Mercury emerging east of the Sun--low

Event Date: September 16th
Time: 7:20 PM


Brief

    The upcoming apparition for Mercury will be interesting, as there are a few pro's as well as a couple of cons.  Normally, as the ecliptic's angle with the horizon shows in the image below, this is one of the worst times to view Mercury in the western evening skies during this time of year.  Of course, as Mercury swings in orbit between superior and inferior conjunctions, it also dims a little each evening. further hindering viewing.
   Before I get to the pro's in the detailed section, let us check out the planet slowly emerging from the Sun, its orbit, celestial equator and the ecliptic.

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

Detailed

Mercury is approaching aphelion over the next three weeks, meaning that we will see a slightly larger separation than we sometimes do, near the time of greatest elongation.  At over 23º, this is short of the 28º separation that we sometimes see when greatest elongation happens during an August evening or April morning.  However, it is still enough to make Mercury a little more visible as the Sun continues to set over a minute earlier each evening for the next few weeks.  Greatest elongation will not happen for over a month, as seen from our perspective and Mercury being further from the Sun during aphelion.  Around the time of greatest elongation, Mercury will start to get closer to the Sun again.  As a result of this change, coupled with Mercury coming closer to us a little each week in its orbit, its rate of dimming becomes very slow in late October.  When it finally reaches greatest elongation, it will still be a good magnitude of -0.1.  By then, the geometry of the western evening sky also starts to very slowly improve, as the declination of Mercury becomes less-south of the Sun.  Unfortunately for the elusive planet, it shortly gets back into the glare of the Sun shortly after that, dimming quickly as we see it wane. 
Long story short, to see Mercury well again, look forward to its next morning apparition later in the season.  In a few weeks, I will revisit Mercury as the geometry gets better and it separates itself more from the Sun.