Measuring the Size of the Universe: How Far to the Mn? 67
Measuring the Size of the Universe: How Far to the Moon?
The Moon is currently moving away from Earth at a rate of approximately 3.8 cm (1.5 inches)
each year. The mean distance between Earth and the Moon is approximately 385,000 km
(239,228 miles), so this distance will increase by 1 percent in approximately 100 million
years, if movement continues at this rate. Isn’t it remarkable that the distances between
heavenly bodies can be measured in millimeters?
Corner Cube Mirrors
We can make such precise mea-
surements thanks to the Apollo
space missions that were launched
beginning in 1969. Apollo 11, 14,
and 15 placed special mirrors on
the surface of the Moon to reflect
laser light emitted from Earth. These
mirrors, which are different from
ordinary household ones, are called
corner cube mirrors. The surface
of a corner cube mirror is con-
structed so that any light that hits
it will be accurately reflected back
to the source in a parallel path, no
matter what direction it comes from.
Scientists can use this kind of mirror to accurately measure distance by shining a light
at the mirror and recording the time it takes the light’s reflection to return. Since the speed
of light is always constant at 299,792,458 meters per second, it is extremely well suited as
a “ruler” for measuring distance.
How a Corner Cube Mirror
If we use an easy-to-understand, two-
dimensional model, a corner cube mirror
would be made up of two mirrors joined at
a right angle, as shown in the figure. Light
that hits the mirror will reflect at the same
angle as the angle of incidence. The working
principle is the same in three dimensions.
The principle behind the corner cube
mirror is not unusual—it is the same principle
used in reflectors on cars or road signs.
The mirror that the Apollo spacecraft
brought to the Moon uses a prism to reflect
light, so it is more accurately called a corner
cube prism.
A mirror for measuring distance, which was placed on the
surface of the Moon
Light ray
Light ra
Corner cube mirror (two-dimensional)

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