Lasers IV 
The first-quarter Moon forms a wide, skinny triangle with a couple of bright companions tonight. Regulus, the brightest star of Leo, the lion, is a little to the upper right of the Moon, with the bright orange planet Mars farther to the right. They're high in the sky at nightfall and drop from view by around 2 a.m.
The Moon, of course, is our closest astronomical neighbor, at an average distance of about a quarter-million miles. But the distance is growing -- by about an inch-and-a-half a year.
Scientists discovered this "inchworm" effect with the help of lasers.
In the 1960s and '70s, Apollo astronauts left three special laser reflectors on the lunar surface. A robotic Soviet probe left another. And over the last four decades, astronomers have routinely bounced laser beams off those reflectors. By precisely measuring the time it takes each pulse to hit the Moon and return, they can plot the distance to the Moon with an accuracy of less than an inch.
The laser shots -- most of which were done at McDonald Observatory -- have revealed much more than just the Moon's distance, though. They've also revealed that the Moon probably has a molten core that's about 400 miles in diameter. And they've provided strong support for Albert Einstein's theory of gravity.
All four reflectors are still in good shape -- and still helping scientists keep track of our next-door neighbor.
We'll have more about lasers in astronomy tomorrow.
Script by Damond Benningfield, Copyright 2010