‘Tah-Tah, Mars!’ Venus Prepares To Depart The Dusk


Aldebaran occultation on March 4, 2017 from Doug Bock

Did you get a chance to see Aldebaran and the moon last night? I caught them early in twilight when thin clouds only bothered a little. Later, they thickened and “occulted” both star and moon for the remainder of the night. I hope your skies were better.

As Venus moves back toward the sun’s direction, it’s leaving Mars behind. Soon the bright planet will depart the evening sky. Created with Stellarium

Have you noticed that Venus is dropping lower in the sky? It’s headed for conjunction with the sun on March 25. You can tell it’s falling away west because the planet’s now 15° or a fist and a half below and right of Mars compared to 8° (less than one fist) just two weeks ago. Sure enough, the goddess planet is retreating from the evening sky. Its big hurry is a matter of perspective, the way we see Venus from observatory Earth.

As Venus orbits the sun, it catches up on the slower Earth and passes it. Right now, we see Venus just under three weeks from inferior conjunction when it passes between our planet and the sun and appears as a large, skinny crescent in the evening sky through binoculars and small telescopes. Credit: ESO with annotations by the author

Venus orbits the sun in just 225 days, considerably faster than Earth. As both planets circle the sun, Venus eventually catches up to Earth and passes it. The passing point is called inferior conjunction, when Venus lines up with the sun in the sky. It happens on March 25. Don’t bother looking for the planet then; it’s only out in daylight and sets and rises with the sun.

This sequence of Venus begins in October 2016 (left) and ends in Feb. 2017 and clearly shows its changing phase and size with time. Credit: Shahrin Ahmad @shahgazer

Several months back, Venus was at greatest eastern elongation (maximum distance east of the sun as seen from Earth) and stood far apart from our star high in the western sky at dusk. Since then, it’s been drawing both closer to the Earth and growing larger in apparent size, while at the same time — from our perspective — moving more in line with sun. That’s why it appears to sink rapidly westward this month.

And I do mean rapidly. From the diagram, you can also see that Venus is closest to Earth at inferior conjunction. The closer an object is to our planet the bigger it appears (compare the Venus crescent to the Venus “full moon”) but also the faster it appears to move. In just a few days, you should really start to see the planet zip downward night to night.

Venus reappears in the morning sky low in the east in early April. Created with Stellarium

While I hate to see this fine planet depart and leave Mars all by its lonesome, here’s a little secret. It won’t be gone for long. Hardly a week after conjunction, Venus reappears as the “morning star” low in the east at dawn. Again, if you look at the diagram, you can see that Venus swings to the right or west of the sun after March 25, so it rises before the sun does, the reason we see it at dawn.

The planet remains a “morning star” through the end of the year, so unless you’re a morning person, get ready to say farewell to Venus soon. Love ya’, baby. We’ll miss you. Before you kiss her goodbye, train a pair of 8-10x binoculars on the planet. If you focus carefully and sharply, you’ll see a tiny crescent. Such a neat sight!

Look a fist to the lower left of the moon tonight to spot Betelgeuse in Orion and from there to Sirius in Canis Major (The Big Dog) and Procyon in Canis Minor (The Little Dog). Together, the three stars form the asterism called the Winter Triangle. Created with Stellarium

One last note: You can use the moon tonight to point you to the Winter Triangle, a trio of stars that includes Sirius, the brightest star in the sky. Look for the triangle about halfway up in the southern sky during the early evening.


Aldebaran seen from the graze line

Let’s do one final video of the Aldebaran occultation. This one was made by Brad Timerson who observed it along the graze line. Watch how many time Aldebaran blinks on and off as it passes between the mountainous profile of the lunar limb.