Illustrated infographic showing the difference (as seen from Earth) between perigee, when a supermoon appears, and apogee, when a micromoon appears.
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If you’re looking to up your minimum monthly requirement of lunar light, tonight’s the night to get a full charge. This month’s full moon occurs Oct. 6 at 11:48p ET. More importantly, it occurs at this month’s lunar perigee, creating an instance of ‘super moon’.
Each month, the moon’s elliptical orbit swings it a bit closer to Earth. The close point is called perigee. The distant point from earth is called apogee. When the perigee occurs around Full moon, the moon appears a up to 14% larger and up to 30% brighter than the smallest full moons of the year. A supermoon, theoretically. The challenge is how to compare the perigee full moon size and brightness to a regular full moon’s size and brightness as you observe on that special night. One way to check the change is to measure the moon’s diameter using a measuring tool, once the moon is well above the horizon, up near the top of the sky. Be sure to record your measurements, you’ll need them next month. Measure the moon’s diameter again at the next full moon using the same measuring tool and with the moon at a similar height as the previous month. Record your measurements again and compare to the previous month’s measurements. Is there a difference?
Here's another moon size experiment to try. Measure the size of the full moon as it’s rising but still down closer to the horizon. Record your measurements. Wait until the moon is near the top of the sky and measure it again. Now check this measurement against the earlier measurement. Is there a difference? Try to explain what your answer indicates.
There is a well-known optical illusion caused by the moon’s appearance when it’s down low in the sky, close to the horizon. Seeing it close to objects of known size, like buildings or trees, makes our brain interpret the moon’s size as enormous. Notice how much smaller it looks later in the evening when seen near the top of the sky. Because of the illusion, we can get the supermoon experience almost every month as the full moon rises or sets. No matter how your measurements work out, enjoy the October moonlight!
About the author
Derrick Pitts, Hon.D
For many Philadelphians, Derrick Pitts is The Franklin Institute. Since 1978, Derrick has been teaching us to look up—and to wonder about what we see up there.