What is the Moon’s “Perigee”?

26 June 2014

The Short Answer (TSA)

            The “Perigee” is that point in the Moon’s orbit when it is closest to 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 closest to the Earth, at its “perigee,” the Moon appears significantly larger. 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.

Difference in Size in the Night Sky -- Apogee & Perigee

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 perigee, the Moon exerts significantly more gravitational pull on the oceans.

In coastal areas, the change in sea level between high-tide and low-tide is greater when the Moon is closer to the Earth.  So, if the Moon is at its perigee, the coastal water levels will drop more at low-tide, than when the Moon is farther away from the Earth.  In the same way, if the Moon is at its farthest point from the Earth, it’s “apogee,” the low-tides will not be as low as usual because the Moon is so far away from  the Earth that the Moon's gravitational pull on the oceans is less.