Moon Musings

Lunar Terminator

As the Moon reaches first quarter phase, I find myself drawn to the Moon's 'terminator', the line that separates dark from light. At first quarter phase (happening this week), the Moon presents Earth a very fine view of the terminator, exposing the highs and lows of the surface of the Moon. As I regularly point out to people during star parties, the region of the terminator during the waxing phase of the Moon is the region of sunrise, the place on the Moon where the Sun is just emerging above the horizon for what will eventually be a long lunar day (this takes 29 'Earth-days' to complete). As such, the illumination on the Moon's surface is much like you would expect on Earth at sunrise: long shadows across the land, with unusual features such as valleys and mountains being partially illuminated. The image attached (courtesy of APOD) shows the view through a telescope of the terminator, with shadows clearly visible in the craters and from the peaks of the mountains.

Tonight will be a fine time to peer at the Moon with anything you have at your disposal. It's the easiest target to find with a telescope or binoculars, and it's always visible even in the worst city lights. And it's a treat to see something unusual yet familiar, the sight of sunrise on another world.