Shoot the Moon
Above is NASA’s Astronomy Picture of the Day from two days ago. (Click on it to download a higher-resolution image.) As explained at NASA’s site, it’s a real photo, with genuine colors, the redness of the moon stemming from its being in mid-eclipse.
By way of explanation, there’s an on-going project at Apache Point Observatory, in southern New Mexico, that uses the observatory’s 3.5-meter telescope in reverse. Usually, a telescope collects light. Tom Murphy of UC San Diego has an on-going experiment that uses the telescope instead to project light, sending a laser beam to the moon.
Astronomers love to give their experiments clever acronyms. Murphy’s is APOLLO, or the the Apache Point Observatory Lunar Laser-ranging Operation. Apollo is an appropriate name because the experiment depends on mirrors left behind by astronauts during the 1971 Apollo 15 lunar landing. By using the APO telescope to focus a laser beam on such a mirror, the APOLLO team can measure the time it takes for the light to return, thereby obtaining a precise measurement of the distance between the earth and the moon. This, in turn, provides a test of Einstein’s Theory of Relativity.
That’s the gist of it anyway. You can read more at the APOLLO page or Tom Murphy’s home page. Or, in the description of the photo that appeared with it two days ago (with links omitted; see original for lots of them):
This is not a scene from a sci-fi special effects movie. The green beam of light and red lunar disk are real enough, captured in the early morning hours of April 15. Of course, the reddened lunar disk is easy to explain as the image was taken during this week’s total lunar eclipse. Immersed in shadow, the eclipsed Moon reflects the dimmed reddened light of all the sunsets and sunrises filtering around the edges of planet Earth, seen in silhouette from a lunar perspective. But the green beam of light really is a laser. Shot from the 3.5-meter telescope at Apache Point Observatory in southern New Mexico, the beam’s path is revealed as Earth’s atmosphere scatters some of the intense laser light. The laser’s target is the Apollo 15 retroreflector, left on the Moon by the astronauts in 1971. By determining the light travel time delay of the returning laser pulse, the experimental team from UC San Diego is able to measure the Earth-Moon distance to millimeter precision and provide a test of General Relativity, Einstein’s theory of gravity. Conducting the lunar laser ranging experiment during a total eclipse uses the Earth like a cosmic light switch. With direct sunlight blocked, the reflector’s performance is improved over performance when illuminated by sunlight during a normal Full Moon, an effect known as the real Full Moon Curse.
It turns out that I’ve been involved with Apache Point Observatory in an administrative role since 2003. My first visit to APO was in October 2005, the month APOLLO started, but I didn’t get to see a laser shot. On my next visit, in April 2008, I was at the observatory during daytime only. In three weeks, I’ll be back for a celebration of the telescope’s twentieth anniversary, which will include some nighttime viewing. Perhaps I’ll see the laser in operation. I hope so.