Tag Archives: Aristarchus

Aristarchus helps us see the wild and woolly moon of long ago

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The bright crater Aristarchus looks like a small but distinct bright patch inside the left or eastern edge of the moon with the naked eye. Photo: Bob King

With the moon turning full tomorrow, let’s take a look at one of its brightest craters, Aristarchus (Aris-TAR-kess). Although only 25 miles in diameter, it’s bright enough to see with the naked eye and positively dazzling in a telescope. The name suits its appearance perfectly. Aristarchus was the Greek philosopher who first proposed the concept of a sun rather than Earth-centered solar system. To this day the crater celebrates this brilliant and ultimately correct idea.

The reason for its radiance has to do with its relative youth. Aristarchus was excavated by a huge meteorite impact “only” 450 million years ago. You’d almost call that recent in lunar years especially considering that most of the moon’s craters are some nine times older, having formed nearly 4 billion years ago. Sunlight and solar radiation darken the lunar surface over time; Aristarchus hasn’t been around long enough for that to happen.

View of the Aristarchus Plateau seen through the window of the Apollo 15 command module. Aristarchus is at left; Herodotus crater and Schroeter's Valley at right. Click for a valley closeup. Credit: NASA

You can see the crater with you unaided eye as a small bright spot in the large dark region that covers much of the eastern half (left side) of the moon called Oceanus Procellarum or Ocean of Storms. It helps that Aristarchus contrasts so well against the dark moonscape. If you have any difficulty seeing it, whip out a pair of binoculars for a second look. Even 5x will bring the shimmering spot into view.

Aristarchus sits on a 125-mile-wide plateau that rises a little more than a mile (at maximum) above the vast “oceanic” plain. The rise is riddles with cracks called rills which long ago carried lava from beneath the moon’s crust into the basin that is now Oceanus Procellarum.

The biggest rill and easiest to see in a small telescope is named Schroeter’s Valley. It begins near the crater Herodotus and winds through the plateau before fading out at the edge of the great “ocean”. If you catch the lighting right, the rill’s as spectacular as the crater. Within its shadowy curves,  it’s not hard to imagine flowing lavas cutting a path through the region and emptying into the vast depression surrounding the plateau. As for the plateau itself, it was lifted to its present height by magmas that welled up from beneath it.

Aristarchus is the bright spot with Schroeter's Valley winding below it. The olive-color of the surrounding region shows up well in this photo. Click to see an overflight of the region by the Japanese Kaguya probe. Credit: Ole Nielsen

While quiet now, the moon once grumbled and roared with vulcanism.

Before we leave Aristarchus, we have to talk about color. Drab grays, whites and the palest of browns tint most of the lunar surface. No so with the plateau. Through a small telescope it has a distinct olive or yellow-brown tinge caused by a 4-12 inch covering of volcanic ash – more evidence of the wild and woolly moon of long ago. See it best around full moon phase.

I like to imagine standing on the Earth 3 billion years ago, watching the flash of crater impacts and glowing lava flows on the lunar nightside through my little telescope. I’d never be able to take my eyes off our companion world. Frankly, I still can’t.

Take the plunge into that burnin’ ring of fire

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Individual dew drops line a blade of grass this morning. Photo: Bob King

Yesterday on a walk in Hawk Ridge Nature Reserve near my home, a flock of migrating nighthawks blew by headed south. I identified them right away by their white wing patches. Birds on the move, cool mornings lavish with dew. I like these hints of fall. Through the telescope last night, the nearly 12-day-old moon served as an illuminated stage for the passage of yet more birds. Over a period of five minutes, I saw some a dozen silhouetted avians zip across the cratered landscape on their way outta here.

Now through September is an ideal time to point your telescope at the waxing moon — especially around full phase — to watch birds migrate at night. We don’t think about it much, but many birds are busy migrating while you and I are out like a light. Hummingbirds, warblers and others not only avoid the heat of day by doing so but are less likely to get nailed by a predator under cover of darkness. To watch the show, all you need is a small telescope and a big moon. Plunk in your low power eyepiece and just wait. I saw my first bird within a minute. They fly by quickly, but since I’m no bird expert, I couldn’t identify the different species. Craters are my forte.

Numerous white rings, splashy bright patches and ray systems are best visible around the time of full moon. Credit: Frank Barrett

Some amateur astronomers scorn gibbous and full moons as worthy of study because they’re too bright, and the landscape is washed out due to the sun shining almost directly above the moon’s surface. When the sun shines from the side, as it does near sunset and sunrise both here and on the moon, everything casts a shadow and shows minute detail. Every bump, wrinkle and hair on a person’s face stands out in glaring detail. But shine a light directly at a person’s face – equivalent to the sun shining squarely over the full or nearly full moon – and shadows and those disturbing wrinkles disappear, lost in a flood of light.

A full moon offers no sense of depth or relief, but does reveal details otherwise invisible at other lunar phases. That includes many small craters which hardly anyone notices during lesser phases, but which are transformed into brilliant rings during the equivalent of lunar high noon.  Last night I lost count of how many of these “hot rings” I saw through the telescope at low power. They’re so brilliant they resemble white flares or fiery white-hot rings of lava. Johnny Cash’s “Ring of Fire” comes to mind.

What you’re seeing is high-angle sunlight reflected off fresher rocks and soils on crater rims and floors or from the tendrils of long rays (splashed rock) surrounding fresher craters. The rays and lighter soils truly shine around the time of full moon. Once you get into seeing these strange lunar lily pads, you’ll be surprised at how alien they look compared to the more familiar peaks and crater holes seen to advantage at other phases.

Proclus crater photographed by astronauts aboard Apollo 15. Credit: NASA

I was particularly struck by the fiery ring of Proclus crater and its peculiar off-center system of rays. Proclus, at 18 miles in diameter, is a young lunar crater, and the brilliance of its rim is clearly the result of fresh rock exposed by impact that has yet to darken under the influence of solar radiation. It contrasts beautifully with the older, surrounding moonscape called Palus Somni or the Marsh of Sleep. The weird forked appearance of the rays suggests that the asteroid that created Proclus struck the moon at an oblique angle.

Several of the brightest craters and their systems of rays (strings of fresh secondary craters formed by rock ejected during the main crater's creation) around the time of full moon include Tycho and its rays, Aristarchus and Proclus. Credit: Frank Barrett

Other brilliant craters and ray systems include Aristarchus (brightest crater on the moon) and Tycho, and there are many more. Like Proclus, they’re relatively fresh craters compared to most. With binoculars you can see all three of the aforementioned as bright spots, while any telescope will show their ring-like forms and feathery rays in far more detail. Between moon and birds, you may find yourself staying at the telescope side later than you thought tonight.