Go crater crazy this week; Venus and Saturn prep for close conjunction

Tonight the 6-day moon displays a concave (inward curving) terminator. Tomorrow night the terminator will swell further to the left and appear convex (curving outward). The southern or bottom half of the moon displays hundreds of craters visible in small instruments. Click photo for a map to identify lunar features by name with a hover of your mouse.  Photo: Bob King

I tried to observe my usual faint comets and galaxies last night but the bright moon reeled in my eyes like bait dangled before a fish. And why not? Now through about Saturday is the best time to enjoy views of the rugged lunar landscape in the smallest of instruments.

As the moon waxes from nearly half tonight toward full, fresh new territory is exposed to sunlight with each passing night. This line of advancing sunlight is called the lunar terminator and marks the boundary between the bright part of the moon we see and the part that will soon be revealed. What makes the terminator advance? Geometry.

This diagram shows the moon orbiting Earth as seen from far above Earth’s north pole. We see different amounts of the lit side depending on where the moon is in its orbit. Credit: Michele Stark

Because the moon revolves around the Earth, its position with respect to the Earth and sun is constantly changing night to night. When it lines up between the two bodies, we see 0% of the moon illuminated by the sun. When it’s a quarter of the way around its orbit and makes a 90-degree angle with the Earth and sun, we see half a moon or 50%. And when it’s directly opposite or 180 degrees from the sun, we see a full 100% illuminated moon. The phases work in reverse after full moon with a new cycle beginning about a month later at the next new moon.

We see the terminator sweeping from one side of the moon to the other during a full lunar cycle of about 30 days. The wobble, called libration, is due to the moon’s changing orbital speed and the tilt of its orbit. Credit: Tom Ruen

Around the time of half moon or first quarter phase, the terminator “pulls the shade back” on a multitude of craters that face us directly. That makes it easy to look straight down inside them. During other phases, we the lunar landscape sidelong and craters appear stretched out and too crowded together to fully appreciate their individuality.

Around first quarter we can take in fascinating details like cracks in their floors caused by the weight of ancient lava floods, concentric ridges in their walls from material that’s slumped downward over time, overlapping craters, craters within craters and majestic mountain pinnacles found in the centers of many craters from crustal rocks that rebounded after being compressed by the power of the impact that formed the crater in the first place.

The craters Aristillus and Alphonsus show features typical of many lunar craters including concentric inner walls (from slumping due to settling and gravity), central peaks and cracked floors. Click photo to see more examples. Credit: Damian Peach

Low-angled sunlight in the vicinity of the terminator creates long shadows that throws these and other features of the landscape into breathtaking 3-D relief. Time to take the bait.

Venus, Saturn and Virgo’s brightest star Spica tomorrow morning Nov. 20 about an hour before sunrise as you face east. Created with Stellarium

Morning skywatchers can now be on the lookout for the planet Saturn below Venus low in the southeastern sky about an hour before sunrise. Saturn’s rings are really opening up this season with a current tip of about 17 degrees. As with the moon, even a small telescope magnifying around 40x or higher will show the planet as a ball surrounded by at least one bright ring.

The two planets will gradually draw together this week as they head toward a close conjunction on Nov. 26 when they’ll be just half a degree apart. More on that later this week.

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