Description

This simulation illustrates Galileo's method for finding the height of a mountain on the Moon, as described in his Siderius Nuncius. While observing the moon with his telescope Galileo noticed bright spots on the dark part of the moon. He interpreted these spots as mountain peaks that were catching sunlight even though the surrounding area was still in shadow. By measuring the distance of these bright points from the terminator line (the line separating the bright and dark parts of the moon), Galileo could use geometry to find the height of the mountain. This simulation assumes the observation is made when the moon is in quarter phase when the geometry is easier (this is the case discussed in the Siderius Nuncius).

The left view shows the Moon as seen from above its north pole. A ray of sunlight skims the lunar surface at the terminator line. A mountain pokes up above the lunar surface. The user can control the angle between the mountain's location and the terminator, as well as the height of the mountain. The view on the right shows the Moon as seen from Earth. When light hits the mountain a bright spot is visible on the dark portion of the Moon.

Options allow the user to display the values for the angle and height of the mountain, as well as the distance of the mountain's peak from the terminator line (measured perpendicular to the Earth-Moon line). In addition, the user can choose to reveal the lines needed to show the geometry for determining the height of the mountain.