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02/18/2019 – Ephemeris – Super Moon, super math

February 18, 2019

Ephemeris for President’s Day, Monday, February 18th. Today the Sun will be up for 10 hours and 36 minutes, setting at 6:15, and it will rise tomorrow at 7:37. The Moon, 1 day before full, will set at 7:52 tomorrow morning.

Ready for some math this morning? This will be easy, because you will only have to think about it. Let’s say the Earth’s radius is 4,000 miles.  That is pretty close to its actual value. OK, it is 3961 miles (6,378 km) at the equator. The Earth is nearly spherical. At 4 this morning the Moon passed perigee, its closest point to the Earth, making tomorrow’s full moon a super-moon. Let’s say it will be at today’s distance of 221,600 miles (356,800 km). That’s center to center. At moon rise or moon set the Moon is near that center to center distance, but if it moves overhead it’s 4,000 miles closer because we are on the Earth facing the Moon. Even though the Moon looks smaller than when it appears on the horizon. It’s an optical illusion that the Moon appears larger when it is rising. Super moon or not.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.


The difference in the Moon's distance at rising (or setting) versus when it is overhead

This illustrates the difference in the Moon’s distance at rising (or setting) versus when it is overhead. BTW today’s perigee puts the Moon at only 56 Earth radii from the Earth. The illustration is mine.

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