The reason why this full Moon is larger and brighter than usual is because we are seeing the Moon near its closest approach to the Earth. What is called a ‘perigee Moon’. This apparent size difference occurs because the orbit of the Moon about the Earth is ellipical in shape.

Its closest approach (perigee) is approximately 226,600 miles (363,104 kilometers), while its furthest point on its orbit (apogee) about the Earth is about 252,100 miles (405,696 kilometers). This results in a difference of about 25,500 miles. Its average distance from the Earth is around 239,000 miles (384,600 kilometers).

Thus, this visually larger full Moon is possible because the Moon seen on October 25-26 is actually about 25,000 miles closer than when it is seen at its furthest point away from the Earth.

The orbital period of the Moon from perigee to apogee and back to perigee is called the anomalistic month.

As a side note, the distance between the Moon and the Earth has just been measured to an accuracy of one millimeter. (See “APOLLO measures Moon distance to 1 millimeter; also helps with gravity”.)

With Halloween coming up and the Moon looking bigger than usual, Thursday night would seem to be a good time to look up in the evening sky and really howl at the Moon.

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