On the night of December 20/21, sky watchers across North America will be treated to one of Nature’s rare and grand spectacles — a total eclipse of the Moon!
On Monday night/Tuesday morning, the full Moon turns rusty red as it glows high in the south in the middle of the star-rich winter Milky Way. Its position in the sky couldn’t be better. The Moon will be as high as it can possibly be! There won’t be a total lunar eclipse this far north on the sky’s dome for several centuries!
To visualize the geometry of a lunar eclipse, picture the sun shining on Earth, which in turn casts a slender cone shaped shadow deep into space. At the Moon’s distance from Earth, the conical shadow is about 2 1/2 times the lunar diameter. When the Moon passes through part of Earth’s shadow, a partial eclipse occurs. On those rare occasions when the Moon, Earth and Sun are all lined up just right with Earth in the middle, the full Moon glides through the core of Earth’s dark shadow and creates a total eclipse of the Moon!
The visible eclipse begins Monday, Dec. 20th, at 10:33 pm PST (Tuesday at 01:33 am EST). At that time, Earth’s shadow will appear as a dark-red bite at the edge of the lunar disk. It takes about an hour for the “bite” to expand and swallow the entire Moon. Totality begins at 11:40 pm PST (02:40 am EST) and lasts for 72 minutes. A partial eclipse lasting for nearly the same period of time will precede and follow the total eclipse. The entire eclipse from start to finish will last about 3.5 hours. By coincidence, this eclipse happens early on the day of Winter Solstice, December 21.
From a dark observing site, you’ll be able to watch the sky darken and stars become more visible as the eclipse progresses. But you don’t need a cloudless sky to watch this natural phenomenon — just a clear view of the Moon. A total lunar eclipse is a splendid sight seen through binoculars or any telescope at low power. This event might be a chilly one to witness, but it will be worth it! So dress up warmly and enjoy the show.
If you’re planning to run out for only one quick look, the best time is very early Tuesday morning at 16 minutes past midnight PST. (03:16 am EST) That’s the midpoint of totality and the eerily glowing reddish Moon will be in deepest shadow then.
Larry Pawlitsky is a local amateur astronomer