Bob Garfinkle
Lunar Bright Spots and Rays on the Moon

General Meeting June 26, 1999

Jane Houston

Students of the Moon have looked up and wondered about the spoke-like features and bright spots for hundreds of years. What causes these rays and bright white spots? Galileo was one of the first to observe them and wrote in 1610 of how the light colored rays "join with neighboring regions of the spots in a gentle linkage, the boundaries mixing and mingling." As the business part of the June SJAA general meeting concluded, friendly mixing and mingling of members was the prelude to Bob's fascinating talk.

Lunar rays come in various shapes and sizes, but they have a lot in common. They all appear and disappear at about the same colongitude each lunation. Long rays are complex and may or may not be radial to the locus crater, and the crater may or may not brighten as its rays brighten. Rays can encircle a crater or be missing from some of the circumference of the crater. They cross over all types of lunar terrain, which reveals the relative ages of the features in a region. Most rays are the same width for their entire length, although many taper at the extended end.

Bob showed an image of the 1874 experiment whereby James Nasmyth and James Carpenter heated some water in sealed glass globes. The heat-cracked globes were used to "prove" that rays were the result of material that extruded up to the surface when the Moon was cracked by internal pressure. Despite the similarity to Tycho's ray pattern, this couldn't work. The lunar crust just isn't strong enough! Another interesting postulation was that the rays were stains arising from highly heated subterranean vapors. In the mid 1960's the first close-up images of the rays were taken by the Ranger spacecraft. Lunar rays are composed of a powder-like material. Tests revealed the reason the rays are light and the soil is dark - it's because the older soil has been bombarded by solar radiation for a longer period, which has blackened the older materials and the newer rays have been exposed for only 2 to 800 million years. Ray material brought back by Apollo astronauts confirmed both the sprinkling over of pre-existing surfaces and that this material is the consistency of flour!

I could go on and describe what Bob had to say about the bright spots. Well, I'll say a little: they are generally small-to-medium sized cone craters. I could talk about the bright ray system known as Cassini's Bright Spot, in the crater know as Hell. To Cassini it looked like a small cloud. I don't want to tell you everything Bob talked about. You'll just have to get a copy of his Lunar Observers' Handbook when it is published. Until then, go out and take a look at the Moon. You'll see many interesting features, and learn that all month long the Moon is a beautiful object to observe, study and sketch. On those nights when the Moon is between first and last quarter, don't despair! Take a break from the dark sky and tour our rocky neighbor. And don't forget to view the beautiful and exquisite rays emanating from craters, and the surprising bright spots. You won't be disappointed, I guarantee!

Bob suggested that you use a number 47 violet eyepiece filter to observe the rays through. This filter makes the rays standout against a darker background.

I hope this has whetted your appetite for adding rays and bright spots to your lunar observing program.

Jane Houston; last updated: October 04, 2007 Prev Next