setting Arcturus: "the ghost of the summer Sun"

Event Date: September 7th
Time: 10:00 PM


Brief

   Arcturus is a little over 19º in northern declination.  Although it sets almost four hours after our Sun, that changes by about 2 1/2 minutes a day at this time of year: the Sun sets about 1 1/2 minute(s) earlier each evening as we approach the September equinox, and Arcturus sets slightly less than 4 minutes earlier.  As shown below with a dark sky, we show Arcturus about 2 1/2 hours before it sets.  The celestial equator and grid are present to clarify its declination.  The set Sun is losing declination quickly, now much closer to the celestial equator than Arcturus.

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

As a result of this gap between sets decreasing quickly between mid August and early October, it seems as if we see Arcturus at about the place in the sky for a few weeks, and then a little lower a few weeks after that.  It isn't until about mid-October that the gap starts to increase again slowly, that we start to have less time to view Arcturus.  With this in mind, along with the Sun being the same declination as Arcturus at Summer's midpoint in late July-early August, there is a coined description for Arcturus: at this time of year, it is called the "ghost of the setting Sun".  After all, Arcturus is orange, which is a little cooler on the temperature scale than our yellow-white Sun.  Also, as a point in the sky instead of a 1/2º disc, it makes us think that the Sun has gone away, and this is what is left as its ghost.