The moon will have completed nearly a quarter of its orbit since new moon tonight, March 1. That’s why it’s called the first quarter moon even when it looks like half. A closer look will show it just shy of first quarter this evening. A boundary called the terminator divides the sunlit part of the moon from the part that’s still in darkness. It will appear concave tonight — curving into the bright part of the moon — and convex (bulging outward) on Monday night.
The next several evenings are excellent for crater observation through binoculars or a small telescope because the terminator cuts through a crater-rich area in the moon’s southern hemisphere. Along that day-night boundary, the sun appears low (if you’re standing on the moon), and makes every crater wall and hill cast long shadows that highlight even the smallest details.
First quarter phase is traditionally the time most of us lose track of the the “earth-lit” part of the moon. That’s the faint illumination on the dark portion of the moon from sunlight reflected off the Earth. Our planet is like a giant night-light in the lunar sky, softly lighting the rocky landscape where the sun has yet to rise. You may not be able to detect earthlight with the naked eye tonight, but I’m almost certain you’ll see it faintly in binoculars.
During crescent phase, earth-lighting is brighter and more obvious for two reasons: the crescent is relatively faint, and the Earth is a brilliant gibbous globe as seen from the moon. By the time the moon reaches first quarter, the Earth is also half-lit and reflects less light, reducing its contribution. The half-moon also produces more glare and makes what little earthlight there is more difficult to see.
Tonight’s moon will point us to three naked-eye and binocular star clusters. The beloved Pleiades or Seven Sisters Cluster glimmers about 7° to the upper right of the moon and the Hyades (HI-uh-deez) 4° to its left. The bright first magnitude star Aldebaran looks like it belongs in the cluster because it neatly completes the letter “V”, but it’s really just a foreground star in the same line of sight. Aldebaran shines from 65 light years away while the Hyades check in at 151 light years. The Pleiades are much further (444 light years), one of the reasons its stars more scrunched together.
On the other hand, the Seven Sisters is much younger than the Hyades — its stars were born in a gravitationally collapsing cloud of dust and gas 100 million years ago. The Hyades are more than six times older, but in comparison to the sun’s age of 4.6 billion years both clusters are infant day care centers.
If you shoot an arrow through the moon past the Hyades you’ll land on a lesser known star cluster, Collinder 69. It one of 471 star clusters in a catalog compiled by Swedish astronomer Per Collinder early last century. Centered on 3rd magnitude Lambda (λ) Orionis, this pretty bunch is suitable for both binoculars and small telescopes.
Its most striking feature is a straight up-and-down row of three 7th magnitude stars directly below Lambda that mimics Orion’s Belt but tipped on end. Collinder 69 is four times as far as the Pleiades, but it’s the youngest of the three at just 5 million years old, roughly about the time the first humans evolved. In a sense we’ve grown up with Collinder 69. To hold it in our gaze reminds us of our own origin.