About Sky Grade

Sky grade takes the current time and moon phase and assigns a letter grade to the sky.  No special software is required to get a sky grade.  Internet Explorer 5 or later is recommended and Java Script must be activated.  An A+ is assigned to a totally dark sky with a new moon.  When it is daylight outside, sky grade is automatically an F.  Of course if you want to observe the Sun or Moon, sky grade does not apply.  The time portion of the sky grade calculation is based on times of civil twilight and astronomical twilight for the 15th of each month.  This means that the rating for daylight is not a continuous function, it is a step function.  So the sky grade for daylight is accurate enough to make a good estimate of the darkness of the sky.  The moon portion of the sky grade calculation comes from a bunch of equations that work with your computer's clock.  These calculations should only vary by a fraction of a second per century so the moon phase calculated is highly accurate. 

There are a few parameters that I have yet to work into sky grade that may be integrated into future versions.  The first is weather.  Numerous weather conditions affect sky conditions, but the big two are clouds and humidity.  If it is mostly cloudy, sky grade is automatically an F.  If if is partly cloudy, I would subtract one letter grade from the score.  And of course if it is clear, no adjustments need be made to sky grade.  At night, especially in the summer time, the air may become saturated meaning humidity is 100%.  Water droplets in the air can sometimes blot out fainter stars.  This is especially true in the urban areas where light from streetlights and signs is reflected off the droplets.  For humidity < 30%, I would not adjust sky grade.  For humidity 30-50%, I would take sky grade down one step (i.e. a B+ becomes a B).  If the humidity is 50-75%, I would take sky grade down two steps (i.e. a B + becomes a B-).  For humidity above 75%, I would drop sky grade one letter grade.  Another factor that comes into play is light pollution.  In urban areas, it is harder to see stars because of all the city lights.  Light pollution's affect on sky grade is hard to judge.  It could be based on population, but two cities of the same size might not have the same number of lights. 

Sky grade is useful for determining when it is best to view meteors, distant planets, and deep sky objects.  I hope to somehow incorporate weather and light pollution parameters into this model so that it will be more accurate.

The chances of seeing a man-made satellite function is based on a least-squares regression line.  This line is based on the number of satellites per hour that are brighter than magnitude 4.5 and that can be seen from the middle latitudes.  The minimum elevation above the horizon is 10 degrees.  These numbers change throughout the year.  This is only an estimation, it is not meant to be an exact probability.  When the probability is between 1% and 9% chances are rated "fair".  Chances between 10% and 19% are rated "good".  Chances 20% and above are rated "excellent".  So when the chances get above 10%, go outside and look up for awhile.  You might catch a satellite within that hour.  Enjoy!

KC9S

Updated May 9, 2005