Mercury and Gemini stars setting

Event Date: June 30th
Time: 9:00 PM


Brief

   Mercury at greatest elongation this evening, has had its best days to be viewed with the eye alone now behind us-- earlier this month that is.  We will end the month with it difficult to see with the eye alone, at magnitude +0.5.  By the time that the Sun is low enough that its glare is not so intense, Mercury is already starting to be deep in atmospheric pollution.  Try binoculars on it, which definitely helps.  If using a telescope, it is already past dichotomy at 40% illuminated.  Aphelion is about two weeks ahead of us, leading to the lower magnitude for greatest elongation.  That is, if compared to the -0.3 that we saw it for such in March; between  +0.1 and -0.1 being an average range.  In March, it was closer to perihelion and reflecting a little more light to our eyes.
Here is the planet seen in orbit, with the celestial guidelines as well.

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

Detailed

   Despite the only-fair showing of the planet for this elongation, the approaching of aphelion means that it is 7º further separated from the Sun than it was in March.  One other small advantage of it moving away from the Sun as it approaches inferior conjunction, is that it is slightly closer to us, and appears slightly larger in a telescope with high magnification.  That angular diameter change is hardly noticeable, and especially when not high in altitude when finally visible.  A powerful telescope using high magnification can distinguish the difference in diameter.
   Try finding Mercury as soon as the Sun's glare allows over the next few days when using a telescope, during evenings when the further failing of the western sky's geometry will not be affecting the finding of it.  If waiting until a week from now and days to follow, not only does Mercury continue to dramatically dim; retrograde motion begins and it starts to fall faster towards the Sun's glare.  This evening, it sets 92 minutes after the Sun, and easily visible to the eye for only about 20-30 of those minutes (skies permitting) before sinking too low.
   Finally, as we watch Mercury sink, so do the same for dimmer stars Castor and Pollux to its right (north, labeled above).  These two require optical aid to see well after the Sun goes down, and will not be easily visible again until late July/early August in the morning sky.