Mercury setting late, gradually

Event Date: June 18th

Time: 8:33(.35) PM

Brief

   It has been a few days since our previous look at Mercury.  It is getting close to greatest elongation, and also close to the end of its best evenings of apparition for this cycle.  With the geometry of the western sky not as good as that during February and March, it also means that Mercury is about the same declination as the Sun; it was about 8-9º further north during the early March greatest elongation week.  Here is Mercury in orbit relative to the ecliptic, looking west-northwest at Sunset (time above). 

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

Detailed

   Mercury sets 97 minutes after the Sun, meaning that if you want to wait for the Sun to get a little lower and less glare in the sky, there is some time for that before Mercury gets too low into atmospheric pollution.  It is 16º in altitude at the time of Sunset, meaning that even in a location with a little obstruction, it is still possible to pick it up with optical aid, being magnitude (-0.1).  Since it is further from perihelion now, that means that it is seen further separated from the Sun and therefore, a greatest elongation at month's end which will be more than the March one (about 26º vs 18º respectively). As it moves closer to aphelion, along with the almost solstice-Sun setting as late as it does, it means that Mercury will not reflect quite as much light to our eyes.  As it continues to wane in phase from its 62% illumination this evening, the dimming rate will increase quickly.  It may still be an object to pick up a week from now in optical aid, although viewing with the eye will be of a small window of time.  Catch it now, while still brighter than magnitude 0, and a decent enough apparition.