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.