The Pleiades embrace Venus in an octennial “cosmic hug” this week. We’ll explore the unique circumstances and offer both observing and camera tips.
While so many of us shelter in place because of the coronavirus we are grateful that the planets can still wander. Whether you've been watching the game of musical chairs played by Mars, Jupiter, and Saturn at dawn or the monthly conjunctions of Venus and the crescent Moon at dusk, the celestial cycles reassure us that despite the pandemic, some things don't change.
I was reminded of this several mornings ago when I awoke to see the planets in a tight clump over Lake Superior. Waves slapped against the pebbled beach as the coming Sun reddened the eastern horizon. All at once I felt this shiver of joy at the realization that the ancient forces that shaped the world are still with us today. Wind moving the clouds, waves gnawing at rocks, and planets endlessly circling. For the rest of the day I walked on air.
You can renew your own cosmic connection on Friday night, April 3rd. That's when the planet Venus will skirt the Pleiades star cluster in a spectacular dusk conjunction visible across much of the planet. Seen from North America the planet will pass only ¼° south of the cluster's brightest star Alcyone. Eastern Hemisphere skywatchers will see it creep within 11′ of 4th-magnitude Merope.
You've probably already noticed Venus nosing in the cluster's direction over the past couple weeks. It will remain within 5° of the sisters through April 9th and within 1.5° from April 1st to 5th. The planet's leisurely approach will offer nightly opportunities for observation and photography.
Connected through resonance
Conjunctions of Venus and the Pleiades happen yearly but the planet passes through the cluster only once every eight years. This happened last on April 3, 2012, and will happen again on April 3, 2028. In fact, wherever you happen to see Venus in the sky on a given night it will return to nearly the same spot eight years later.
Over time, repeated gravitational interactions between the Earth and Venus have brought the two bodies into a near resonance such that for every eight Earth years, Venus circles the Sun almost exactly 13 times. This 8:13 ratio means that the two planets return to nearly the same positions in their orbits at eight-year intervals, and Venus repeats it course across the sky. Happily, that includes a stroll across one of the most iconic open clusters!
Because both Venus and Earth orbit in ellipses rather than perfect circles, and Venus's orbit is inclined 3.4° to the plane of the ecliptic, the resonance isn't a perfect one. This is why each of Venus's passages through the Pleiades is similar yet unique. In 2028 Venus will cut a more central path across the cluster while on April 5, 2060, it will graze the western edge.
Finding Venus on the special night will take little effort. Face the sunset direction during late evening twilight and look up. The planet blazes at magnitude –4.6 and through a telescope presents a fat crescent that's 45% illuminated. As the sky darkens you'll see a few stars of the Pleiades without optical aid, glittering about the planet like moths around a streetlight.
But to really appreciate the sight use a pair of 3-mm or 50-mm binoculars, or even better, a small telescope with a wide-field eyepiece. The extra light-gathering power will reveal many more cluster stars and the large field of view in both instruments will frame the duo in a beautiful way. If bad weather intervenes you can still watch the event via livestream at Gianluca Masi's Virtual Telescope site starting at 1:30 p.m. Eastern Daylight Time (17:30 UT). Masi aptly describes the infrequent pairing as a "cosmic hug" — a wonderful description.
Take a picture
While you can carefully hold up a cellphone to photograph Venus and the brighter Pleiads at dusk, the image scale will be small. Unfortunately, zooming in rarely helps because the resulting images are often grainy and of poor quality. Instead, mount a pair of binoculars on a tripod and hold the cellphone camera against the rubber eyecup of one of the eyepieces. Keep it absolutely still and press away.
Exposure times will vary depending on when you take the picture — shorter exposures in early twilight and longer ones when the sky darkens. For starters, set the ISO to 800 and expose between 2 to 6 seconds. Check the screen and adjust the exposure as needed for the best image.
DLSR owners have more flexibility and offer much higher resolution than most cellphones. You can compose a scene with a medium-wide lens like a 35-mm or go in tight with a telephoto. To avoid star trails use a high ISO and short exposure. I abide by the 400 Rule, a more stringent interpretation of the 500 Rule. Divide 400 by the focal length of the lens to determine the longest exposure in seconds you can make before stars start to trail. For example: 35 mm = 11 seconds; 200 mm = 2 seconds. Wide-angle lenses allow for lower ISOs (less grain and noise); long lenses will require high ISOs to capture an image in a brief amount of time.
Attach the camera to a tripod. Set the shutter speed to "M" (manual), click the switch on the side of the lens barrel to "M" and open the lens to its widest setting — usually f/2.8 or f/4. Next, press the live view button (found on the back of the camera) and carefully focus on Venus. You will only need to focus once per lens for the entire session. I recommend a 35-mm lens to include a scene, an ISO of 800 or 1600, and an exposure between 5 and 10 seconds in mid- to late twilight.
Whether you watch alone or with family I hope this cosmic hug will buoy your spirits during a time of crisis.