Comet we should’a visited

An interesting comet will be very near to Saturn this evening, which may help in finding it with a good telescopes or binoculars.

Comet 22P Kopff near Saturn

22P Kopff (the 22nd recognized periodic comet) should be only about 0.1 of a degree north of Saturn on Oct. 4 at 4 Universal Time, which by eastern North American clocks is midnight between Oct. 3 and 4, and on the west coast is back in Oct. 3 at 9 PM. So after sunset the comet is approaching this closest moment, and should be just northwest of Saturn.

They are 51 degrees from the Sun, but at a low angle for us in the northern hemisphere, so only about 16 degrees above the horizon at this time of the evening. For Australians they will be vertically above the Sun, so about 40 degrees high.

This comet (described and charted more fully in the “Comets” pages of Astronomical Calendar 2015) was discovered by August Kopff in 1806.  About 1983 it was selected as the main target for a space probe called CRAF (Comet Rendezvous / Asteroid Flyby), which after visiting two asteroids would go into orbit around the comet and accompany it for two years, down to its 1996 perihelion. Since it is a highly “active” comet, scientific instruments would have gathered buku information on how a comet throws out clouds of gas and dust as it is warmed by the Sun. Alas, in a budge cut, the mission was scrapped.

Comet Kopff, as my 3-D picture shows –

Comet 22P Kopff orbit in space

– was at opposition in April, and as near to us as it would get, but still far out from the Sun (beyond Mars’s orbit), so dim. By now it is almost at its perihelion (Oct. 25), but we have left it far behind (it is 2 AU or Sun-Earth distances from us, as against 1.5 in April) and at a decreasing angle eastward from the Sun. The combination has it about as bright as it gets for us in this visit, perhaps magnitude 10, possibly better. (Remember that comet brightnesses are notoriously unpredictable; also that a cloudy object has lower surface brightness than a star of the same magnitude.)

Meanwhile, in the part of the sky that comes into view around midnight, the Moon is approaching Last Quarter. It will reach that exact phase at 21 Universal Time on Oct. 4, which is 10 PM by British clocks, 5 PM by Eastern and 2 PM by Pacific North American ones. But slightly before then, at the hour we show, local midnight for eastern North America –

Last Quarter Moon 2015 October 4

– the Moon displays its greatest libration for the whole year, 10.51 degrees. What that means, as we explained about this time last year (, is that the Moon “rocks” toward us, so that with telescopes we can peer that far past its average limb (edge) into the landscape of the “Luna Incognita,” the Unknown Moonland that is usually turned away from Earth.

Moon libration 2015 October 4

Or we could, if it weren’t that that part of the limb is now in darkness. A great libration isn’t much use unless it happens at a sunlit part of the limb. I’ve removed the Moon’s darkness at least enough to show the maria or “seas” in the night side. It will be interesting to know whether you can possibly make anything out; the only light available to the Moon’s dark side is “earthshine”, and that usually doesn’t become noticeable till nearer to New Moon. You are most unlikely to discern anything brought into the line of sight by libration at the extreme northeastern edge, north of the circular Mare Crisium.  For a libration less great but more convenient, we have to wait till the nearly Full Moon of November 24.

One thought on “Comet we should’a visited”

  1. I’ll pass on trying to observe Kopff’s comet, but I’ll be interested to hear others’ reports.

    I’ve been struck by how much of the Moon’s sunlit western* limb that I’m used to seeing during her waning phase is hidden from view recently. Grimaldi was barely visible on the Moon’s limb this morning.

    *I mean lunar western, the left-hand edge as seen by an observer on Earth’s northern hemisphere. Oceanus Procellarum is west of Mare Serenitatis in the same sense that North America is west of the Atlantic Ocean. Lunar western is the same direction as celestial eastern.

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