Moon Essentials: Parallax

Parallax is the apparent shift in the position of an object when viewed from two locations. You can easily see this effect just by alternately closing your left and right eye. The effect is larger for objects closer to you and for observer locations that are farther apart. As the closest celestial object to the Earth, the Moon dramatically exhibits this effect when viewed from opposite sides of the globe.

The two-frame animated GIFs on this page demonstrate the difference in the apparent orientation and position of the Moon as viewed from Tokyo, Japan and from Houston, Texas at the same moment. The first image pair is a close-up of the full Moon showing that observers in the two locations see a slightly different set of features at the eastern and western edges of the Moon. The second pair of images is a wider field of view showing how much the Moon's position is shifted against the background stars when seen from the two locations.

A single observer can see these same parallax effects by viewing the Moon at different times during the night. Between moonrise and moonset, the Earth's rotation carries the observer from one side of the globe to the other. The resulting change in the apparent orientation of the Moon during the night is called diurnal libration. The Moon's position against the background stars also changes, but this is the combined effect of parallax and the Moon's motion in its orbit, which partially cancel each other out: In 12 hours, parallax can move the Moon's apparent position 2° to the west while its orbital motion moves it 6° to the east.

Lunar parallax was used for over 2000 years to measure the distance to the Moon. Although the specific techniques vary, the essence of the method is the solution of the triangle formed by the Moon and two widely separated observers. By the early 1960s, this approach had produced a distance accurate to within a mile. It's now measured with lasers to within a fraction of an inch.