Geminids meteors to impress under moonless skies

Published on Monday 30 November 2009 21:55

WEATHER permitting, the coming month holds plenty of interest for stargazers. Not only is Orion, the centrepiece of our winter's sky, on view at a reasonable hour of the evening, but we have two conspicuous evening planets and probably the best meteor spectacle of the year.

There is even a lunar eclipse on Hogmanay.

The nearly-full Moon stands low in the east and 2 below-left of the Pleiades in Taurus at nightfall this evening, though we probably need binoculars to glimpse the star cluster against the Moon's glare. By our map times, they are well up in the south-east and Orion leans on his side clear of the horizon below them. Gemini, with the star Castor directly above Pollux, lies to Orion's right and stands much higher in the south-east as Orion strides across the meridian at 01:00 tonight.

The Sun reaches its southernmost point in the sky at 17:47 GMT on the 21st, the moment of the winter solstice. Sunrise/sunset times for Edinburgh change from 08:19/15:44 today to 08:42/15:40 on the solstice and 08:44/15:48 on the 31st. Nautical twilight lasts for about 95 minutes at each dawn and dusk.

The Moon is full on the 2nd, at last quarter on the 9th, new on the 16th and at first quarter on the 24th. The Moon makes another close approach to the Pleiades on the night of the 28th, occulting a few of the southernmost stars of the cluster in the small hours of the morning.

During the month's second full moon, on Hogmanay evening, the Moon lies in the middle of Gemini when it passes through the northern part of the Earth's shadow. The Moon begins to enter the Earth's outer penumbral shadow at 17:15 and between 18:52 and 19:54 a thin sliver, no more than 8 per cent of the Moon's diameter, makes it into the dark central umbra. Expect to see a considerable contrast between the Moon's bright northern region and its southernmost limb, especially near the time of mid-eclipse at 19:23. The Moon finally leaves the penumbra at 21:30.

It was disappointing for Europe, but the Leonids meteor shower produced rates in excess of 100 meteors an hour over parts of Asia for just a few minutes around 21:30 GMT on 17 November. If the flurry had arrived a few minutes later, and if our weather had been kinder, we might have spotted some super-swift long-trailed meteors streaming away from our north-eastern horizon. As it was, Leonids were around later that night and during the previous night, but they were few and far between.

Just as for the Leonids, December's Geminids shower benefits from no moonlight interference this year. However, the Geminids are the richest and most reliable of showers, with a large proportion of relatively slow and bright meteors. Given clear skies, we are surely in for a treat.

As the name implies, meteors from Geminids diverge from a radiant point near Castor in Gemini, which climbs from low in the north-east at nightfall to pass high in the south in the early hours. Meteors stream from this point between the 7th and the 16th, but are expected to be most prolific during the morning of the 14th, with sustained rates of 100 or so for someone observing from a dark location.

Of course, we need not look towards Gemini to see them; they burn up at 35km per second in all parts of the sky, although their trails point back towards Gemini. Uniquely among the major showers, the parent of the Geminids stream seems to be an asteroid, Phaethon, rather than a comet, though it is possible that Phaethon is the extinct hulk of a former comet.

Jupiter, brighter than any star at magnitude -2.3, stands 19 above Edinburgh's southern horizon at nightfall this evening and moves to set in the west-south-west about 30 minutes after our star map times. It is a little further to the west at nightfall by the year's end, and has faded a little to magnitude -2.1 as its distance increases. Look for Jupiter 4 below the young Moon on the 21st, when the dim magnitude 7.9 distant planet Neptune lies only 0.5, or one Moon-breadth, above-right of Jupiter.

Mars performs an about-turn near the border between Cancer and Leo before backtracking into Cancer to reach its closest and brightest in January. December sees it rise in the east-north-east at our map times and climb through the eastern and south-eastern sky to pass high in the south a few hours before dawn, where it stands above the Moon on Saturday. As Mars approaches from 142 million to 111 million km this month, it brightens from magnitude -0.1 to -0.8 and swells in diameter from 9.9 to 12.7 arcseconds.

Saturn, bright at magnitude 1.0 and on the other side of Leo in Virgo, rises in the east by 01:40 at present and two hours earlier by the year's end, climbing to approach the mid-southern sky by dawn.

Telescopically, it is 17 arcseconds wide and set within rings 38 arcseconds across, with their northern face tipped 4 towards the Earth.

Look for Saturn above the waning Moon before dawn on the 10th.

Despite its brilliance at magnitude -3.9, Venus is becoming increasingly difficult to spot very low down in the south-east before dawn and is unlikely to be seen beyond mid-December. Mercury, though, might just be glimpsed though binoculars very low down in the south-west as an evening star as it moves to stand 20 east of the Sun on the 18th. Forty minutes after sunset on that evening, it shines at magnitude -0.5 and is a mere 3 high and a similar distance below-right of the extremely slender crescent Moon.