Mercury orbits every 88 days, Venus in 225 days, and Mercury passes Venus every 145 days.
If Earth were motionless, Mercury and Venus would therefore meet up in our sky every 145 days. From the orbiting earth, the average time between Mercury-Venus conjunctions is longer than 145 days, and the interval varies considerably.
Mercury’s maximum angle from the sun is 28 degrees, while Venus ranges up to 47 degrees. So, whenever Venus is more than 28 degrees from the sun, a Mercury-Venus conjunction is impossible.
Venus spends around 234 days within 28 degrees of the sun when it’s on the far side of its orbit, but only 38 days when on the near side. Earth and Venus return to the same relative positions every 584 days, so Venus is in position to meet up with Mercury a little less than half the time.
The last Mercury-Venus conjunction, on April 25, was unobservable, because the planets were only 8 degrees from the sun and lost in its glare.
Mercury’s and Venus’s orbits are each tilted a bit relative to Earth’s, so some conjunctions bring them closer together than others. A little more than one degree separated the planets on April 25, but tomorrow they’ll be half as far apart.
To observe them, find a spot with an unobstructed view of the west-northwest horizon (where the sun is currently setting).
Start looking very low to the horizon around 9:30 p.m. Venus will come into view first in the twilight. As the sky darkens, much dimmer Mercury will appear about a pinkie’s width at arm’s length to the lower left of Venus. Binoculars will help.
If you miss this one, the next observable Mercury-Venus conjunction will be a wide one (4.5 degrees) on December 29.
Next column: How to find the faintest constellations.
Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or email@example.com.