Published May 22, 2010, 10:50 AM

# Astro Bob blog: Geometry made simple

Step into a triangle of bright lights with the moon, Saturn and Spica tonight.

By: Bob King, Duluth News Tribune

 Astro Bob A look at celestial happenings in the Northland and beyond Bob King Click here to view previous posts or additional resources

A simple geometry lesson livens up the southern sky tonight when three bright celestial bodies form a right triangle -- defined as a triangle where one of the angles is 90 degrees. Created with Stellarium

Let's see. How many different types of triangles are there? There are only six so I shouldn't have too much trouble remembering them all. You've got your right triangle, equilateral, scalene, isosceles, acute and obtuse. I'll leave it to you to click on the link above for their individual descriptions. For the moment, we're interested in one of the most familiar -- the right triangle. One will be put on display for us this evening at dusk when the moon, Saturn and Spica, Virgo's brightest star, gather together in the southern sky. You could quibble that it's not a perfect right triangle where the square of the hypotenuse is equal to the sum of the squares of the two sides but for a casual gathering with no forethought, it's a solid effort.

The 9-day moon shows off hundreds of craters in a small telescope but even binoculars will show the craters Copernicus and Plato. Photo: Bob King

The moon is nine days old tonight and casts enough light to easily see your way about in the dark. I always look forward to the moon at this age because the crater Copernicus makes such a stunning presentation near the lunar terminator. Both Copernicus, which is 58 miles across, and nearby Plato, 68 miles, are easily viewable in a pair of steadily-held 7 or 8-power binoculars. The two are similar in size but of very different ages: Plato was carved out by a small asteroid 3.8 billion years ago while Copernicus is only 800 million years old. You can see its relative youth in the crater's more rugged walls and central mountain peaks. Erosion occurs on the moon over the eons as micrometeorites "sandpaper" its surface. Moonquakes, temperature extremes, the solar wind and the slumping of crater walls through gravity and settling contribute as well.

Copernicus (left) shows many terraces due to slumping of its walls. The crater is 2.3 miles deep and the central peaks rise 1300 feet above its floor. Plato's floor (right) has been filled with lava that welled up from below and is only 3300 feet deep. No doubt it too had central mountains, now submerged beneath dark, smooth lava flows. Credit: Damian Peach

Posted by: rking@duluthnews.com on 5/22/2010 at 9:32 AM | Comments (0) | Permalink | Edit