Adventures around Aristarchus

The moon reaches last quarter at 6.49pm on Tuesday. At that precise time, it will be invisible as it will be below the horizon here in New Zealand.

To observe the moon for yourself, you must wait until moonrise, which won't occur until 2.31am on Wednesday.

Because it is highest in the sky at dawn, the last quarter moon is much less observed than the first quarter moon, which is visible after sunset.  This is a shame since many intriguing features are visible on the moon's surface during this lunar phase.

This week I would like to tell you about the crater Aristarchus. 

Sometimes called the lunar lighthouse, Aristarchus is named after Greek astronomer Aristarchus of Samos. It is the brightest crater on the lunar surface. The crater sits in the middle of Oceanus Procellarum (Ocean of Storms) and is visible to the naked eye in the moon's northwestern quadrant.

Even small telescopes reveal fascinating details in the surrounding regions.

Aristarchus is bright because, by lunar standards, it is fairly young. Geologists believe the 40km crater, which is deeper than the Grand Canyon, was created 450 million years ago when a giant asteroid impacted the moon's surface.

Debris from this cosmic collision can be seen surrounding the crater as a system of bright rays that extend hundreds of kilometres from the impact site.

Aristarchus seems to be geologically active and may still be volcanically active. Since the invention of the telescope, astronomers have reported more than 100 observations of so-called Transient Lunar Phenomena, which are short-lived changes in light, colour or appearance in the region around this crater.

More recently, the Apollo 15 mission in 1971 and the Lunar Prospector mission observed evidence for or detected Radon-222 gas.

With a half-life of under four days, radioactive Radon-222 must have been released recently either through brief explosive outgassing events or by slow diffusion from the surface.

So, why not turn your telescope towards the moon and see if you can contribute to science by observing evidence for activity around Aristarchus?