Exploring the Full Moon

There’s plenty to see even when the lunar disc is fully lit.

Seronik-Moon and clouds

The full Moon rises over a bank of clouds. Courtesy Gary Seronik

It’s often been said that full Moon is the least interesting time to use a telescope to view the lunar surface. That may be true, but even at its worst, there’s still lots to see — including a host of features that are at their very best.

The reason lunar observers give the full Moon a pass is because there’s no shadow contrast and as a result, most lunar features appear flat and lifeless. Even big, prominent craters like Alphonsus and Morteus can be a tremendous challenge to find under these conditions.

The real action during full Moon is the complex shadings of light and dark ranging from dark grey to brilliant white. Several of the full Moon’s most noteworthy features are highlighted here, but if you take the time to inspect the lunar disk, you’ll find many other intriguing little splotches and luminous rings. The real trick is identifying them.

Seronik-Full Moon map

Click on the image for a full-size version. Moon image courtesy Gary Seronik

Tycho: The most eye-catching crater during full Moon is 85-kilometre-wide Tycho. It features impressively long impact rays that span fully two-thirds of the lunar disc. Tycho’s rays are a superb target for steadily held or mounted binoculars. Inspect Tycho itself with a telescope and you’ll be able to make out a dark halo that encircles the crater. This dark stuff is glassy impact melt — testament to the incredible energies released when the crater formed 109 million years ago.

Maria Serenitatis and Tranquillitatis: Think the lunar seas are a uniform shade of grey? Think again. Compare Mare Serenitatis with its neighbour, Mare Tranquillitatis, especially where the two meet. If you look closely you’ll see that Serenitatis is a distinctly warmer hue than Tranquillitatis. These differences indicate slightly different compositions and ages for the two maria.

Mare Imbrium: Like Serenitatis and Tranquillitatis, Imbrium isn’t a uniform grey. Indeed, the Moon’s biggest sea looks like it’s made of a patchwork of different lavas. Notice, for example, how the mare in and near Sinus Iridum is a different shade of grey from the surrounding lavas. This demonstrates that the Imbrium impact basin was filled with lavas from several floods instead of a single, large-scale event.

Proclus: Full Moon is when most observers “discover” Proclus, a 28-km-wide crater situated on the western edge of Mare Crisium. When the terminator is nearby, Proclus is an inconspicuous, normal-looking crater, but under high-sun illumination, the crater’s rays attract attention. What makes the Proclus rays interesting the large “zone of exclusion” where the rays are absent. This zone exists because Proclus was formed by a shallow-angle impact — the incoming rock struck the Moon at nearly a grazing angle.

Copernicus: Another of the Moon’s finest ray craters is 93-kilometre-diameter Copernicus. Notice that its rays are relatively stubby and somewhat faded compared with Tycho’s. This is because rays are short-lived features that disappear over time. And since Copernicus is about eight times older than Tycho, its rays have had that much longer to fade away.

Cassini’s Bright Spot: North of Tycho is a small, brilliant patch of whiteness — one of the brightest features on the lunar disc. This tiny spot is known as Cassini’s Bright Spot, after the astronomer Gian Domenico Cassini, who first called attention to it. Although the spot was suspected of undergoing dramatic changes, in reality it’s simply an ordinary, fresh, 3-kilometre-wide crater with a tiny ray system. The real challenge here is to try and find the crater when the Moon isn’t full.

Aristarchus: One of the Moon’s brightest craters is 40-kilometre-wide Aristarchus. The crater’s brightness and rays indicate it’s relative youth — it’s perhaps only 500 million years old. It’s eye catching for sure, but the diamond-shaped region the crater appears to be sitting on is even more interesting. This is the Aristarchus Plateau. The Plateau lies about two kilometres above the surrounding mare and features a subtle, reddish hue. Can you see it?

Reiner Gamma: This tadpole-shaped bit of lunar lightness is a challenge to pick out, but if you do get your scope aimed at it, you’ll see one of the Moon’s oddest features. It looks like an oddly disfigured ray, but one lacking a source crater. It turns out, Reiner Gamma isn’t a ray at all, rather, it’s the only nearside example of a swirl. Little is know about swirls, but this one has the most intense magnetic field of any feature on the Earth-facing side of the Moon.

Messier and Messier A: Perhaps the most difficult item to pin down in this selection are the two tiny craters named Messier and Messier A. During full Moon they look like a pair of small dots in northern Mare Fecunditatis. Closer inspection reveals that the left-hand dot (Messier A) has pair of parallel, thin rays emanating from it. How did this oddness form? Probably from a single impactor speeding in from the east and striking the Moon at a very shallow angle.

Categories: Moon and Planets
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