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Ian Morison tells us what we can see in the northern hemisphere night sky during May 2015.
The winter constellations of Orion, Taurus and Gemini are setting in the west after sunset, with Auriga close behind. Leo is higher in the sky to their left, and further over is the bright star Arcturus in Bootes. Between them, in an otherwise fairly blank part of the sky, the Realm of the Galaxies offers 18 Messier objects to telescopic observers. The Summer Triangle rises in the east later in the evening, consisting of the bright stars Vega in Lyra, Deneb in Cygnus and Altair in Aquila. The constellation of Hercules is between Arcturus and Vega, and its four brightest stars make a trapezium called the Keystone. Two-thirds of the way up the Keystone's right-hand side, binoculars can locate the globular cluster M13. The asterism of the Plough is overhead, its hindmost stars, Merak and Dubhe, pointing towards Polaris in the north. Below Polaris is the W-shaped constellation of Cassiopeia.
- Jupiter is in the south-south-west after dark. Receding from us, its brightness declines from magnitude -2.1 to -1.9 during the month, while its angular diameter decreases from 38 to 35". It moves eastwards from Cancer towards Leo, and a telescope shows its equatorial bands, Great Red Spot and Galilean moons. From 22:00 BST (British Summer Time, 1 hour ahead of Universal Time) on the 20th, observers can see the shadow of the moon Ganymede passing across Jupiter, followed by the moons Callisto and Io themselves.
- Saturn reaches opposition this month, when it is opposite the Sun in the sky and due south at around 01:00 BST.
- Mercury passed superior conjunction (behind the Sun in the sky) on the 10th of April, and can be seen low in the west after sunset in early May, a few degrees from the Pleiades Cluster in Taurus. It is best seen on the 7th, 22 degrees from the Sun in the sky at eastern elongation. Its brightness falls from magnitude -0.4 at the beginning of the month to +3.0 in the middle, when it becomes lost in the Sun's glare. At the same time, its angular size grows from 6.8 to 11".
- Mars may be just visible on the 1st, 9 degrees below the Pleiades Cluster and shining at magnitude +1.4, but is lost in the twilight after this. With an angular size of 3.8", it is too small for surface details to be seen.
- Venus shines brilliantly in the west after sunset, climbing from Taurus into Gemini and passing the star Epsilon Geminorum on the 16th. On the way to its greatest separation from the Sun in the sky on the 6th of June, it brightens from magnitude -4.2 to -4.4 and grows from 16 to 22" during May, while its illuminated fraction drops from 67 to 53 percent.
- Saturn is well placed for observation this month, as it reaches opposition on the 23rd and is therefore visible for most of the night. The rings are inclined at 24 degrees to the line of sight, almost as visible as they can ever be. Saturn brightens to magnitude 0 during May, its brightest for 8 years, with the planet spanning 18.5" and the rings 42". Around the time of opposition, the lack of shadow on the ring system makes them appear brighter than usual, a phenomenon known as the Seeliger Effect. Saturn is moving retrograde (westwards) away from the double star Beta Scorpii and towards the constellation of Libra, which it reaches on the 12th. Unfortunately, it cannot currently be seen higher than 18 degrees above the horizon from the UK, making atmospheric scintillation a problem for viewers.
- Mercury can be seen between the Pleiades and Hyades Clusters shortly after sunset on the 2nd.
- The Eta Aquariid meteor shower peaks before dawn on the 6th. The radiant, in Aquarius, is low in the south-eastern sky, but some meteors may be spotted around 90 minutes after sunset. A waning gibbous Moon in the west may hinder the view slightly.
- Venus and a crescent Moon are close together about an hour after sunset on the 21st and 22nd. Venus is below the stars Castor and Pollux in Gemini, with the Moon to its lower left on the 21st and to its left on the 22nd.
- Jupiter appears near the crescent Moon around 22:00 BST on the 23rd and 24th, close to the Mane of Leo the Lion. The Moon is below Jupiter on the 23rd and to its left on the 24th.
- Around the 23rd, Saturn is just above the fan of stars that marks the head of Scorpius. It is in the south-east at around 23:00 BST and rises higher over the following two hours.
- Comet Lovejoy is still visible as a faint, fuzzy ball this month, and can be found using binoculars or a small telescope when it passes within 1 degree of the star Polaris on the 28th.
Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during May 2015
Three bright planets are visible in the early evening. Venus appears low in the north-west as the Sun sets, outshining everything except the Sun and Moon as its atmosphere reflects almost 70 percent of the sunlight that falls on it. Jupiter appears soon after Venus, just to the left of the head of Leo the Lion in the north. It sets around midnight NZST (New Zealand Standard Time, 12 hours ahead of Universal Time) at the beginning of the month and 22:00 at the end, and is near the Moon in the sky on the 24th. Saturn rises a little later in the east, officially in Libra but close to the Claw of Scorpius and to the left of the star Antares. It is at its brightest and closest to us around the time of opposition on the 23rd, and appears high in the north at midnight. Its rings are inclined favourably for viewing, and the Moon passes within 2 degrees of it on the 6th.
On the same night, the Earth's passage through debris from Comet Halley causes the peak of the Eta Aquariid meteor shower, so named because the meteors appear to radiate from a point (the radiant) near the fourth-magnitude star Eta Aquarii in Aquarius. The radiant rises around 02:00 NZST in New Zealand, and up to a meteor a minute may be spotted streaking across the sky - although the Moon will obscure the fainter ones. The more minor Alpha Scorpiid meteor shower peaks on the 13th, its radiant near to Antares and Saturn. Although visible throughout the night and largely unhindered by the thin crescent Moon, it provides no more than 5 meteors per hour.
To the lower-left of Saturn, in the constellation of Serpens, is the globular cluster M5, also called NGC 5904. At magnitude +5.7, binoculars can be used to view it, while a small telescope picks out some of its hundreds of thousands of stars. It is home to over one hundred variable stars, the brightest of which is called Variable 42 and changes from magnitude +10.6 to +12.1 and back every 26.5 days. M5 is around two-thirds of the way from the red star Antares to the orange star Arcturus in Bootes, which rises in the north-east after dark. Above Arcturus is the bright blue star Spica, in Virgo, which is actually a double.
On the opposite side of the sky, Comet C/2015 G2 (MASTER) is expected to reach a peak brightness of magnitude +5.4 on the 14th, making it easily visible in binoculars. The comet begins May in the constellation of Sculptor, resides in Fornax from the 9th to the 14th, moves through Eridanus, Lepus and Canis Major and ends the month in Monoceros. It is visible above the south-western horizon after dusk on the 14th, setting just after 21:00 NZST.Compiled by Ian Morison