Triangles For Lunatics, Crater Riot

An eye-catching triangle will greet your gaze this evening at dusk. Created with Stellarium

If you missed last night’s amazing occultation, the moon has more in store tonight. Look up at dusk and you’ll see three very different objects form a right triangle high in the southwestern sky. The moon occupies one corner and the Jupiter and Spica the others. Spica, the faintest of the three, is intrinsically the brightest — by far. Located 250 light years away, if put in place of the sun, it would shine 1,900 times brighter. We’d also be amazed to find out that instead of one star, Spica would appear double. What appears to be a single star is really two suns only 11 million miles apart — less than a third of Mercury’s distance to the sun — that revolve around each other every 4 days.

Jupiter in a small telescope this evening (July 1) accompanied by its four brightest satellites. Time is 9:45 p.m. CDT. North is up. Created with Stellarium

Both are enormous, 7 and 4 times the size of the sun, and so hot that they appear slightly blue to the eye. Compare Spica to Jupiter to see the color difference. Jupiter looks pale yellow in contrast from chemical compounds that lace its bands of ammonia clouds. The planet’s four brightest moons will be out tonight and easily visible even in a spotting scope.

A telescope as small as 4-inches will show the moon like this tonight. The terminator, at left, cuts across a crater-saturated landscape. Credit: Virtual Moon Atlas

Through 7x or higher magnification binoculars the left (east) side of the moon looks crinkly with craters. If you had to pick one evening when the maximum number of craters were on best display, the day or two after first quarter phase would be it. The terminator, the line of advancing lunar sunrise during the waxing phases of the moon, slices across a huge region of ancient lunar crust called the lunar highlands. The highlands preserve a record of bombardment by asteroids and meteorites from more than 4 billion years ago. You could travel more than 2 billion years back in time, whip out a telescope, and the terminator view would appear virtually the same.

Portrait of the 51-mile-wide (82 km) crater Tycho. Collapsed, terraced walls formed in the impact’s wake as did the central mountain peak, made of material that rebounded after it was compressed by the impactor. Look around crater and you’ll hundreds of parallel gouges in the landscape where material blasted from the crater scoured the area. Credit: NASA/LRO

The reason the craters show more clearly now than at full moon is because we see them shortly after local sunrise, when the sun’s low rays cast shadows that clearly reveal their bowl-like outlines, central mountain peaks and bumpy floors strewn with debris and melted rock from the heat generated by impact.