Mercury is the most elusive of the bright, naked-eye planets. This week is a good opportunity to see it.
Mercury spends most of its time in the sun’s glare, emerging for a few days a little more than six times per year in alternating evening and morning apparitions.
Because Mercury’s orbit is the least circular of the planets, the extremes of these swings (called “greatest elongations”) make the maximum sun-Mercury distance vary from 18 to 28 degrees, depending on where Earth is in its orbit (i.e. whether we’re viewing Mercury’s elongated orbit end on or from the side, respectively). Morning elongations are greatest in late March; evening elongations are best in mid-August.
To see Mercury in a sufficiently dark sky, its solar elongation is important, but so is separation from the horizon. Mercury is hard to see in the morning near the Vernal Equinox, when its orbital plane lies close to the horizon, so it rises in bright twilight shortly before the sun. Similarly, Mercury is difficult to see in the evening near the Autumnal Equinox because it sets in bright twilight shortly after sunset.
Between its varying maximum elongations and the tilt of its orbit, Mercury’s best evening apparitions are when it reaches greatest elongation between mid-April and late-May, and its best morning apparitions occur when it reaches greatest elongation between mid-October and late-November.
Mercury’s best morning apparition of 2019 will arrive on Nov. 28, standing seven degrees above the southeast horizon one hour before sunrise. Second best is Friday morning, when Mercury will be five degrees (the width of three fingers at arm’s length) above the east-northeast horizon one hour before sunrise (5:30 a.m.).
Telescope owners: Mercury, now at greatest elongation, appears a tiny “half-moon” shape in the eyepiece.
Next column: Stellar pairs in close orbits.