In late December the Sun follows a southerly path across our Western Slope skies, and daylight hours are short. December 21 at 9:48 p.m. MST marks the solstice, when the Sun shines directly overhead at 23.44 degrees south latitude. Solstice means “still stand of the Sun” in Latin, which refers to the fact that the Sun appears to linger far to the south this time of the year. But the Sun starts moving slowly northward just after the December solstice. This fact was celebrated in ancient Rome around December 25 as Dies Natales Solis Invicti, which translates as: “The Birthday of the Unconquered Sun.” Some historians have suggested that early Christians adapted this date for Christmas.
The last full Moon of 2015 occurs at 4:11 a.m. MST on Christmas morning, December 25. The full Moon stands exactly opposite the sun in our sky, so it will rise around sunset on Christmas Eve, December 24, climb very high in our sky by midnight, and set around sunrise on Christmas Day. If it’s clear, watch the full Moon rise at about 4:46 p.m. on Christmas Eve. Does the rising Moon look especially large to you?
Most people perceive the rising Moon to be much larger than the Moon as seen high in the sky. This effect is called the “Moon illusion.” The Moon is actually about 4,000 miles further away from us when it’s on the horizon, than when it’s high in our sky. Those 4,000 miles correspond with the radius of the Earth. So, the Moon’s apparent size is actually 1% to 2% larger when it’s overhead. Even though this is easily proven by photography, the full Moon still looks huge to me when it’s rising. But, the explanation for the “Moon illusion” lies in the field of psychology, not astronomy.
You’ve been listening to Western Slope Skies, produced by members of the Black Canyon Astronomical Society. This episode was written & recorded by Art Trevena.