogue of unusual or interacting galaxies, entry number 317. How did Charles Messier not see 3628 at the same time? We'll never know.

Wolf 359, a star a mere 8 light-years distant, is moving rapidly through space, at 5 arc-seconds/year. Photograph or sketch it each year to monitor the rapid proper motion. The author is imaging it with Burke-Gaffney Observatory every year and plans to make a short animated movie.

Double star 54 Leo is a somewhat tight couple at 6.6". It is official designated STF 1487 in the Washington Double Star catalogue, a discovery entry by Mr Struve. It is a fun target with bright and colourful stars. A showpiece, it is easily split at 70x magnification. The author first learned of this double target from a Sky & Telescope list.

τ (tau) Leo is wide and easy. aka 84 Leonis or STFA 19 (that's Struve again but from his addendum). Another neat system.

μ (mu) Leo aka Rasalas has an exoplanet. Of course, we can't see exoplanets with amateur telescopes. In fact, this planet was found by the radial velocity method. What is most fascinating is that the planet orbits the star every 358 days at a distance of 1.1 astronomical units (AU), slightly more than the Earth-Sun distance. Isn't that interesting? However, the naked eye magnitude 3.9 star is an evolved K2-class star. That's significantly cooler and dimmer than our G-class home star. So probably mu Leo b is an ice world.

Check out R Leo, a pulsating variable star. It's magnitude ranges from 4.4 to 11.3 over a period of 300+ days. Currently, the M class star is near maximum brightness putting it easily in naked eye range. Variable stars, like double stars, are viable targets for citizen scientists.

The ecliptic runs through it. Planets, the Moon, the Sun pass through Leo on a regular basis. The only bright solar system object in the neighbour at the moment is Vesta, asteroid number 4, making a little curved arc night-by-night. It's at mag 6.5 which is the limit of human vision but look for it with binoculars or bigger and sketch the change in position. It's near 51 Leo.


Leo Minor

Above or north of the big Leo is little Leo or the constellation Leo Minor. Easily overlooked between the great bear and great feline star patterns.

Take in galaxy NGC 3344. This is called the "Sliced Onion" in Stellarium as the face-on galaxy has a concentric ring appearance. It harbours a double star. Well, that’s not the right way to say it. The double is in our galaxy but it appears superimposed atop the distant one.

NGC 3432 is near alpha and beta Leonis Minoris. It is a RASC Finest and Herschel 400 target. This interesting edge-on galaxy that looks bent! Is it disturbed by nearby but very dim MCG 6-24-27 galaxy? The unusual spiral is also known as Arp 206. Stellarium calls it "Knitting Needle."

7 LMi is a wide double at 63.3" on the western edge of the constellation of faint stars. aka SAO 61529, HIP 46652, and HJ 1166. It is fantastic with colourful components. Do you see another star? It is clearly a triple.

An intriguing double is A2152 aka HD 90698 or HIP 51320. It is in a parallelogram of stars but only two of them are an official double. A fun arrangement of different stars, different brightness, different colours. Very interesting gathering! Another two-in-the-view thing.


Ursa Major

We conclude our trip along the meridian at Ursa Major with of course includes the Big Dipper. This time of year, for Canadians, the big constellation is nearly overhead. Six months from now, it is low in the sky below Polaris. That means now is the time. This is the best time to look at deep sky objects here.

ETU targets in UMa include Dubhe and Merak, the "pointer" stars. We also have the famous double of Mizar and Alcor that works at every level: unaided eye, binoculars, and telescope.

The Owl Nebula, a planetary nebula, is often disappointing when low in the sky. Visit NGC 3587 now for best viewing through the least amount of air.

Messier 108 is not far from Owl, also below the bowl. Stellarium calls it "Surfboard galaxy" which is apropos, given it's long elongated oval shape. This is Arp 18. It is very faint, not symmetrical, and angled north-east to south-west. NGC 4085 is in the same field at 78x.

Many enjoy viewing or imaging Bode's Galaxy (NGC 3031, Messier 81) and Cigar Galaxy (NGC 3034, Messier 82, Arp 337) to the north. M81 is dim and diffuse compared to the mottled thin M82. Again, this is the best time of year to get these interesting island universes.

disturbed galaxy NGC 4088 in luminance

NGC 4088, a Finest, a Herschel, is on the south-eastern edge of the constellation. It too is mottled and bent.

Check out the "fast mover" Alula Australis or xi UMa. It is another binary star system with a short orbital period. Do an annual sketch to plot the movement of the tight pair.



We venture off piste for a nova, an exploding star, in Cassiopeia. While low, it is bright at magnitude 8.2. That's in binocular range! Look for the Bubble Nebula to the east and Messier 52 to the north. Compare to good star charts or good planetarium software for a bright object not on the charts.



The Sky This Month presentation is made possible by many people and tools.

Software used includes SkyTools, SkySafari, Stellarium, Excel, PowerPoint, OccultWatcher. Paper resources used include SkyNews magazine, Sky & Telescope magazine, and the RASC Observer’s Handbook. Other web sites consulted were by NASA and the ESA, Images were captured using the freely accessible, command by social media BGO and MRO telescope systems provided by SMU. I thank the humans Tom Luton, David Mitsky, Ron Baalke, Chris Vaughan, Paul Markov, Dave Lane, Hilding Neilson, Dave Chapman, Curtis Michael, and Rhonda Gribbon. Thanks to Betty, Andrew, Ward, and Ennio for video streaming and recording. Thanks to Zoom and YouTube and OBS during these "interesting" times! The QR code was quickly produced with

See the author's lumpy darkness blog for astronomy information, astrophotos, tips and tricks, guides and videos on astronomy software, and so on. Contact Blake Nancarrow (astronomy at computer-ease dot com) for errors, omissions, questions.

Onward and upwards.