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Telescope House May Sky Guide

Telescope House May 2021 Sky Guide

We begin May under two months from Northern Hemisphere Midsummer  - and of course Midwinter for those in the Southern Hemisphere.  This begins to have some knock on effects for readers at higher mid-northern latitudes, such as much of Northern Europe and North America.  True darkness begins to rapidly diminish, for those at mid-to-high northern latitudes, due to the Earth’s angle of rotation in relation to the plane of our solar system.  For those around 50 degrees latitude N, astronomical dusk begins at just before 11pm on the 1st May and ends at just past 3 in the morning.  Fast forward two weeks to the 15th and astronomical dusk begins at 11.46pm and ends at 2.11am.  By the end of the month, there is no astronomical dusk and dawn for those at this latitude, as the Sun never sets more than 18 degrees below the horizon - and true astronomical darkness is never achieved.  This naturally has a knock-on effect for observation and astrophotography of fainter deep sky targets, but thankfully has no effect on brighter solar system objects. 

The Solar System

The Moon

Our natural satellite begins May at a 71% illuminated Waning Gibbous phase in Sagittarius, sitting low to the horizon for those observers in the northern hemisphere.  Rising at a little after 1.20am (from 51 degrees N), the beginning of the prior evening will be unencumbered by moonlight, though this will be offset somewhat by Astronomical dusk occurring at just before 11pm, giving only around 2 hours of true darkness to observe and image the deep sky in.  The further south you are the earlier the astronomical twilight will end, but this is a sure sign for the of us further north that the lighter summer months are bearing down on us.

The Moon reaches Last Quarter on the 3rd May and will rise in Capricornus at just after 3am.  The following morning it can be found between Jupiter and Saturn in the pre-dawn sky, though will be pretty low for observations.  Beyond this point, the morning Crescent phase of the Moon kicks in as it will appear to draw closer and closer to the Sun, foreshortening its illumination as it does.

New Moon occurs on the 11th, sliding to the south of the Sun in Aries.  After this occurs, the Moon becomes an evening target, high in the northern part of the Ecliptic (the plane of our solar system in the sky, as viewed from Earth).  Rising in a pretty steeply setting part of the ecliptic (when observed from the higher temperate northern hemisphere), the Waxing Crescent Moon is (contrary to some received wisdom) very much observable from only a few hours “old” within the monthly lunar cycle.  However, it tends not to be seen before a couple of days old, due to its proximity to the Sun and the lightness and lack of contrast of the bright evening sky.  This month, the planet Venus can provide a useful guide to the location of the emerging Moon, at its earliest phase, but sky conditions and visibility will have to be kind and binoculars will be needed in order to seek it out.  It may just be possible to make out the New Moon as a very faint darkening circular area of sky sitting underneath Venus on the evening of the 12th, but at under 1% illumination, don’t expect to pick up any detail and make sure the Sun has set before trying to seek it out.  You can try again the evening after, when the Moon will sit higher in the sky at sunset and will be flanked by Mercury, which itself will be reasonably difficult at +0.1 magnitude.  The Moon will be 3.5% illuminated by this point in time and will be about 15 degrees high in the NW at sunset.  Again, binoculars will probably be essential to locate it.

A narrow 2.7 day old Crescent Moon, taken through an Explore Scientific Maksutov Newtonian Comet Hunter.  Image credit: Kerin Smith

May’s Moon is really the last of the “High Spring Evening Crescent” phases visible from the northern hemisphere.  While next month’s Crescent Moon will actually reach very reasonable height in the sky, once the Sun has crested over the top of the most northerly part of the Ecliptic, the angle at which it appears to set from the northern hemisphere gets significantly shallower and the season of lunar observation is definitely over.

The Moon reaches First Quarter phase on the evening of the 19th, while in Leo - setting at a little before a quarter to three in the morning.   Continuing its slide through the southern part of the ecliptic, it will eventually come to be Full in the constellation of Scorpius on the 26th.  This month Full moon will coincide with a Total Lunar Eclipse, which occurs when the Moon passes through the shadow of the Earth as cast by the Sun.  Sadly for those of us in Europe and Africa, the Middle East and much of the Western part of Asia, no eclipse will be visible as it occurs when the Moon is set from these parts of our planet.  However, those bordering and in the Pacific will get a grandstand view, with the Moon high in the sky from Eastern Australia and New Zealand and Micronesia, where it will probably be seen best from.  Much of India, , and the rest of the Far East won’t see the eclipse in full, as the Moon will be rising as it has started.  Conversely, the Pacific coasts of the Americas will experience the even as the Moon is setting - though much of the beginning of the event it will still be seen by some of those on the eastern seaboard of the USA, Mexico and the Caribbean.  If you find yourself in these parts of the world, don’t miss the opportunity to experience one of nature’s loveliest spectacles. 

This Full Moon is also a Perigee-Syzygy Moon, otherwise more popularly-known as a Supermoon, which occurs when the Moon is closest to us during the process of its slightly elliptical orbit.  As stated in previous sky guides, there’s no great significance to this and the synchronisation with an eclipse is mere coincidence.  The Moon itself will appear slightly larger than average, especially when viewed low in the sky when atmospheric lensing also appears to expand it somewhat.  The irony is - eclipse aside - Full Moon is the worst time to observe the Moon telescopically, as full illumination bleaches out many of the most interesting features to observe.  Expect to see plenty of headlines about the Supermoon Eclipse closer to the May’s Full Moon, but rest assured there’s no greater significance to the event as a result of it being a “Supermoon”.

After the excitement of the eclipse is over, the Moon will continue its path through Ophichus, Sagittarius and Capricornus, until we reach the end of May with the Moon sitting just a little to the south of Saturn in the morning sky at a 73% illuminated Waning Gibbous phase, much where we picked it up at the beginning of the month.


The Solar System’s innermost planet starts May very well-placed for observations from the temperate northern hemisphere.  This is one of the best opportunities off the year to catch Mercury in the evening sky after the Sun goes down.  On the 1st, the planet will be a bright -1.1 magnitude and sit some 11 1/2 degrees above the horizon in the west after sundown.  At 5.7 arc seconds across and 81% illuminated, it will be a naked eye target, though easier in binoculars and small telescopes, which will resolve its tiny disk.  Mercury is still climbing north out of the Sun’s glare and as such conditions will remain kind for those trying to find this illusive world over the next couple of weeks, as it approaches maximum elongation from the Sun on the 17th May.  By this point, the planet will have faded to +0.5 magnitude and show a 35% illuminated crescent phase, 8.2 arc seconds across.

Finding Mercury will be made a little more straightforward by the presence of the much brighter Venus in the same are of sky, acting as a beacon to guide observer’s to its fainter neighbour.  Mercury will sit higher in the sky than Venus until much later in the month, towards the end of May.  Beyond the 17th, Mercury will start to fade significantly as it draws round towards us on its orbit, foreshortening illumination and dropping in brightness as it does so.   By the time the two planets draw alongside each other in close conjunction at the end of May, sitting just half a degree apart on the 28th, Mercury will be a rather pitiful +2.2 magnitude and while it will have made it into double figures in terms of arc second’s diameter - 10.6 - the phase of the planet will be a very low 12.6% illumination, making it impossible to see in the glare to the evening sky.

The advice as far as May’s evening apparition of Mercury is to catch it early.  Although common sense suggests it will be easier to find Mercury the higher it is in the sky, this separation from the Sun comes at a sacrifice of brightness.  Sometimes it is easier to find a brighter Mercury lower in the sky, than it is a fainter one, higher up.  What ever method you use to try and find Mercury, as ever, we caution to make sure the Sun has set before doing so - especially when using binoculars or a telescope.

Mercury, sunset, 14th May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,


As discussed earlier in this sky guide, Venus provides a useful waypoint for both the very New Moon and Mercury this month, appearing with both in the same area of sky.  As far as Venus itself is concerned, it appears to move at a much more sedate pace than its fleet-of-foot planetary neighbour and is very slowly inching its way away from the Sun.  At -3.9 mag at the month’s beginning, it will be seen by those with clear westerly horizons after the Sun sets, though will not even attain a height of seven degrees (from 51 degrees N) at this point in time.

Mid-month will find Venus a tad higher in the sky at sunset - just over 9 degrees - but no brighter.  The planet is still very much over on the farthest part of its orbit around the Sun, as seen from our perspective on Earth, so change is very gradual.

By May’s conclusion, there’s no great change in brightness or illumination of Venus.  The planet sits a little over 11 degrees high in the sky and although it will prove a useful waypoint for finding other objects this month, it will need to be higher in the sky before we recommend higher power observations with telescopes.

Venus at sunset, 31st May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,


The Red Planet is a rather pitiful sight in a telescope this month.  Although hanging on in there as a potential target for observation, the planet’s disk is really tiny - just 4.6 arc seconds on the evening of the 1st and showing a brightness of +1.4 magnitude.  While we would never discourage anyone from using their telescope to observe anything in the night sky, rest assured there are much better and more interesting objects to look at than Mars, at present.

As the month continues, the situation deteriorates yet further.  By the 15th Mars’ brightness has shrunk to +1.7 mag and disk down to 4.4 arc seconds diameter.

By the end of May, Mars is shrunk yet further to 4.2 arc seconds diameter, but is no fainter at +1.7 mag, though at this brightness it is now fainter than either Castor and Pollux, the two brightest stars in Gemini, where the planet is currently resident.  Mars will not be back to close to its best until the latter part of 2022, when it returns to Opposition on December 8th.

Mars comparitive angular size October 2020 to May 2021.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,


In stark contrast to Mars, Jupiter’s situation is slowly improving.  The Solar System’s largest planet is so big, that even at the extremes of separation from us, it is never faint and always presents a good-sized disk for observation.  Early May finds Jupiter in Aquarius, continuing its trend as an morning target, rising a little over two hours before the Sun.  At -2.2 magnitude and 37.5 arc seconds diameter, the planet will be easy to spot in the sky, as long as you have reasonable easterly horizons, though it will only attain a height of a little under 16 1/2 degrees altitude (from 51 degrees N) as the Sun rises.  At the beginning of the month, Jupiter is to be found just under 72 1/2 degrees separation from the Sun.

By mid-May, Jupiter will have brightened fractionally to -2.3 mag and will now display a 39 arc second diameter disk.  It will now sit over 84 degrees separation from the Sun and rises just under 2 hours 40 minutes before the Sun.  It now sits just over 19 1/2 degrees high at daybreak (from 51 degrees N).

At the end of May, Jupiter will have improved yet further, displaying a -2.4 mag, 41.4 arc second disk.  Rising at 1.48am (BST), the planet will attain a height of just under 23 1/2 degrees altitude as the Sun comes up.  We are still some way off Jovian Opposition, which occurs on 20th August this year, by which time Jupiter will appear to be -2.9 magnitude in brightness and nearly 50 arc seconds across.

Jupiter with dual Great Red  Spot and Io transit, 4.16am (BST), 13th May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,


Found in a similar area for sky, in Capricornus, but further west in the Ecliptic than its neighbour Jupiter, Saturn rises earlier and attains a somewhat greater separation from the horizon at the beginning of the day.  On the 1st, the Ringed Planet is to be found at a modest +0.7 mag and displaying a 16.7 arc second disk.  Rising at just past 3am on the 1st, Saturn attains a separation from the horizon of just under 17 3/4 degrees (from 51 degrees N) at sunrise.  

Saturn, Jupiter and the Moon just before dawn, 5th May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,

Saturn reaches its furthest prograde point in its path around the sky on the 23rd/24th May, after which it begins a retrograde motion - appearing to travel backwards in the Ecliptic to its “proper” apparent motion.  This is a precursor to Opposition for all of the Outer Planets and it is caused by our “catching up” with the outer body on our faster interior orbit, causing the angle at which we observe the outer body in relation to the background stars to change.  The analogy of catching up and undertaking a car and noting its apparent motion, relative to the car you are travelling in, is a simple version of what’s happening here.  Of course, the outer planet never actually changes its orbital course, but apparent retrograde motion is a clear demonstration of the visual effects of orbital dynamics at work and as such a fascinating demonstration of the way our Solar System works and our place within it.  While we’re still a little way off Saturnian Opposition, which occurs on the 2nd August this year, retrograde motion of any of the outer planets is always a sign that this is upcoming.

Saturn ends the month a little brighter at +0.6 mag and just slightly larger at 17.6 arc seconds diameter. It sits at an altitude of just over 21 1/2 degrees high (from 51 degrees N) in the east as the Sun comes up.

Saturn and moons, sunrise, 31st May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,

Uranus and Neptune

Uranus is just emerging from April 30th’s Superior Conjunction and as such will be unobservable for some time to come as it is too close to the Sun, in Aries.  Neptune, having been through Superior Conjunction in mid March, while considerably further west of Uranus, in Aquarius, is still in a very poor part of the sky to observe a planet this faint, especially as much of the temperate upper northern hemisphere will be experiencing permanent astronomical twilight from late May.  It will be a little while longer before both planets are in a better position to be observed.

Uranus and Neptune, relative positions at sunrise, 15th May.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,


C/2020 R4 Atlas is visible in May, though will be faint.  Those with telescopes or large binoculars may wish to hunt out comet as it is making its way through Canes Venatici, Coma Berenices, Ursa Major and Leo during the month, though it will be a difficult target.

C/2020 R4 Atlas path May 2021 (comet position shown 1st May).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp.,

As reported in previous sky guides, C/2021 A1 Leonard looks like it will be a comet to look forward to, as it passes Earth by 0.23 AU in December 2021, just prior to perihelion.  It is predicted that it may reach around the 4th magnitude, so could well be an interesting binocular and telescopic sight.  This is naturally still some way off and we must be cautious in regards to how this comet may develop, however potentially promising it looks at present.


Reaching their upper range of activity between the 4th-6th May, the Eta Aquariids meteor shower peaks on the night of May 5th this year.  While the zenith hourly rate of this shower - around 30-40 at maximum, this year - is not as high as some of the major annual showers, this event would be worth staying up for (or at least attempting to record photographically), where it not for the pernicious influence of the Moon, the perennial upsetter of meteor shower observation, which will be at 36% phase and hanging around all night in Aquarius itself.  As meteors from any radiant can be seen in any part of the sky, observers do not have to solely concentrate on the part of the sky the radiant is located in to catch some decent meteor activity.  As such, one could try starting off observations or photography before 4am when the Moon rises.  This probably cuts the potential for observable meteors by a factor of 50% or very likely more, but this year will probably be a better strategy than waiting until later when the radiant has risen.   As the radiant rises so late, it is often commented that the Eta Aquarius are better seen in the southern hemisphere, but those in the northern hemisphere should still try and catch a few meteors, if possible.  This shower is seeded by the famous Halley’s Comet, whose debris is quite fast-moving, resulting in bright, energetic meteors.  The best of these will be visible, especially if you try to observe before the Moon rises.


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