Astro Bob: A fine night for moon-watching
A striking crater triplet and massive fault highlight the crescent moon Sunday night, June 5. In addition, the moon will pass very close to the star Eta Leonis, hiding it from view in some locations.
The moon is a wonderful place to visit with a pair of binoculars or a small telescope. As it approaches half or first-quarter phase, thousands of craters come into view, several of which are highlighted on the map above. Perhaps the most striking are Theophilus, Cyrillus and Catharina, which together form an imposing trio.
Theophilus is the youngest, with an age between 3.2 and 1.1 billion years. It partially overlaps its neighbor, Cyrillus, which dates back to 3.9 billion years. Through a telescope you'll notice that Theophilus displays a much sharper crater rim — a sign of relative youth — compared to slumped and worn-down periphery of Cyrillus. It also sports a prominent central mountain peak 5,000 feet (1,400 meters) high — almost a mile!
Just below (south) of Catharina, another ancient impact crater with a heavily-eroded rim, look for a long, scalloped "wrinkle" in the lunar crust. That's the Altai Scarp , named for the Altai Mountains in Asia. The colossal fault stands 3,000 feet (1,000 meters) high and extends for some 300 miles (480 km).
The incoming asteroid that excavated neighboring Mare Nectaris (Sea of Nectar) 3.8-3.9 billion years ago created a powerful shock wave that caused the lunar crust to rise up and crash back down, forming waves of shattered and liquefied rock that over time cooled and solidified in place. Think of ripples of water expanding outward from a rock dropped in a pond. The Altai scarp is the outermost of these waves and one of the best preserved relics of this ancient cataclysm.
Rogue asteroids slammed into the moon during a period of intense bombardment about 4 billion years ago and gouged out additional seas or "maria" (mare is Latin for sea). Later, dark, titanium-rich magma, channeled by impact-generated cracks in the crust, flooded these deep basins, which cooled to form the dark plains we see today. The best known is the Sea of Tranquility (Mare Tranquillitatis), where the Apollo 11 astronauts landed in July 1969.
Besides providing a great display of its tumultuous past, on Sunday night, the crescent moon will shine very close to the 3rd magnitude Eta Leonis in Leo the Lion. Eta is a white supergiant star 20,000 times more luminous than the sun 1,270 light-years away.
Binoculars should show the star hovering just above (north) of the moon's edge as soon as it gets dark out. From southwest of a line passing from Cuba to British Columbia and cutting through the cities of Denver and Dallas, skywatchers will see the moon occult (cover) the star, a fun event to watch in a small telescope.
The occultation occurs at the moon's dark edge, making for a dramatic view. Observers will see the moon "snap up" the star like a chameleon capturing an insect with its long tongue. If you live within a mile of that line (Denver, Dallas and n. Houston), you'll witness a grazing occultation, with the star flashing in and out of view as it passes behind mountains and craters at the moon's extreme northern edge. Very exciting to watch!
Here's a list of cities that where the occultation will be visible, along with the times the star disappears and later reappears — on the moon's opposite edge. Enjoy the show and happy moonwalking!
"Astro" Bob King is a freelance writer for the Duluth News Tribune.