What is the “Apogee of the Moon”?

26 June 2014

The Short Answer (TSA)

            The “Apogee” is that point in the Moon’s orbit when it is farthest away from the Earth. The Moon’s orbit around the earth is not perfectly circular, but is more like an oval (or ellipse). So, sometimes, the Moon is closer or farther away from the Earth.

 The Moon's Orbit Around the Earth

             When the Moon is farthest from the Earth, at its apogee, the Moon appears significantly smaller in the sky. If you compare photographs, it’s surprising just how much the distance of the Moon from the Earth can affect the Moon’s apparent size in the night sky.

 Perigee & Apogee -- Change in the Moon's size in the Night Sky

              However, both the Moon’s apogee (farthest point from the earth) and perigee (closest point to the earth) have been precisely calculated for centuries. But why?

              What makes the Moon’s distance from the Earth so important?

              The “tides.” Tides are the rise and fall of sea levels in coastal areas caused by the gravitational pull of both the Sun and Moon on the Earth’s oceans.  At its apogee, the Moon exerts significantly less gravitational pull on the oceans.

               In coastal areas, the change in sea level between high tide and low tide is less when the Moon is farther away from the Earth.  So, if the Moon is at its apogee, there will still be a low tide, but the water level won’t drop as far as it does at other times. In the same way, if the Moon is at its closest point to the Earth, it’s “perigee,” the low tides will be significantly lower than usual because the the Moon is so close to the Earth that it’s gravitational pull on the oceans is greater.