In late April 2018, the sun’s innermost planet Mercury swings to its greatest western elongation from the sun, that is, its farthest point west of the sun in our sky for this morning apparition. Mercury is 27 degrees west of the sun at its farthest, this time around. It’s low in the eastern sky before sunrise. Farthest from the sunrise … sounds easiest to view, right? That’s true for the Southern Hemisphere, and the coming few weeks offer Mercury’s best apparition in the morning sky for all of 2018 for southerly latitudes. Yet at northerly latitudes, this is Mercury’s poorest showing in the morning sky for the year.
The featured sky chart above is especially for the Southern Hemisphere. It’s specifically for Cape Town, South Africa, which is roughly the same latitude as Sydney, Australia; Auckland, New Zealand; and Santiago, Chile. Given an unobstructed eastern horizon, all these places should be able to view Mercury with the eye alone in a predawn sky. At temperate latitudes in the Southern Hemisphere (South Africa, southern Australia, New Zealand), Mercury rises better than two hours before the sun.
Meanwhile, at mid-northern latitudes (United States and Europe), Mercury comes up about an hour before sunrise, and mid-northern latitudes will find Mercury deeply buried in the glow of twilight.
That’s in spite of the fact that this present morning apparition will last for yet another month.
Click here for recommended almanacs; an almanac can give you Mercury’s rising time in your sky.
Why is this morning showing of Mercury so outstanding from the Southern Hemisphere, but so poor in the Northern Hemisphere? Mercury’s distance west of the sun (27 degrees) is the same for the whole Earth. Here’s what’s different. The ecliptic – the roadway of the sun, moon and planets across the sky – makes a narrow angle with the predawn horizon in spring. Meanwhile, in autumn, it makes an exceptionally steep angle (nearly perpendicular) with the predawn horizon … and it’s autumn now south of the equator.
Bottom line: If you live in the Southern Hemisphere, late April and early May 2018 present your best opportunity of 2018 to spot Mercury, the innermost planet, in the east before sunrise.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.