May sky at night — click on the image below to see a larger version

Everyone with more than a passing interest in the sky has a list of events or objects that they hope to see one day. My list started with the realisation that I would be able to see the total solar eclipse in 1999 from home. But I did not realise how lucky I had been to tick one of the most sought-after of all celestial sights, Comet Halley, which I saw on its last return to the inner solar system in 1986.

Unlike other comets, Halley is not named after its discoverer but after Edmund Halley (1656-1742), the Oxford astronomer who calculated its orbit and predicted its return. It passes by only every 75 years or so. So if you missed it last time you have more than 50 years to wait before it comes back. If that seems too long, you can see at least part of the comet this month, thanks to what could be an excellent display of Eta Aquarid meteors.

Meteor showers occur as the Earth passes through debris spread along a comet’s orbit, and the Eta Aquarids, like the Orionids in October, have their source in Comet Halley. The shower is already active, and will continue until May 20, with a peak just before dawn on Bank Holiday Monday, May 5.

The Eta Aquarids are not as famous as illustrious showers such as August’s Perseids or December’s Geminids, but there are two good reasons to look this year. First, May 5 is the date of the new Moon; moonlight can drown out faint meteors so its absence will greatly improve your view. Secondly, the shower has a reputation for being variable and while the consensus seems to be for a Zenith Hourly Rate of 40, some are predicting rates of almost double that figure for the next few years — worth looking out for, particularly in the hour or so before dawn.

Returning to the evening sky, the Plough is still prominent overhead, while the great zodiacal constellations of Virgo and Leo, which I discussed last month, dominate the south and west of the sky.

Trace the line made by the handle of the Plough (or, more classically, the tail of Ursa Major, the Great Bear) to a distinctly orange star.

This is Arcturus, the fourth-brightest star in the sky and the second-brightest visible from British latitudes. It owes its brightness to its proximity, lying 37 light years from Earth.

This distance (rounded to 40) meant that Arcturus was selected to open the 1933 World’s Fair in Chicago; the 1893 fair had also been held there, and the light from the star was duly used to flick a switch that turned on the lights.

Such frivolity aside, Arcturus is primarily of interest to astronomers as a nearby example of a star lagging behind its neighbours in orbiting the galactic centre, suggesting that it belongs to an older population of stars.

Look east to find the bright star Vega, one of the Summer Triangle, but combining here to point the way to the quintessential spring constellation, Hercules. The “keystone”, which is the most prominent feature of this group of stars, lies approximately halfway along a line joining Arcturus to Vega. This asterism is less prominent in the sky than it appears on the map, but, once found, is easy enough to locate.

The principal attraction of Hercules lies in its globular clusters; M13 and M92 are the best known in the Northern sky. These immense systems, containing hundreds of thousands of stars, are more like small galaxies than normal star clusters. M13, the brighter of the pair, can be found on the right-hand side of the keystone. It was discovered by the hero of this month’s column, Halley, whose description can hardly be beaten: “This is but a little patch, but shows itself to the naked eye when the sky is serene, and the Moon absent.” M92, a little way north, is fainter, but both are fine objects in small telescopes, and even binoculars may be enough to begin resolving individual stars from the multitude.

The planetary outlook continues to be depressing for all but the naked-eye (or non-UK-based) observer. Mars crosses from Gemini, where it begins the month in line with the twin stars of Castor and Pollux, into Cancer. If you have a telescope and are farther south than Aberdeen, look for the crescent Moon between 12.15 and 13.15 on May 10; Mars will disappear behind the Moon and reappear approximately 45 minutes later. Saturn is well placed at the start of the month, due south at dusk, but will become increasingly engulfed in the summer twilight and hence harder to observe. Jupiter is a morning object, but in the constellation of Sagittarius remains low in the sky. Finally, Mercury in the evening sky is well placed, reaching greatest eastern elongation on May 14, providing the best opportunity to view the elusive inner planet until 2009.