Mercury reaches tomorrow its greatest eastward elongation –
– that is, angular distance from the Sun.
The elongation is 27 degrees, which is greater than for any others of Mercury’s seven excursions out into view this year. And yet, for us in northern latitudes, it is the worst of the year! That is, Mercury climbs least high above the sunset horizon – because it climbs out at a low angle.
This is a classic instance of the perversity of the little planet’s performances. They differ greatly because of its eccentric orbit; they cheat northern countries by always being least favorable when they should be most; and instead are generous to south-hemisphere countries by showing steeply above the horizon when they also venture far out from the Sun.
Because of another factor, Mercury’s curve from below to above the ecliptic, it will show slightly higher for us two or three weeks after the greatest-elongation moment.
Detail from the Mercury section of Astronomical Calendar 2015.
There is one positive aspect for us northern hemisphereans regarding Mercury’s perverse behavior. Mercury’s steep (yet close) elongations from the Sun are fairly easy for us to see, but the shallow, far elongations are difficult, but they can be seen more often than not. Even when I lived in Germany in the 1980’s, at a latitude of over 48N, I was able to almost always pick Mercury out on at least one day of each of its spring-morning or autumn-evening apparitions. Since 1984, I have been able to see Mercury at more than 90% of its elongations, living here in the United States around 38N most of the time. If we were in the southern hemisphere, I bet we’d miss most of the shallow-near elongations completely. Unfortunately, the most recent morning apparition, in June of this year, was one that I failed to see (totally due to our bad summer weather; the elongation and elevation above the horizon should have made it an easy one to see).