Sky and Telescope - November 2017 - 48

NOVEMBER 2017 OBSERVING
Celestial Calendar by Alan MacRobert

The Poleward Trek of Comet ASASSN1
Heading far north, this telescopic target should be at its best in October and November.

C

p Comet ASASSN1, or more formally C/2017 O1, already showed a condensed
nucleus and a lovely green coma when Rolando Ligustri took this image on July 22nd
as the comet was crossing Cetus.

48

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

omets often hug the dawn or dusk horizons frustratingly low in the twilight. And
sometimes they're completely out of sight at
their peak. The reason, of course, is that comets
shine brightest when they're closest to the Sun in
space. Not only is this when they're heated the
most and release the most gas and dust, it's also
where sunlight illuminates these sheddings the
most intensely.
The newly found Comet ASASSN1 won't act
like that. Formally named C/2017 O1, it was
discovered by the All-Sky Automated Survey for
Supernovae on July 19th at 15th magnitude. It
brightened rapidly and is now high in the dark
night sky for small-telescope users at mid-northern latitudes. It should glow at about 7th magnitude from early October through mid-November,
and it could reach 6th in mid-October.
It will come only as near to the Sun as 1.5 a.u.
(perihelion is October 14th), and Earth is coasting along more or less between it and the Sun -
affording us a picture-window view of it against a
backdrop of dark space.
Try binoculars if you have them, but expect to
need a telescope. A diffuse thing like a comet is
always less easily visible than a star of the same
magnitude, especially through light pollution. In
early August, ASASSN1 had a bright, condensed
nucleus, a good sign that it may hold to its predicted brightness. It was not showing a tail, and
with its distance from the Sun, it may never.
The comet travels northward between Perseus
and Auriga for much of October, as shown at
right. This part of the sky will be high in the east
to northeast by late evening and passes overhead
in the early-morning hours. The comet spends
the rest of October and November trekking north
through the dim polar wastes of Camelopardalis
and northern Cepheus. By then it will be well up
in early evening.
Can you see the comet's green color in your
eyepiece? It's caused by hot diatomic carbon (C2)
ﬂuorescing in the ultraviolet sunlight. I see at

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017

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Sky and Telescope - November 2017 - Cover1
Sky and Telescope - November 2017 - Cover2
Sky and Telescope - November 2017 - 1
Sky and Telescope - November 2017 - Contents
Sky and Telescope - November 2017 - 3
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