Comet ISON sweeps close to the Sun on the 28th
A brace of comets and the year’s best apparition by Mercury might be enough to tempt us outside during the chill predawn hours this month.
Let us start with the evening, though, where Venus remains poorly placed for our northern latitudes. Although it blazes brilliantly at magnitude -4.4, it stands less than 6° above Edinburgh’s south-south-western horizon at sunset on the 1st and sinks to set in the south-west only 91 minutes later. It is also at its greatest angle of 47° to the east of the Sun. On the 6th, it is 8° below and left of the young Moon and at a more southerly declination (celestial latitude) than at any time since 1930. Turning northwards again, it is 9° high in the south at sunset on the 30th, when it remains visible for 157 minutes and is brighter still at magnitude -4.6.
Our charts show the stars of the Summer Triangle, Vega, Altair and Deneb, tumbling into the west as the Square of Pegasus crosses the meridian. To the south of Pegasus are Pisces and Aquarius where the two most distant planets, Uranus and Neptune, shine dimly as binocular objects of magnitude 5.7 and 7.9 respectively.
The winter constellations are beginning to climb in the east, with their centrepiece, Orion, just rising below Taurus and the Pleiades. Jupiter, magnitude -2.4 and brighter than any star, rises 35 minutes before our map times and climbs to pass 56° high in the S before dawn. By the month’s end it has improved to magnitude -2.6 but has hardly shifted in position in Gemini, below and to the right of Castor and Pollux. The Moon is nearby on the night of the 21st/22nd when Jupiter’s cloud-banded disk appears 44 arcseconds wide if viewed telescopically.
Mars rises in the east at 01:17 on the 2nd and shines at magnitude 1.5 well up in the south-east, 10° below and left of Regulus in Leo, before dawn. Speeding eastwards into Virgo, it brightens to magnitude 1.3 by the 30th when it rises only 16 minutes later. Catch it below the Moon on the 27th.
Comet ISON is on track to graze within 1,100,000 km of the Sun’s surface when it reaches perihelion at 18:35 GMT on the 28th. However, whether it will survive the encounter is anyone’s guess. Sadly, hopes that it would blossom into the Comet of the Century have been dimming by the day as its performance has fallen further and further below what most comet experts were predicting after its discovery more than a year ago. Even this summer, the expectations were that it would be an easy binocular object by now and that it would surpass Venus at perihelion. As it is, it has still not been sighted through binoculars and it is even being suggested that it may never be visible to the unaided eye.
Claims earlier in October that the comet was already on the brink of disintegrating were soon countered by Hubble telescope observations showing it still to be intact. Images show a greenish head or coma surrounding ISON’s nucleus, with a tail pointing away from the Sun and narrowing along its length. It is brightening, but only by as much as can be explained by the intensifying sunlight and there are few signs that it is actually growing larger and more active.
Plunging sunwards, Comet ISON crosses the Earth’s orbital distance today and may be a telescopic object near magnitude 8.5 some 8° below-left of Mars in the south-east before dawn on the 2nd. It may be as much as a magnitude brighter and Moon’s breadth right of the star Beta Virginis before dawn on the 7th and perhaps near the sixth magnitude, and visible through binoculars, when it lies 0.7° below-left of Virgo’s brightest star, Spica, on the 18th.
Mercury, meantime, is emerging from the Sun’s glare to become conspicuous low down in the morning twilight in the south-east. Between the 10th and 29th, it rises more than 100 minutes before the Sun, climbs to stand between 8° and 11° high 30 minutes before sunrise, and brightens from magnitude 0.6 to -0.7. It is joined before dawn by Saturn (magnitude 0.6) which stands just 0.5° above Mercury on the 26th.
We can use Mercury to locate ISON later in the period. The comet may yet become a naked-eye object with its tail slanting back towards Spica as it sinks from 7° to the right of Mercury on the 21st to lie 5° below-right of Mercury three days later. That may be our final view of it until after perihelion. Comet Encke, a much more predictable and reliable binocular fuzz-ball at about the fifth magnitude, sinks from 6° above-right of Mercury on the 13th to pass 1.5° right of Mercury on the 18th before we lose it in the twilight.
With Comet ISON no longer expected to excel at perihelion, and because of the serious danger to our eyesight, do not attempt to observe it close to the Sun. If its tail does unfurl spectacularly, though, it may climb steeply from our south-eastern horizon before dawn on the 29th and 30th.
As the Sun tracks southwards, sunrise/sunset times for Edinburgh change from 07:20/16:32 on the 1st to 08:18/15:45 on the 30th. New moon on the 3rd is followed by first quarter on the 10th, full on the 17th and last quarter on the 25th. The new moon brings a solar eclipse on the 3rd which begins as an annular or ring eclipse over the western Atlantic but soon evolves to a total eclipse whose narrow path tracks eastwards to cross equatorial Africa from Gabon to Somalia. Surrounding areas, but not Britain, see a partial eclipse.