The theory of continental drift had to wait decades before it could be tested through actual tracking of plate movements. Continents move only millimeters per year, and the techniques necessary for tracking them with such precision did not exist in 1912 when the theory was first proposed. The technique used to make such precise measurements is known as laser ranging.

In the late ’50s and early ’60s, scientists discussed the idea of bouncing bright pulses of light off of satellites and measuring the amount of time the light took to return. If they knew the amount of time the light took to travel, they could calculate the distance to the satellite.

The first problem with this is the idea of using bright pulses of light. Normal light emanates from a “point source” like a light bulb and spreads out over distance. When you shine a flashlight on your hand a few inches away, the beam is smaller than when you shine it on the wall a few yards away. The beam on the wall is also dimmer than the beam shining on your hand, because the same amount of light has been spread out over a larger area. Scientists would have the same problem trying to reflect a flashlight off an orbiting satellite.

The second problem scientists would have to solve would be getting their beam of light to bounce directly back at them from the orbiting satellite. In the end, both of these problems were solved.

First, the introduction of the laser meant that scientists could use coherent light to keep their “flashlight” beam from spreading out. Light is made up of tiny “particles” called photons. Actually, there is nothing in everyday life that is really comparable to a photon. They sometimes act like waves and sometimes like particles. For the moment, though, think of photons as waves. Normal light is made up of waves that are of all different sizes, traveling in all different directions away from a source. Coherent light, on the other hand, is made up entirely of waves that are exactly the same size, travel in the same direction and all go up and down in unison. Because of this, the light does not spread out in the way that normal light does.

Producing coherent light only became possible with the introduction of the laser. For now, it’s enough to know that a laser is a device that produces lots of identical photons and only allows them to escape if they are lined up and traveling in the same direction.

The problem of bouncing light directly back to its source can be solved by using corner-cube reflectors. These are precisely what they sound like: sets of three mirrors arranged into corners, like the intersection of two walls and the ceiling in a room. In this case, it’s more useful to think of light as a particle. If you throw a tennis ball against the corner of your room, it will bounce back in the same direction it came from. This also works for photons bouncing off a corner-cube reflector.

Scientists use laser ranging regularly today. Special satellites are designed for these experiments. They are covered in mirrors. Picture a giant orbiting disco ball, and you’ll have a pretty accurate picture of what they look like. To set up one of the first laser ranging experiments, scientists used the moon landing.

The moon always faces the earth. Like the flashlight beam, the Earth’s gravity is weaker the further away you get. This means that the pull of gravity is stronger on one side of the moon than the other. The effect of this is that the earth is constantly trying to stretch the moon out in a straight line. It actually manages to make it slightly football-shaped. If it were made of liquid, the football bulge would move across the surface as the moon spun on its axis. The moon actually does this to the earth as well, which is why we have tides. Instead, the moon is made of solid rock, and rather than let the rock swell and fall like the tides, the moon actually has adjusted to rotate so that the same side always faces the earth. This means if you are pointing a laser at the moon, you will always know exactly where your target is.

Knowing how to laser range between the earth and the moon is one thing. Getting your equipment there is another.

Josh Braun is the Daily Nexus science editor.