Mercury east of Sun and eventually, north of

Event Date: January 19th

Time: 5:18 PM

Brief

Mercury reached superior conjunction yesterday, and has started what will be eventually be an excellent apparition east of the Sun.  Why?  Each day that Mercury emerges from the Sun, moving faster in prograde motion for the next few weeks, it will be further north of the Sun than the day before.  This gap will increase until about the time of greatest elongation.  We will get more into those measurements as we get closer to that mid-February week.  For now, it is a slow "climb" north of the Sun for the planet, over the next four weeks.  Here we see it with its orbit, setting just before the Sun, being slightly south of it.

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

Detailed

   As Mercury becomes a little more visible each evening, it also moves one date closer to its perihelion, which happens at the time of its greatest elongation (g.e.).  As a result, being one of Mercury's smaller g.e.'s as a result of perihelion, it will be one of its brightest.  Looking at the orbit now, we can see the little line between it and the ecliptic, which marks perihelion.  Mercury will swing around--as we see it--from far to near, and reach the that marker in the orbit, which it does about every three months.  There are times however, when it reaches this point closer to conjunction than g.e., and other times when we see it reach it at g.e. when seen in the morning.  While we are fortunate to have perihelion near g.e this time when north of the Sun, the other extreme is to have Mercury at aphelion when far south of the Sun during g.e.; this often happens in the evening during mid-summer and mid-fall, and in the morning during the late winter and late spring.  Whenever either happens, we can use Mercury's brightness at g.e. to have a pretty good idea of (a) how far north it is of the Sun and--not as noticeably-- when a slightly brighter Mercury during bright twilight is actually at perihelion.  If you don't know for sure, look up its elongation value, or the dates of perihelion itself.  Once again, a smaller separation from the Sun as seen from here, means it is closer to perihelion (17.5-18.5º); aphelion has it between 27.5º and 28.5º.