Clear skies this evening? Face west about 45 minutes to about an hour after sunset and you’ll see — from top to bottom — the 3½ day old crescent moon, Saturn, Venus and Jupiter. Jupiter will be leaving the scene soon. Venus sticks around and gets higher and brighter. Saturn follows Jupiter’s lead and slowly moves westward in the coming nights, dropping lower and lower in the direction of sunset. And of course the moon will wax and move upward and to the left as it orbits the Earth.
All these cycles interweave effortlessly to create tonight’s scene. If you have a pair of 7x to 10x binoculars be sure to look at the mysterious, semi-dark two-thirds of the moon tonight lit by earthlight. Earthlight is sunlight light reflected from the Earth to the moon and back. Since Earth only reflects the sun’s light, the earthlit moon appears a dim, almost smoky blue-gray to the eye. But in binoculars there’s enough light to pick out several of our satellite’s most prominent craters. It’s fun to explore the moon this way and appreciate how our planet is the moon’s night light.
Through a small telescope, one of my very favorite “crater arcs” makes its appearance tonight: Langrenus, Vendelinus, Petavius and Furnerius. Just rolls off the tongue, doesn’t it? Because the moon is a sphere and these four splendid holes are located relatively near the edge of the moon they appear foreshortened. Face-on we’d seen them as circular bowls but from the side they’re almond-shaped.
The wonderful, symmetrical arc begins with Langrenus at the north end and terminates at Furnerius. Furnerius is the smallest in the group at 78 miles (125 km) across and Petavius the biggest with a diameter of 110 miles (177 km). If you could build a highway across the latter it would take about 2 hours to travel from one end to the other.
Look at how crisp and distinct they are. That’s because they all lie near the moon’s terminator, the borderline between day and night where the sun is just starting to rise. Low sunlight brings their many details in beautiful relief. Enjoy!