Introduction to the Moons Perigee and Apogee

The distance from moon’s perigee to its apogee and back varies from day to day, if not minute to minute. The distance changes primarily because of the elliptical path the moon traverses on its orbit around the earth. To determine these intervals, one starts with the distance, in miles (or kilometers), from the precise center of the moon to the inner core of the Earth. On average, this equals about 238,000 miles. Astronomers use special mathematical formulas for computing the center-to-center distance. Computing the complexity of the changing perigee-to-apogee spread, should one need to know this, necessarily begins with the center-to-center calculation.

Visually comparing the difference between the moon at apogee and the moon at perigee presents a challenge few can meet with the naked eye or even by telescopic observations. Against a star strewn background, at widely spaced junctures in time, the moon will appear to have much the same relative “size” to the casual viewer, even though in actuality the moon will hold a position many miles farther from Earth at its apogee than at its perigee. A camera may offer one the best means for capturing the difference in order to make a “visual” comparison. With the camera firmly fixed in place, a picture of the moon’s two elliptical stages should reveal a definite difference in apparent image sizes.

The gravitational pull of the sun increases the elliptical peculiarity of the lunar satellite, for instance pulling it farther from Earth during the new moon phase. This gravitational attraction will have the greatest effect during periodic stages of closer proximity of the Earth with the sun. This will result in an exaggerated differential between the moon’s perigee and its apogee. A camera-generated comparison of the moon between this period of the Earth’s rotation around the sun and that when the distance between the two has narrowed should display definite differences in the moon’s apparent dimension.

Except for amateur and professional astronomers, the foregoing observations may hold but transitory interest for most people. For anyone living near large bodies of water, however, the perigee-apogee dynamics can prove of significant importance. Since the moon exercises considerable influence on the tides of oceans as well as large lakes, the effect on boaters, clam diggers, sunbathers and other water-loving people can prove substantial.

Now you know: It’s primarily because of perigees and apogees that people study tide books before they go down to the beach.