From the September 2013 issue

Is Venus always either a “morning star” or an “evening star” during certain times of the year?

Ray Montello, League City, Texas
By | Published: September 23, 2013 | Last updated on May 18, 2023
Venus and the Moon 2010
Venus was an “evening star” when this photograph was taken, May 15, 2010.
Unfortunately, Venus’ path through the sky is a bit more complicated than that. This autumn, Earth’s neighbor shines brilliantly in the west after sunset. It reaches its greatest elongation from the Sun — its greatest angular distance from the Sun on the sky — on November 1, when it lies 47° east of our star. The planet grows brighter as it falls back toward the Sun in our sky, reaching its peak in early December. This “evening star” then shines at magnitude –4.9 — some 25 times brighter than the night sky’s brightest star, Sirius.

Venus will pass between the Sun and Earth in January and then quickly rise into view before dawn. It will remain the “morning star” until next September. After passing on the far side of the Sun, it will return to view after sunset as an “evening star” in December 2014 and stay there until the following summer. The pattern repeats itself every 583.92 days (approximately 19 months). This is Venus’ “synodic period,” or the time it takes for the planet to return to the same alignment relative to Earth. Compare this with its 224.7-day shorter “sidereal period,” the time it takes to return to the same position relative to the background stars.

Although Venus doesn’t appear in the same spot in the sky year after year, it does show a more subtle pattern. If you multiply its synodic period by 5, you come up with 2,919.6 days. What does this ungainly number have to do with anything? If you divide it by 365.26 days (the time it takes Earth to revolve around the Sun), you get 7.993 — just about 8. So nearly every eight years, Venus returns to the same area of sky at the same time of year. Its fifth greatest eastern elongation after the one November 1 will occur October 29, 2021.

Richard Talcott
Senior Editor