Jupiter outstanding as the Geminids meteors fly
December brings our longest nights of the year and what may be 2014’s richest meteor shower. Indeed, there is an argument for ranking December nights as the most spectacular of the year if only because Orion, and the sparkling constellations that attend him, stand at their highest near the meridian at midnight. Of the bright planets, Jupiter outshines every star and is well placed from mid-evening onwards, but the others are lurking shyly near the Sun and require a little more effort.
Jupiter is unmistakable from the moment it rises in the east-north-east some 35 minutes after our star map times. Improving in brightness from magnitude -2.3 to -2.5 this month, it climbs to pass high in the south and onwards into the south-west before dawn. We find it in Leo, to the right of the Sickle and less than 8° above-right of Regulus. It is here that it reaches a stationary point on the 9th before beginning a westerly motion which carries it back into Cancer just a day before its opposition in early February.
With its large disk and changing cloud-patterns, Jupiter is always an rewarding telescopic sight while the motions from side to side of its four main moons may be followed using nothing more than decent binoculars. When Jupiter lies near the Moon on the night of the 11th-12th, it is 717 million km distant and its globe appears 41 arcsec in diameter.
Orion stands clear of the horizon in the east-south-east at the map times. Its main stars, the blue-white supergiant Rigel at Orion’s knee and the contrasting red supergiant Betelgeuse at his shoulder, are among the ten brightest. the trio of stars between them form Orion’s Belt while hanging below the Belt is Orion’s Sword and the fuzzy glow of the Orion Nebula where new stars and planets are forming, albeit slowly, before our eyes.
A line upwards along the Belt extends to Aldebaran (close to the Moon on the 5th-6th) and onwards to the Pleiades or Seven Sisters star cluster. Carry the line downwards towards Sirius which rises one hour after our map times and is our brightest star after the Sun.
North and east (above-left) of Orion lies Gemini with its twins Castor and Pollux, while close to Castor (see chart) is the radiant point for the annual Geminids meteor shower. Bright medium-slow meteors streak in all parts of the sky between the 8th and 17th but all radiate away from this point as they follow parallel paths into the upper atmosphere. The radiant climbs from the north-north-east horizon at nightfall to pass high in the south at about 02:00. Meteor rates are expected to be highest during the 24 hours around 07:00 on the morning of the 14th when more than 80 Geminids per hour might be counted under ideal conditions. The Moon is much less obtrusive than during the Geminids last year.
The Square of Pegasus crosses the high meridian in the early evening and shifts to the south-west by our map times as Andromeda stretches up from its upper-left corner. High in the south are the two smaller constellations of Triangulum the Triangle and Aries the Ram. Aries’ main star, Hamal, is identical in brightness to Polaris, the Pole Star, but lies perhaps five times closer to us at 66 light years, It also appears to have a planet that is larger than Jupiter and takes 381 days to orbit at a distance slightly greater than that between the Earth and the Sun.
Aries also gives its name to the celestial counterpart of the Greenwich meridian. Longitudes in the sky are measured eastwards from the so-called First Point of Aries where the Sun crosses the sky’s equator at the spring or vernal equinox. When the Greek astronomer Hipparchus assigned the name more than two thousand years ago this point was located in Aries. However, the Earth wobbles on its axis over a period of 26,000 years with the result that the First Point of Aries has slipped more than 30° westwards against the stars and now lies to the south of the Square of Pegasus in the dim constellation of Pisces.
The Sun is furthest south in the sky at 23:03 GMT on the 21st, the moment of our winter solstice. Sunrise/sunset times for Edinburgh change from 08:19/15:44 on the 1st, to 08:43/15:40 on the 21st and 08:44/15:48 on the 31st. Nautical twilight persists for around 94 minutes at dawn and dusk. The Moon is full on the 6th, at last quarter on the 14th, new on the 22nd and at first quarter on the 28th.
Mars, the best of the planets after Jupiter, is the brightest object low in the south-south-west at nightfall and climbs a little higher from night to night as it slides northwards in relation to the Sun. It does, though, dim from magnitude 1.0 to 1.1 as it tracks eastwards through Capricornus. It sets at about 19:15 and stands left of the young earthlit Moon on Christmas Eve.
By mid-month, and provided we have a clear south-western horizon, we may be able to spot the brilliant (magnitude -3.9) evening star Venus just after sunset. At Hogmanay, Venus stands 6° high at sunset and sets itself 76 minutes later. Mercury slips around the Sun’s far side on the 8th and is destined to join Venus as an evening star in the New Year.
Saturn is emerging as a pre-dawn object low in the south-east where it shines at magnitude 0.5 as it tracks from Libra into Scorpius. Catch it 7° below-left of the waning Moon on the 19th.