The Night Sky

Welcome to Neil Bone's bi-monthly review of the night skies and what to look out for.

Neil is Director of the British Astronomical Association Meteor Section and has over 30 years experience observing the sky. He has written many books on Astronomy including Philip’s Mars Observer’s Guide and Philip’s Deep Sky Observer’s Guide; and is a contributing consultant to the popular magazine, Astronomy Now.

Sky Notes for Philip’s December 2006

Winter arrives, bringing long nights. Saturn is now a late-evening object, and Venus might be glimpsed from Christmas onwards in the western evening sky. The Geminid meteors put on a fine display on 13th December.

As a result of Earth’s orbital motion around it, the Sun appears to move eastwards against the star background, by roughly one degree (about twice its apparent angular diameter) each day. The Sun’s apparent path, the ecliptic, is a great circle on the sky, inclined at an angle of 23.5 degrees – Earth’s axial tilt – to the plane of another great circle, the celestial equator (a projection onto the sky of Earth’s equator). On 22 December at 00h 22m GMT, the Sun reaches its most southerly position on the ecliptic, marking the northern hemisphere Winter Solstice. At this time, the hours of daylight are at a minimum for those at the latitudes of the British Isles, end even when it is above the horizon the Sun cuts a low arc across the southern sky. Around the solstice, the Sun is above the horizon at London for only 7 hours 40 minutes, while from Edinburgh it is up for a mere 6 hours 52 minutes!

The Sun’s annual apparent motion around the sky is described in Philip’s Practical Astronomy.

Newcomers to observational astronomy hoping for a Christmas telescope with which to further pursue their interest can find guidelines to choosing the best instrument in Philip’s Stargazing with a Telescope.

The Geminid meteor shower – the most reliably prolific of the year, with peak observed rates reaching three or four per minute in bursts – is active from 7 to 16 December. Maximum is on  the Wednesday evening to Thursday morning of 13-14 December, and dark sky conditions will prevail until the waning crescent Moon rises, around 01h GMT.

Geminids emanate from a radiant near the magnitude +1.6 star Castor, the more northerly (upper) of Gemini’s ‘twins’. The meteors are quite slow and often bright; the brightest are prone to flickering and fragmentation in flight. Geminids are unsual in having an asteroid – 3200 Phaethon – rather than a comet as their parent body.

Saturn is well-placed from mid-evening onwards, high in the southeast against the stars of Leo by midnight. In mid-December, Saturn’s apparent eastwards motion against the star background stops, and the planet begins to move westwards – retrograde – as Earth in its faster orbit closer to the Sun catches up to the planet.

The rings remain the main attraction for observers using medium sized (80-100 mm aperture) telescopes, and should be readily resolved. Titan, Saturn’s brightest satellite – at 8th magnitude easy in a small telescope – lies due west of the planet by about four ring-spans on 7 and 23 December, due east eight days later.

Venus finally begins to show as an ‘Evening Star’ around Christmas, setting about an hour after the Sun. At magnitude –4, the planet is brighter than anything else in the night sky except the Moon, but for the moment its southerly position means it will be low down and quite difficult to spot for a few weeks yet. The waxing crescent Moon is nearby on the evening of 21 December.

Jupiter emerges from conjunction behind the Sun into the morning sky during December, rising around 06h GMT at the year’s close. Now at a southerly position against the stars of Scorpius, the giant planet will be best placed for observing in the summer of 2007. Mars, meanwhile, is lost in the bright morning sky close to the Sun, and won’t be at its best again until this time next year!

For details on all the planets, consult Philip’s Solar System Observer’s Guide.

Winter is here, and in the starry sky that can mean only one thing: Orion is at his best! The figure of the Hunter is already clearing the eastern horizon as dusk falls on a December evening, and come midnight he stands proud on the meridian, dominating the southern sky. Orion is one of the sky’s most distinctive figures, with his waist (Belt) marked out by a slanting line of second-magnitude stars. His broad shoulders are formed from red giant magnitude 0 Betelgeuse, and second-magnitude Bellatrix, while his knee are blue-white Rigel (another magnitude 0 giant star) and second-magnitude Saiph.

Orion is a good starting-point for learning the constellations of the winter sky, and Philip’s Guide to the Night Sky will help you to do so.

Perhaps the most famous object I this rich region of sky is the Orion Nebula (M42), a great cloud of gas and dust within which new stars are being born. Appearing to the naked eye on a dark night as a slightly fuzzy ‘star’ in the sword dangling from Orion’s Belt, the Orion Nebula is seen as an extended haze in binoculars. A small telescope further increases its apparent extent, and shows wispy structure in the nebulosity. A prominent dark ‘bay’, popularly known as the ‘Fish’s Mouth’ is tipped by the Trapezium, a bright quadruple star system well seen in 80-100 mm aperture telescopes. 

There’s much to explore here in the depths of the winter sky; Philip’s Deep Sky Observer’s Guide will point you in the direction of some of the more interesting objects in Orion and the surrounding constellations.

If you have any comments you would like to make, or questions that you would like to ask about Philip’s astronomy books, please contact our astronomy editor, Daisy Leitch, by email at daisy.leitch@philips-maps.co.uk.