May 9 – This Little Light Of Mine

Today’s Factismal: Scientists first “pinged” the Moon with a laser beam in 1962; you can repeat their experiment today.

Of all of the experiments carried to the Moon by the Apollo astronauts, only one is still running, mainly because it has no moving parts. Called the Laser Ranging Retroreflector (LRR) and looking a lot like a reflector for a giant bicycle, the experiment was carried up by astronauts on Apollo 11, 14, and 15. The experimental apparatus on the Moon consisted of a set of mirrored cubes placed on a large flat table in a hexagonal pattern; because any light entering the cube is reflected back at the source, these are called retroreflectors. When a light from the Earth hits the LRR it is reflected back at the place on Earth where it was emitted. With an appropriate sensor, astronomers can detect the reflected light.

The Lunar Ranging Retroreflector at Apollo 11 (Image courtesy NASA)

The Apollo 11 Lunar Ranging Retroreflector
(Image courtesy NASA)

But why would anyone want to shoot a light at the Moon? The answer to that is both simple and complex. Astronomers are interested in the light because it tells us how far away the Moon is; by dividing the travel time of the light by the speed of light (and remembering that the light goes twice the distance {there and back again}), we can get a very accurate measure of how far away the Moon is. And that is important because the Moon is slowly moving away from the Earth.

Close-up of the Apollo 15 Lunar Ranging Retroreflector (Image courtesy NASA)

Close-up of the Apollo 15 Lunar Ranging Retroreflector
(Image courtesy NASA)

That happens because the Moon’s gravity creates a slight bulge on the Earth. But, since the Earth is rotating faster than the Moon orbits, that bulge is always moving slightly ahead of the Moon. That then causes a very small increase in the Earth’s gravity just ahead of the Moon, which then makes the Moon speed up very slightly. That increase in speed then moves the Moon further out in it orbit; in effect, the Earth is dragging the Moon into a higher orbit. Currently, the Moon is moving away from the Earth at 3.8 cm per year. Eventually (in a hundred million years or so), the Moon will be so far away that it falls into one of the libration points between the Earth and the Sun and will get stuck there.

moon1 Moon2

Of course, the energy to speed up the Moon has to come from somewhere. It turns out that it comes from the Earth’s rotation; moving the Moon into a higher orbit slows down the Earth’s rotation. Over the past 350 million years, the Earth’s rotation has slowed from 23 hours to 24 hours. But that slowdown has increased in the past few million years; in another 50 million years or so, we will have a day of 26 hours.


The LRR also provides incontrovertible proof that we landed on the Moon. If you shoot a laser at the Moon’s surface, it is too dull to reflect enough light to detect. But if you shoot a laser at one of the LRRs, it will send the beam right back to you. Ping! Proof that we actually went there!

If you’d like to play with more Moon science, then head over to Moon Mappers:

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