Mercury separating further, yet low

Event Date: October 20th

Time: 6:45 PM

Brief

   Mercury is getting close to greatest elongation.  As a result of poor geometry during the first month of fall for the western evening sky, coupled with our perspective of Mercury's plane from our point of view, it has maintained a declination that has been further south of the Sun.  That, since we first started seeing it finally emerge far enough from the Sun.  Looking west-southwest, we see it separated from the Sun by about 23º, which is less than a degree from greatest elongation.  At magnitude -0.1, it is just slightly brighter than Arcturus to its upper right.

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

Detailed

   Although just slightly brighter than Arcturus now, that will change by month's end, when Mercury starts to dim faster, reaching inferior conjunction not long after.  On average, the change in magnitude of Mercury can decrease quickly between emerging from the Sun after superior conjunction and greatest elongation, and then speeds up even more to the days just before inferior conjunction.  This time however, Mercury started approaching the Sun a little over a week ago.  As it came closer the Sun and therefore us, it means that we have seen the planet increasing in angular size at a rate fast enough to almost override its rate of waning.  Simply put, there is enough light still reflecting back to our eyes.  As Mercury approaches inferior conjunction, it will further approach perihelion with the Sun and therefore, will not be quite as close to us at that point.  That is why we will start to see a big drop in magnitude.  Overall for the best Mercury apparent magnitudes, it is best to see it just after superior conjunction just short of perihelion, while Earth is also near perihelion with the Sun; the latter happening in early January, when that is also a good time to see Mercury in the evening with a good apparition.  Look forward to that, then!