Mercury, Moon and "twin" stars

Event Date: June 22nd
Time: 9:00 PM


Brief

   Mercury is close to greatest elongation (g.e.), setting 69 minutes after the Sun.  This means that is does not set below the horizon until 10:13, which is about 15 minutes short of of the end of astronomical twilight.  As we look towards the west-northwest during the first 30+ minutes after Sunset, there will be too much glare to see +0.1 magnitude-Mercury.  By the middle of nautical twilight, Mercury is finally easily visible.  At dimmer magnitudes to its right and a little further north, is the star Pollux, and its twin Castor, marking the two Gemini heads.  The namesake constellation is hard to see during this second half of June, with all other stars Sun-washed out or not bright enough to see with the eye.  Below, are the twin stars with Mercury and the waxing crescent Moon to Mercury's left and up a bit.

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

Detailed

   Notice that at its current phase, that the Moon is low to the horizon.  With the Sun just slightly less than its best declination of the year, being two evenings past the June solstice, the Moon is not as far north.  It's orbit (not included this time to avoid interference with Mercury's) puts it at its most southern ecliptic latitude at this position.  Although it will gradually gain ecliptic latitude in a few days, it will move through the part of the sky that the Sun does at the time of the September equinox.  Since at that time, it moves south quickly, as will the Moon.  By the time the Moon reaches gibbous phase and then almost full, it will be seen in the same part of the sky as the last couple of full Moons.
   For one other note on Mercury, look for it forming a slight arc with Castor and Pollux.  If you read this entry early and look outside at the trio 2evenings earlier, the arc seems less obtuse.