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                          The SOCIETY for POPULAR ASTRONOMY
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                  Special Electronic News Bulletin  2009 February 26
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        COMET LULIN
        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
        visible.
        
        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
        http://www.ast.cam.ac.uk/~jds.
        
        
        
        
        Bulletin compiled by Clive Down
        (c) 2009 the Society for Popular Astronomy
        
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