AN INCREDIBLY dim comet flared up in brightness last week to change the naked-eye appearance of the constellation Perseus, ideally placed as it climbs in our eastern evening sky. The brightening, by a factor of a million or so, is unprecedented, though the same comet had been enjoying a lesser outburst when it was first spotted in 1892 by Edwin Holmes from London.
Comet Holmes takes almost seven years to orbit the Sun at a distance that varies between 327 million and 775 million km, in a path that criss-crosses the asteroid belt between Mars and Jupiter. It passed perihelion, its closest point to the Sun, in M
ay and appeared near its predicted brightness at the 17th magnitude until last Wednesday. This was so faint that only the largest of telescopes could hope to show it.
On Thursday, though, something caused it to brighten spectacularly to become a second magnitude object in Perseus. Initially starlike in appearance, it was only a little inferior to Mirfak, the brightest star in Perseus, which stood a few degrees away and shines at magnitude 1.9. The new "star" could easily have been mistaken for a true stellar outburst, a nova, which might not be so unusual in a Milky Way constellation like Perseus.
Indeed, what was the brightest nova seen until then flared in Perseus in 1901 and was discovered by Thomas Anderson, a clergyman and amateur astronomer in Edinburgh. That nova reached magnitude 0.2 to rival the bright star Capella and, now called GK Persei, it is still studied as it flickers erratically near the 13th magnitude. Interestingly, Anderson made an independent discovery of our comet just two days after Holmes in 1892, so we might now be watching Comet Anderson rather than Comet Holmes.
The cause of the comet's startling behaviour must lie in its icy nucleus. Stresses caused by the warming and evaporation of ices beneath its crust may cause the nucleus to fracture catastrophically from time to time, exposing fresh ice to the Sun's heat and ejecting a blizzard of dust that gives the comet its yellowish hue.
By Sunday, Comet Holmes was still bright but distinctly fuzzy, appearing through binoculars as a diffuse blob about six arcminutes across and with a more intense core. I also suspect that it was a little fainter overall than it had been on Saturday, though the bright moonlight didn't help. If it is fading, don't despair, for during 1892 it was bright for several weeks and brightened again to the fifth magnitude more than two months later, just shy of its best during that performance.
An arrow on the eastern side of our "Looking North" chart plots the motion of Comet Holmes during November as it moves from 3.4° east (below-left) of Mirfak this evening to pass only 0.3° (less than a Moon's breadth) north of the star on the 19th. Expect the comet's head or coma to inflate and, perhaps, to sprout a tail. Note, though, that any tail will point away along our line-of-sight and appear foreshortened.
Looking below Perseus at our map times we find Auriga and Capella, to the left of Taurus and the Pleiades. Just rising in the east is the mighty Orion, while to its left is Gemini, where Mars blazes like a bright orange beacon.
The Square of Pegasus stands on the meridian in a relatively uninspiring region of the heavens. The Plough is turning below Polaris, the Pole Star, in the north while the top of the Summer Triangle has fallen beneath the midpoint of the western sky.
The Milky Way arches from west to east, flowing through the "W" or "M" of Cassiopeia in the zenith. This month the Sun dips a further 7° southwards while sunrise/sunset times for Edinburgh change from 07:19/16:33 GMT on the 1st to 08:17/15:46 on the 30th. Nautical twilight lengthens from 84 to 93 minutes. The Moon is at last quarter today, new on the 9th, at first quarter on the 17th and full on the 24th.
There may just be time to catch Jupiter (magnitude -1.9) very low in the south-west at nightfall, but only if you can see down to the horizon.
Mars, though, becomes our premier evening planet when it rises at Edinburgh's north-eastern horizon at 19:29 tonight and by 17:19 on the 30th.
The Red Planet is creeping eastwards close to the star Mebsuta in Gemini and slows to a halt on the 15th before heading westwards in a retrograde arc that will carry it through opposition at Christmas, when it will be nearest and brightest. This month it approaches from 115 million to 93 million km as it doubles in brightness from magnitude -0.6 to -1.3.
A telescope shows that Mars's dust storms have subsided, permitting better glimpses of the markings on its small disk, which grows from 12 to 15 arcseconds in diameter.
Mars passes high in the south, above and to the left of Orion, as the brilliant morning star Venus climbs above the eastern horizon. In fact, Venus rises about four hours before the Sun and is unmistakable at magnitude -4.4 to -4.2 as it climbs to stand high in the south-east at dawn.
During November, it shows a shrinking gibbous disk through a telescope and tracks east-south-eastwards in Virgo to lie 4° above-left of Virgo's leader Spica by the month's end. It stands even closer to some of Virgo's other bright stars on the mornings of the 6th, 13th, 18th and 26th, and it lies to the left of the waning earthlit Moon next Monday.
As Venus speeds through Virgo, it leaves behind the ringed planet Saturn (magnitude 0.8) which lies a few degrees below-left of the (fainter) star Regulus in Leo and appears just above the Moon on Sunday.
The only bright planet I haven't mentioned is Mercury, which is beginning its best morning apparition of 2007. As seen from Edinburgh, it rises in the east 92 minutes before the Sun this morning, and a maximum of 123 minutes before sunrise on the 9th, when it shines at magnitude -0.6 and stands 9° high in the ESE 40 minutes before sunrise. Of course, it might be better to look for it earlier against a slightly darker sky, but, in any case, binoculars aid its discovery in the twilight. Mercury slips back into the Sun's glare by about the 21st.
The full article contains 1098 words and appears in The Scotsman newspaper.
