(This observing exercise is geared for middle and high school students.)


Even though we are not on the Moon, we can determine the sizes (diameters) of some of its features. The procedure is to measure the length of the feature and ratio it to the full diameter of the Moon.


The diameter of the Moon is 2200 miles (3500 km), and this can be used to compute the sizes of its surface features. All you have to do is (a) measure the size of the feature with a ruler and (b) measure the full diameter of the Moon with the same ruler. Measure the feature's dimensions as precisely as you can. This exercise works best if the image is projected onto a board or wall, but it can also be performed on a computer screen.


Some of the larger surface features are

Mare Imbrium                  Crater Tycho

Mare Serenitatis             Crater Copernicus

Mare Tranquillitatus       Crater Plato

Mare Crisium                   Crater Eratosthenes

Mare Nectaris                  Crater Aristarchus

Mare Fecunditatis           Crater Kepler

Mare Nubium                   Apennine Mountains

Oceanus Procellarum


You can use the links for the Lunar Phase Days to see which surface features are well visible for the phase during which you are observing.


1. Measuring the full diameter of the Moon may be tricky for two reasons. (A) Unless the phase is a Full Moon, then the illuminated disk of the Moon is not a circle. You will need to measure from the north pole to the south pole. If the Moon is a crescent, this would be from one “horn” to the other one. (B) The other difficulty is that your view is of only part of the Moon. You will need to move the telescope to one pole, and then measure the full size of the displayed region of the Moon. Next, move the telescope to the edge of the previous view. Repeat the measurements and movement of the telescope until you have moved across and measured the full diameter.


2. You can now compute the Scale of your image of the Moon. The full diameter is 3500 km (or 2200 miles) and you have measured the image in cm, mm, or inches. The Scale is equal to the full diameter divided by the measured size of the image.


Scale (km/cm)  =  3500 km / _________________ cm  =  __________________ km/cm


3. Measure the lunar features of interest using the same ruler. Make certain that the same units, for example centimeters, are used for both the Moon's diameter and the features' sizes.


4. Compute the sizes of the features you measured.


Feature size = Scale (km/cm)  x  Feature's diameter (cm)


  Feature              Scale (km/cm)            Measured Size (cm)               True Size (km)        

1.  |                                |                                             |                                                           |                                                     |

2.  |                                |                                             |                                                           |                                                     |

3.  |                                |                                             |                                                           |                                                     |

4.  |                                |                                             |                                                           |                                                     |


These sizes will be most accurate for the features nearest to the center of the Moon's disk. Because the Moon's shape is a sphere, the features toward the edge are curved, so you are not seeing the full extent of their sizes.



James Sowell, 2013