Moon and Spica 
The Moon is passing through a busy section of sky this week. Over the next few nights, it’ll snuggle up to two of the brightest stars in the night sky, plus two planets that are even brighter.
The action tips off tonight with the closest of the four encounters. The target is Spica, the brightest star of the constellation Virgo. They rise in late evening, with Spica just below the Moon. Bright orange Mars is close to their lower left.
Under the scale that astronomers have used to define brightness for centuries, Spica is a first-magnitude star. Under this scale, a smaller number equals a brighter object. So a first-magnitude star is brighter than one of second magnitude.
You might expect that a star of first magnitude would be twice as bright as one of second magnitude, but that’s not quite the case. A difference of five magnitudes equals a 100-fold change in brightness. So a first-magnitude star is actually a bit more than two-and-a-half times brighter than one of second magnitude.
Just to complicate things a bit, Spica actually changes brightness. It consists of two large stars in a tight orbit around each other. The stars are so close together that their gravity distorts their shapes. Instead of nice and round, they’re shaped like eggs. So as they spin around each other, we see them at slightly different angles — a change that causes Spica to vary in brightness by a few percent.
More about the Moon’s next companion tomorrow.
Script by Damond Benningfield, Copyright 2014