Venus Sprints from “Evening Star” to “Morning Star”

Venus is about to go bye-bye. If you haven't looked at the dazzling evening planet recently, time's running out. On March 12th its solar elongation will be just under 18° and decreasing daily by more than 1°. As it departs, Venus pals up with Mercury. The two inner planets will be closest at 5.5° apart on March 12th. Binoculars will help in spotting fainter Mercury to the left of its scintillating sibling.

On March 23rd, Venus will be in conjunction with the Sun and transition into the dawn sky, reprising its role as Morning Star. In classical times, it went by the Latin Lucifer, when observed at dawn, and Vesper, at dusk. While these names are rarely used to describe the planet in our time, the ancient duality persists in the traditional appellations of Morning Star and Evening Star.

Normally, Venus is hidden in the solar glare for a week or so around the time of conjunction and invisible with the unaided eye. During this time, determined observers are keen to track it telescopically in the daytime sky while taking care to steer clear of the Sun. There's great motivation to do so. Backlit by the Sun and closest to Earth, the planet reaches its greatest apparent size and displays a jaw-droppingly thin crescent.

Around the time of conjunction, when we view Venus in nearly the same line of sight as the Sun, the cusps of the crescent extend beyond 180° due to forward-scattering of sunlight by the planet's atmosphere. I've often seen the fragile horns continue as tentative threads of light for about 270°. In fleeting moments, they reach even farther and touch horns to form a complete circle. Watch for the extensions to appear when the crescent dips below about 2.5%-illuminated, from March 16-29. Minimum illumination of 1.0% illumination occurs on March 22–23.

Venus reaches its great latitude north of the ecliptic on March 18th at 8.5°. Since this occurs just a few days before inferior conjunction, the planet will pass well north of the Sun, making it relatively easy (and safer!) to hunt in daylight. I use a small refractor equipped with a safe solar filter on an equatorial mount. After looking up the right ascension (RA) and declination (Dec) for both the Sun and Venus, I center the Sun in the field of view of a low magnification eyepiece and bring it to focus. Then I offset the scope in RA and Dec toward Venus, remove the filter and look through the eyepiece.

Naked-eye crescent quest

If you have exceptional vision you may even be able to discern the Venusian crescent without optical aid. The human eye has a visual acuity of about 60" or 1 arc-minute. Venus comes tantalizingly close to that size during the current apparition with a maximum diameter of 59.5"on March 22-23. Unfortunately, aberrations of the eye like astigmatism can warp the true shapes of objects of such small angular size.

To reduce ocular distortions, Sky & Telescope senior editor Alan MacRobert suggests "sighting through a clean, round hole in a thick piece of paper about 1 mm, 2 mm, or 3 mm in diameter held right next to your eye (try them all). It will mask out optical aberrations that are common away from the center of your eye's cornea and lens. Try each eye." The best time to attempt this is when Venus is in a bright sky (preferably before sundown) to minimize the planet's glare.

Amateur astronomer Jeff Stevens of England has been following Venus with the naked eye and binoculars for many weeks. Recently, he took MacRobert up on the suggestion. Stevens used a multi-diameter hole punch to make 2 mm, 3mm, and 4mm diameter holes in a card, and a separate pointed tool to create the 1 mm hole. On March 1st, despite his best attempts, the result was inconclusive. On March 4th he detected an elongation in the planet using the 2 mm and 3 mm holes. Five evenings later, on March 9th, he met with success:

"The 2mm, 3mm, and this time the 4mm, all allowed me to see a very subtle but clear tiny curve of light representing the crescent Venus," writes Stevens. "The phase was 7% illuminated and 55″ in diameter."

"Weather permitting I will continue to view Venus, until it is lost to the Sun," said Stevens. I encourage you to do the same. If possible, observe the planet before sunset when its higher altitude makes for a sharper image. For naked-eye Venus-spotting in daylight, I use an app to pinpoint its location, then (carefully) sweep it up in binoculars. I'd love to hear how your crescent quest turns out — good luck!

See the evening star before sunrise?

Because Venus lies so far north of the ecliptic, mid-northern-latitude observers have a unique, if challenging, opportunity to observe the planet in the dawn sky before conjunction when it's still officially the Evening Star. Expect Venus's last gasp at dusk around March 18th. At the end of civil twilight, when the center of the Sun is 6° below the horizon, the planet will stand about 2.5° high.

Then on March 21st, we can start watching for Venus at dawn, but beware — it'll be in the mud! I'm hopeful a few of us will still succeed in pulling it out with binoculars. I've set the map time to the start of civil twilight, but it may be possible to spot Venus a bit later when it's higher above the eastern horizon. I did say challenging, right?