The bright planets Mercury and Saturn will meet for a quasi-conjunction on February 23, 2021. Although both of these worlds shine as brilliantly as the sky’s brightest stars, Mercury and Saturn will be hard to see with the eye alone in the glow of morning dawn. Look eastward as the predawn darkness is giving way to morning twilight. Binoculars will be helpful.
What’s a quasi-conjunction? Two planets are said to be in conjunction if they’re north and south of one another (in right ascension) on the sky’s dome. If two planets come to within 5 degrees or less of one another without being in conjunction, then it’s called a quasi-conjunction. At their closest, these two worlds will be 4 degrees apart on February 23, at about 08:00UTC. For reference, the width of two fingers at arm’s length approximates 4 degrees of sky.
The Southern Hemisphere has the advantage for spotting Mercury and Saturn before sunrise. We give the approximate rising times for Mercury and Saturn at various latitudes (given an absolutely level horizon):
40 degrees north latitude: Mercury/Saturn rise about 1 1/6 hour (70 minutes) before the sun
Equator (0 degrees latitude): Mercury/Saturn rise about 1 1/2 hours (90 minutes) before the sun
35 degrees south latitude: Mercury/Saturn rise about 1 5/6 hours (100 minutes) before the sun
From mid-northern latitudes, Mercury and Saturn shine pretty much side by side over the eastern horizon, and rise at roughly the same time. Farther south, Saturn rises before Mercury does, so Saturn is the higher up of these two planets at southerly latitudes. (See sky chart below.) The rising times listed above are for the planetary pair, or when both planets finally climb above the horizon.
If you miss Mercury and Saturn on February 23, don’t give up. Mercury and Saturn will remain fairly close together for the rest of February, and Mercury will meet up with the king planet Jupiter for a conjunction on March 5, 2021.
Bottom line: Will you catch Mercury pairing up with Saturn in the eastern sky before sunrise on February 23, 2021? Southern Hemisphere viewers have the best shot at seeing these planets.