Shooting the Blood Moon reveals the source of the Full Moon Curse

No, we're not witnessing the start of an interstellar war here, although that's definitely a blood-red moon and a real laser beam.

On April 15th, when the moon slipped through the dark umbra of Earth's shadow, many skygazers — professional and amateur alike — were anxiously waiting for this 'Blood Moon eclipse' to reach totality. However, not everyone was waiting for the same reasons. Dan Long, at the Apache Point Observatory, was poised and at the ready, with a powerful laser pointed at a very specific part of the lunar surface.

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This wasn't going to be the 'shot heard round the solar system' though. The target he had lined up is of human origin, known as the Lunar Laser Ranging RetroReflector (LRRR) array, which was set up at the Apollo 15 mission landing site in 1971. This is the largest of four reflector arrays — composed of multiple clear glass prisms laid out in a grid pattern — put there by the Apollo 11, Apollo 14 and Apollo 15 missions, and another positioned on Russia's Lunokhod 2 rover. Using the speed of light, anyone can find the exact distance to the moon by firing a laser beam at the LRRR and timing how long it takes for the light to return.

However, there's something that causes problems with these lunar ranging experiments, because the amount of light being received back from the reflectors has been slowly dropping off over time. Lunar dust accumulating on the prisms can certainly account for some of the loss, even the tiny amount that gets kicked up into the tenuous lunar atmosphere. Part is due to some of the laser light being scattered by Earth's atmosphere and the surface of the prisms themselves. However, these don't account for it all, and even stranger, the effect is worst on full moon nights, so that the lasers only return a tenth of what they should (even accounting for all the other losses). Scientists jokingly call this the 'Full Moon Curse.'

Tom Murphy, the director of the observatory's APOLLO program (Apache Point Observatory Lunar Laser-ranging Operation), has an idea of what's causing this 'curse' though. He told UC San Diego News that it's heat.

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Murphy's idea is that the heat from the sun's rays is causing the prisms to become thermal lenses — the surface of the prism becomes warmer than the core, the molecules in the surface are thus slightly further apart than at the core, so a lens is produced. The more direct the light, the more heat there is, and the stronger the lens becomes. The most direct light received on the moon is during a full moon, when the sunlight can shine straight down the tiny wells the prisms are recessed in. An eclipse allows them to test this, though, as it gives them the exact same 'full moon' conditions, but without the full direct light from the sun. When they fired their laser at the array during lunar eclipses, they found that the returning laser completely recovered that ten-fold loss of photons it experienced during the full moon.

Why bother with all this? The extremely precise measurements they've made have given them the distance to the moon, accurate to within about a millimetre. Using this, and what's known about the moon's orbit so that they can use the moon's orbit (and how it changes with time) to test some of the most fundamental aspects of physics used today — Newton's gravitational constant, G, and Einstein's theory of general relativity.

(Photo courtesy: Dan Long/Apache Point Observatory)

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