Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during May 2013.

The constellation of Taurus is setting in the west at dusk, with Leo moving across the southern sky. To the south-east are Virgo and Coma Berenices, between which the 'Realm of the Galaxies' provides a rich area for telescope observations. The planet Saturn is near to Virgo's brightest star, Spica. Bootes, with its bright star Arcturus, lies to the east, above Hercules. The four brightest stars in Hercules make a trapezium known as the Keystone, on one side of which is the largest and brightest globular cluster in the northern hemisphere, M13. Lyra, Cygnus and Aquila rise later in the evening, with their bright stars Vega, Deneb and Altair forming the 'Summer Triangle'. Near to Vega, in Lyra, is what appears through binoculars to be a double star. A telescope reveals that each component is itself a double star, lending the system the name of the 'Double Double'. Jodrell Bank's Astronomical A-list gives details of some the night sky's nicest objects.

The Planets

• Jupiter is low in the west at sunset, shining at magnitude -2 at the beginning of May, lying between the horns of Taurus the Bull, 9 degrees above the star Aldebaran. Its angular diameter drops from 33.6 to 32.4" during the month, but a small telescope will show the planet's atmospheric bands are four largest moons.
• Saturn reached opposition (on the opposite side of the sky to the Sun) on the 28th of April, and so is visible all night at the beginning of the month, in the constellation of Libra. It moves into Virgo in the middle of the month, travelling westwards (retrograde motion) as the Earth overtakes it in its smaller orbit around the Sun. The rings are inclined at 18 degrees to the line of sight and extend to a diameter of 43". We are seeing Saturn's southern hemisphere, as the northern half is largely covered by the rings. Its largest moon, Titan, can be seen with a small telescope. It reaches a maximum elevation of 28 degrees.
• Mercury passes behind the Sun (superior conjunction) on the 11th, becoming visible again around the 19th, when it joins Jupiter and Venus in the evening sky. It then has a magnitude of -1.4, but binoculars will be needed to see it just after sunset.
• Mars reached superior conjunction on the 18th of April and will not reappear into the pre-dawn sky for several months.
• Venus emerges from behind the Sun in the second week of May, shining at magnitude -3.9. Its disc spans 10", with its phase (illuminated fraction) dropping slightly from almost 100 percent to 96 percent by month's end.

Highlights

• Saturn is again prominent this month. It dims slightly from magnitude +0.1 to +0.3 and its angluar size declines from 18.8 to 18.5". The gap between its A and B rings, called Cassini's Division, is easily visible using a small telescope, but a larger telescope shows the fainter inner C ring. The rings will continue to become more visible for the next two years as their angle to our line of sight increases.
• The Eta Aquariid meteor shower peaks before dawn on the 6th, with around 10 meteors per hour potentially visible. Its radiant is low to the east-south-east, near to a thin crescent Moon which should not hamper visibility much. The shower results from dust particles released by Comet Halley as it approached the Sun around 4,000 years ago.
• Two comets can be seen this month. C/2011 L4 (PANSTARRS) is now circumpolar, passing through Cassiopeia and visible throughout the night. It is visible in binoculars or a telescope and shows a nice tail. C/2012 F6 (Lemmon) arrives in the northern hemisphere, moving up the outside of the Square of Pegasus and visible before dawn. Binoculars will be needed to see it as it dims from magnitude +6.5 to around +8 during the month.
• The Moon is closer than usual this month, and therefore larger and brighter when it is full on the night of the 24th to 25th.
• Mercury, Venus and Jupiter will form a tight group just above the western horizon after sunset from the 24th to the 28th. You may need binoculars to spot Mercury, but don't use them until after the Sun has set.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during May 2013.

May sees Orion the Hunter in the western sky in the evening. Above it is Sirius, the brightest night-time star, which often twinkles in different colours as shifting air currents refract its light. Sirius is known as the Dog Star because it is part of Canis Major, one of Orion's two dogs. To Maori it is Takarua, the Winter Star, while to Egyptians it is Sothis, whose dawn rising heralds the annual floods of the river Nile. Orion is on his side, with a line of three stars forming his Belt. These three are part of the Bird Snare, Te Manu Rore, in Maori star lore. A fainter line of stars above the Belt marks the Orion's Sword. The Sword and Belt together are nicknamed the Pot or the Saucepan.

On the opposite side of the sky, Scorpius and Sagittarius rise in the east after sunset, containing a wealth of night-sky objects. Crux and the Pointer Stars are high overhead in the south. Near to the star Beta Crucis is a star cluster called the Jewel Box, visible as a hazy glow to the naked eye. Individual stars can be seen with binoculars, while a telescope reveals more detail. The constellations of Carina, the Keel, and Vela, the Sails, run along the Milky Way between Crux and Sirius, and host many bright stars, clusters and nebulae. The brightest nebula, the Eta Carina Nebula, covers a larger area of sky than the Orion Nebula, and binoculars show star clusters nestling among glowing clouds of gas that are intertwined with dark lanes. The star Eta Carinae is there, and has interested astronomers by brightening from magnitude +8 in 1940 to +4.6 today.

Now is a good time to observe the Aurora Australis, or Southern Lights. The phenomenon is caused by the interaction of the solar wind with the Earth's magnetic field and atmosphere, and its colourful glow is sometimes visible from southern parts of New Zealand, Australia and South America. It is governed by the Sun's activity, which is currently increasing towards a probable peak in the spring. You can check a number of websites for an aurora forecast.

The Planets

• Saturn is visible throughout the night, appearing as a yellow star in the east during Twilight.
• Jupiter sets shortly after the Sun.
• Venus becomes visible later in the month, climbing higher as the month progresses. On the 28th, it is just over 1 degree below Jupiter in the sky.
• Mercury completes the planetary trio by appearing in the west at sunset, producing a conjunction that can be seen for about an hour after sunset late in the month. Binoculars may be required to see Mercury, 3 degrees to the right of the other two planets, but don't use them until after the Sun has set.

Highlights

• Observers in northern Australia have the chance to see an annular eclipse of the Sun for a number of hours around midday on the 10th. Observers nearby will see a partial eclipse. In Wellington, less than 1 percent of the Sun's disc will be covered, while North Cape will see 13 percent coverage.
• Comet C/2012 S1 (ISON) has reached 17th magnitude this month as it brightens on its approach to the Sun. The Hubble Space Telescope has imaged a very active nucleus, and the current prediction is that the comet will be visible in binoculars from August and with the unaided eye in October - although this is subject to change!