Are All Full Moons The Same Size?

Are All Full Moons The Same Size?

The Moon cannot change in size, but the way that it appears from Earth can vary. The Moon illusion, a phenomenon where the Moon appears bigger because it is lower on the horizon than one that is higher up in the sky, is one example. The same is true of constellations. A constellation that is viewed low in the sky will appear larger than one that is high in the sky. The actual size does not vary, but it appears to be different.

For one example, the Harvest moon is thought to be bigger, brighter, and more colorful than other Full moons, but is simply the moon that occurs closest to the autumnal equinox. An equinox happens twice a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun. At this moment in time, the Sun can be observed directly overhead from the equator. An equinox happens at one moment in time, but the coinciding day, termed an equilux, is when sunrise and sunset are closest to being exactly 12 hours apart.

Although very rare, when a full moon happens on the same night as an autumnal equinox, it is called a Super Harvest Moon. The Moon generally rises about 50 minutes later each day, but during the time of the Harvest moon and the Hunter’s moon, the moon rises only 30 minutes later each successive night. The result is that the Moon rises shortly after sunset, is lower on the horizon, and changes in color. All celestial bodies appear reddish in color when they are low on the horizon because the light must pass through more atmospheric particles than when overhead. The Harvest moon was given its name because the supposed larger size and bright color allowed farmers to continue to work even after the Sun had set.

Finally, the Moon’s orbit is egg-shaped, so there are times when the Moon is at its shortest distance from Earth, at perigee, and also times when it is at its furthest from Earth, at apogee. The orbit of the Moon does change slightly though, so each perigee is not the same distance from Earth every time. On March 11, 2011, the affectionately called “Supermoon” lit up the night sky. The Moon was 221,566 miles from Earth, its closest approach since 1993. It was forecasted to look about 20% brighter and 15% bigger than a typical full moon, but these differences wouldn’t actually seem noticeable with the naked eye. The total amount of light was a little greater, so the illumination of the ground would be more than most nights, but experts guessed that a casual observer would not particularly notice. There are a number of varying factors such as the Moon’s orbit, the proximity to Earth, and the time of rising that can influence how the Moon appears from Earth, but these differences are small.

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