Tuesday, October 2, 2012

Prepare to be disappointed (or amazed)

   A word of caution - comets are notoriously fickle, spectacular predictions based on early observations often lead to big letdowns.

      On November 28, 2013, Thanksgiving Day in the U.S., the midday Sun may have a companion as it travels across the sky.  Comet ISON (  C/2012 S1) will execute a hairpin turn as it passes 1.8 million km above the Sun's surface.  Considering the diameter of the Sun is 1.3 million km this is an extremely close encounter. If the comet can survive the heat and tidal forces it has been estimated that it may shine as bright or brighter than the full moon.  If it attains that brightness it could be visible during the day one degree away from the Sun.   Of course you will have to block the sun out (by standing in the shadow of a building perhaps), to view it.   The potential of this comet has been compared to the Great Comet of 1680 and the Great Comet of 2007.  It has an uncannily similar orbit to the 1680 comet which sported a 70-90 degree long tail and its head was briefly visible during daylight in December of that year.  It has been suggested that the current comet and the 1680 comet may have split from one object in the distant past.

   The Great Comet of 2007 you ask?  How did you miss that one?  Comet McNaught (C2006 P1) brightened rapidly as it approached the Sun in January 2007.  This blogger found out about it on January 4th of that year and after making 3 unsuccessful attempts to view it through mostly cloudy skies in bright twilight I finally succeeded on January 10 and captured a few photos.
1/10/2007  Comet McNaught  5 minutes after sunset

1/10/2007 Comet McNaught in bright twilight
     As winter weather goes in southeast Michigan the skies clouded over for the next week and I never saw the Great Comet of 2007 again.   Here is a link to Spaceweather.com archive from January 2007. http://spaceweather.com/archive.php?month=01&day=13&year=2007&view=view   In the middle of the page there is a photo gallery of the comet.  Perusing through the gallery you can see photos taken in midday (around page7) in which the comet is clearly visible while only 5 degrees from the Sun.    After its close approach to the Sun the comet went on to become a spectacular sight in the Southern Hemisphere as shown from page 11 onward in the above link.

   On January 10, 2007 Comet McNaught brightness was estimated at magnitude -3 or -4.  The brightness of celestial objects is measured on an inverted exponential scale., where  lower numbers indicate greater brightness.  For example an object of magnitude 3 is roughly 2.5 times brighter than a fourth magnitude object and every five steps lower on the scale equals one hundred times brighter.  For example the full moon shines at -11 magnitude, while the Sun shines -26. The 15 steps (or 3 steps of 5) of magnitude difference makes the Sun 100 x 100 x 100 or 1 million times brighter than the full moon.   The preliminary estimates of Comet ISON's predicted maximum brightness  next November range from -6 to -16.
    Comet experts say there really won't be any tweaking of predictions possible until August of next year. For now it's just a waiting game.   I tend to be a pessimist, that way I'm never disappointed.   I don't have time to be disappointed because I'm usually too busy disappointing others.  Therefore, I foresee the grand expectations fizzling out as the comet burns up before it's solar encounter.  I will however be ready to document the apparition if the Oort cloud visitor gets anywhere near it's predicted brightness. 
   To be continued......

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