Put two pieces of masking tape 1 inch apart horizontally,
with one above the other, on a window through which a full
moon can be seen. Move back from the tapes until the moon's diameter exactly fills the space between the pieces of tape, then measure the distance from the eye to the tapes.

Knowing that the moon is about 240,000 miles (actually
238,900 miles) from the earth, and realizing that the
distance of the moon from the earth is to the diameter of the moon as the distance of the tapes from the eye is to the diameter of the image between the tapes, you can calculate the diameter of the moon by simple proportion.

For example, if the average distance to the tapes is 110
inches, then:

diameter of image diameter of moon
----------------- = ----------------
distance to eye distance to moon

and
1 inch n miles
---------- = -------------
110 inches 240,000 miles

Similarly, you can measure the diameter of a coin, hold it
between an eye and the full moon, then measure the distance
it is held (from the eye to the coin) when it exactly covers the moon. The ratio of the diameter of the coin and its distance from the eye will be the same as the diameter of the moon and its distance from the earth.