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Thursday, 26 February 2009

SPA Comet Special Bulletin

                 The SOCIETY for POPULAR ASTRONOMY
         Special Electronic News Bulletin  2009 February 26
By Jonathan Shanklin
Comet 2007 N3 (Lulin) is now visible in the evening sky,and reaches
opposition (when it is due south at midnight) on Feb. 26.  If the
clouds part,it should be visible to the naked eye at around 5th
magnitude from a sufficiently dark site, and through a telescope will
show a rather unusual tail.  The comet has a retrograde orbit
(i.e. opposite in direction to the planets) that is inclined at 178
degrees, which means that it moves nearly in the plane of the
ecliptic.  The gas tail will barely be visible as it will be pointing
directly away from us; the dust tail, however, is a broad fan lying in
the plane of the comet's orbit, so both a tail and anti-tail should be
The geometry also means that the comet's brightness is enhanced around
the time of opposition owing to the smallness of the angle between the
Sun, comet and the Earth.  The brightening in such a situation is
known astronomically as the 'opposition effect', and reveals something
about the character of the surface of the body concerned.  It is
frequently conspicuous in everyday life when the Sun is shining and is
at a low altitude; in its terrestrial manifestations it is known as
the heiligenschein effect.  If the observer's shadow falls on a rough
surface, the surface appears to brighten progressively towards the
shadow of the observer's head.  That happens because the rugosities in
the surface shade one another where the light arrives at an angle, but
in the exact anti-solar direction one sees only the illuminated parts
of the surface.  The effect is particularly striking on a surface such
as a cornfield, where the stalks shade one another substantially when
the obliquity of the illumination is only a few degrees.
Images show the coma of the comet to have a green hue, which is due to
emission of light from certain molecular bands of cyanogen and the
Swan bands of diatomic carbon.  People with sensitive colour vision
may see the colour through a telescope, but others will see only a
white glow.  Colour vision fails below a certain minimum surface
brightness, so to have the best chance of seeing the green colouration
it is necessary to maximize the apparent surface brightness of the
coma by using the lowest magnifying power appropriate to the
telescope being used.  That means a power that makes the exit pupil
from the eyepiece as large as will enter the eye -- about 6 or 7
millimetres, implying a magnifying power of about four times the
aperture of the telescope in inches.
Now is the best time to observe the comet, as the waxing Moon will
start to interfere with observations in early March.  The comet is
currently in Leo; it will fade quickly after opposition, and by the
time the Moon is out of the way in mid-March it can be expected to
have faded by two magnitudes.  To compensate however, it will by then
be high in the sky in Gemini, and so may be easier to see, even in
light-polluted areas.  This relatively bright comet gives beginners a
chance to practise their techniques.  Visual observers can attempt
magnitude estimates.  Use a technique similar to observing variable
stars, but de-focus the stars to make them appear nearly the same
diameter as the comet.  Better still, try remember the brightness of
the in-focus comet, and compare it with the out-of-focus stars.  You
can sketch the comet using techniques similar to those for drawing
deep-sky objects.  Imagers will need only short exposures to record
the comet, but you can then stack them to try to bring out more
detail.  For the latest information about this and other comets, and
some guidance on making observations, see the Section web page at
Bulletin compiled by Clive Down
(c) 2009 the Society for Popular Astronomy

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