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The night sky for May 2017

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during May 2017.

The Stars and Galaxies

The Planets


JupiterThis is a great month to observe Jupiter which came into opposition on April 7th so, during May, will be visible in the south during the evening. It is moving down the ecliptic and lies in Virgo. It now reaches an elevation of ~36 degrees when crossing the meridian. An interesting observation is that the Great Red Spot appears to be diminishing in size. At the beginning of the last century it spanned 40,000 km across but now appears to be only ~16,500 Km across - less than half the size. It used to be said that 3 Earths could fit within it, but now it is only one. The shrinking rate appears to be accelerating and observations indicate that it is now reducing in size by ~580 miles per year. Will it eventually disappear?

May: Look for the Great Red Spot on JupiterA list gives some of the best evening times during May to observe the Great Red Spot which should then lie on the central meridian of the planet. The times are in UT.

May 4/5th - after midnight: The Moon occults the double star 49 LeonisJust after 00:20 BST, the dark limb of the Moon will occult the double star 49 Leonis (magnitudes +5.8 and +7.9 separated by 2 arc seconds). As a result, the starlight will take longer to be extinguished than from a single star. If the seeing were good, it might be possible to split the pair using a high magnification and see each component disappear.

May 5th and 6th before dawn: The Eta Aquarid Meteor ShowerThe Eta Aquarids are one of the finest meteor showers that can be seen from the southern hemisphere, but, in the northern hemisphere, may be glimpsed in the pre-dawn sky in the south-east around 90 minutes before dawn. Sadly, this year the peak is when the Moon is coming towards full - but happily low on the western horizon - so there will be some moonlight to hinder our view.

May 7th - evening: The Moon and JupiterThis evening the Moon, three days before full, will pass just 1.5 degrees above Jupiter.

15th May - evening: Observe the Galilean Satellites.If clear on the evening of the 15th and using a small telescope, one could observe the 4 Galilean Moons lined up on one side of the giant planet.

22nd May - dawn: Venus and a thin crescent MoonBefore dawn on the 22nd, Venus will be seen over to the left of a very thin waning crescent Moon.

22nd to 31st May - all night: Observe Comet 2015 V2 (Johnson)During the last 10 days of May, with no Moonlight to hinder our view, binoculars or a small telescope could be used to spot Comet Johnson as it moves down through the constellation Bootes closing in on the bright star Arcturus. It might reach magnitude +6 so should be easily visible with binoculars. The chart shows its position during this time.

27th May - ~23:20 BST: a shadow transit of Jupiter.After sunset on the last three day of the month, one can observe, if clear, a thin waxing crescent Moon passing the Beehive cluster (M44) as it moves up to pass Regulus, in Leo, on the 31st.

May 3rd and 16th, evening: The Hyginus RilleThese evenings, should it be clear, are a superb time to view the Hyginus Rill as it will lie close to the terminator. For some time a debate raged as to whether the craters on the Moon were caused by impacts or volcanic activity. We now know that virtually all were caused by impact, but it is thought that the Hyginus crater that lies at the centre of the Hyginus Rille may well be volcanic in origin. It is an 11 km wide rimless pit - in contrast to impact craters which have raised rims - and its close association with the rille of the same name associates it with internal lunar events. It can quite easily be seen to be surrounded by dark material. It is thought that an explosive release of dust and gas created a vacant space below so that the overlying surface collapsed into it so forming the crater.

Southern Hemisphere

Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during May 2017.

Kia ora and welcome to the May Jodcast from Space Place at Carter Observatory in Wellington, New Zealand.

Our evening skies this month are dominated by Jupiter and Saturn, along with some of our brightest stars. Jupiter will be one of the first objects to appear, visible in the north east shortly after the Sun has set. A bright waxing gibbous moon will pass within 2 degrees of the planet on the evening of May 8th, that’s 4 moon diameters apart. Both will be visible within the same binocular field of view.Just to the right of Jupiter is Spica, the brightest star in the constellation of Virgo, and below, just above the horizon is orange coloured Arcturus, which at magnitude −0.05 is the brightest star in the northern celestial hemisphere and the 4th brightest in the night sky.

Arcturus has a similar mass to the Sun, but it is further along in its lifespan and has already expanded to become a red giant, with 25 times the diameter and 170 times the luminosity of our own star. When close to the horizon it often appears to twinkle red and green as its light is broken up by our atmosphere.

All three of the brighter stars are in the southern hemisphere, and are also visible in our evening skies this month. The brightest, Sirius , sits halfway up the western sky at the beginning of the month, with Orion’s belt, now almost vertical, below. Rigel and Betelguese , the seventh and ninth brightest stars sit to either side of the belt. Both Orion and Sirius will soon be disappearing from our evening skies, before reappearing before the Sun in the morning skies over the coming months.

The second brightest star Canopus is circumpolar here in New Zealand, never dropping below the horizon. This month it sits a little higher than Sirius, further around towards the south west. To complete the trio, the third brightest star, Alpha Centauri is high in the southeast, pointing the way to the Southern Cross.

Below Alpha Centauri, rising in the twilight sky is the curve of stars making the body of Scorpius, the scorpion. Its brightest star Antares is a variable star, which ranges in brightness from magnitude 0.6 to 1.6, and is on average the 15th brightest in the night sky. Antares is a red supergiant, , one of the largest stars known, almost 900 times the diameter of the Sun. If it were placed at the centre of the Solar System, its surface would extend to the middle of the asteroid belt. The name Antares means rival of Mars, because of its striking red colour. To Māori the star is known as Rehua and represents a drop of blood Maui pulled from his nose to bait his fishhook. In Aotearoa, the constellation of Scorpius in seen as Maui’s hook, te Matau a Maui, which was used to pull up a great fish which became the north island of New Zealand.

Scorpius/te Matau a Maui is our winter constellation and will be dominating our skies over the coming months, visible throughout the night.

Sitting just 1.3 degrees to the west of Antares, and visible in the same wide field telescope view, is the 5.9 magnitude globular cluster Messier 4, just about visible to the naked eye in a clear, dark sky. M4 is one of the nearest globular clusters in the sky, at just 7,200 lightyears away and is the only globular cluster that Messier was able to resolve with the modest equipment he was using, 20 years before William Herschel was able to resolve all of the Messier globular clusters with his much larger telescopes.

M4 is one of the loosest, most open globular clusters and features a central bar of 11th magnitude stars, which can be resolved in a 10cm telescope. Through binoculars the cluster is seen as a round hazy patch.

Messier 4 contains some of the oldest stars known in our galaxy, with an estimated age of 13 billion years. Discovered by the Hubble Space Telescope in 1995, these White Dwarf stars are the remnants of ancient solar like stars that have already shed their outer layers into space.

In 2003 a planet was discovered orbiting one of these White Dwarfs and its Pulsar companion, the first circumbinary (ie orbiting both stars)”planet ever found, and the first planet detected in a globular cluster. With a mass of around 2.5 Jupiters and an estimated age of 12.7 billion years, it is also one of the oldest known extrasolar planets, earning it the unofficial nicknames “Methuselah” and “The Genesis Planet”.

A little over half way from Antares to Arcturus is another globular cluster, M5, or NGC 5904, in the constellation of Serpens. At almost 6th magnitude it is also tough to spot with the naked eye in anything but the darkest conditions, but with binoculars it is easy to find, although you will need a small telescope to begin to resolve it and start to pick out a slightly elongated shape and a few edge stars.

With a calculated age of around 13 billion years, M5 is one of the oldest globular clusters known, and at 165 light years in diameter and containing hundreds of thousands of stars, it is also one of the largest. It also contains 105 known variable stars, with the brightest and most easily observed, Variable 42, changing from magnitude 10.6 to 12.1 in just under 26.5 days.

To find M5 you can star hop from the faint star 109 Virginis to 110 Virginis and then around twice the distance again to find 5 Serpentis, and M5 is just 20' to the North West.

Below Antares, and rising a little later in the evening, is bright, cream coloured Saturn. It’s a great time to observe Saturn through a small telescope at the moment with its rings at close to maximum tilt. Saturns largest moon , Titan, can be spotted orbiting around for ring diameters from the planet, with several smaller, closer moons also visible in larger telescopes.

In the morning sky Venus rises in the east around 4am and is joined by Mercury around an hour and a half before sunrise. Mercury reaches its greatest elongation west on the 18th of May and is making its best morning appearance of the year, rising before twilight begins throughout the month.

If you’re out planet spotting in the early hours then take a look for the eta aquarids meteor shower, which peaks around the 6th of May. This shower is caused by the Earth passing through the trail of debris left behind by the famous comet Halley. The shower appears to radiate from a point near the fourth magnitude star Eta Aquarii, one of the brightest in the zodiac constellation of Aquarius, representing the water bearer.

The radiant won’t rise in the eastern sky until around 2am here in New Zealand, so the best time to go meteor spotting is in the few hours before sunrise. At the shower’s peak you may be able to spot up to a meteor a minute, many of them fast and bright and leaving glowing trails behind them. Viewing conditions this year will be much better than last, with the Moon setting on the other side of the sky in the early hours of the morning, giving you a good few hours of observing time before the Sun rises just after 7 am.

You may also have a chance for some binocular comet hunting this month as Comet PanSTARRS (C/2015 ER61) heads towards Perihelion (its closest approach to the Sun) on the 10th of May. At the start of the month it will be moving from Aquarius through to Pisces and may be visible with binoculars in our morning skies. On 4th April the comet experienced an outburst, brightening from 8th to 6th magnitude almost overnight, and whilst it is always hard to predict how bright a comet is going to get, this one is definitely worth keeping an eye on.Wishing you clear skies from the team here at Space Place at Carter Observatory.

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The Night Sky This Month is one part of the Jodcast. The full show contains the latest news, interviews with astronomers, answers to listener questions and more.

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