The most exciting astro event of 2021 is set for early tomorrow morning here in Colorado. As projected by the Fiske Planetarium, the Moon will be low in the western sky when the eclipse begins at 5:11 a.m. MDT, while the sky is turning twilight blue with the approach of sunrise. The eclipse will reach its maximum at 5:18. The Sun will then rise at 5:36 and the moon will set at 5:43.
Early-rising observers can see the partial eclipse begin at 3:44 a.m. MDT, progressing into full eclipse from 5:11 a.m. until 5:25 a.m. Sunrise will come and the Moon will be full, and also a “super moon.” That means the Moon will be closer to the Earth than for any other full Moon this year.
"Supermoon” is not a scientific term but a popular name referring to full moons occurring when the Moon is relatively close to the Earth, thus visibly larger than normal. Some observers report there are two a year, following in successive months, while others claim three. But there’s no dispute that this one will be the closest full Moon of the year at 222,048 miles from Earth (16,800 miles closer than the moon’s average distance).
Add to that an optical effect that will make the Moon appear even larger when near the horizon. Then there is the bending of the red light (through Earth’s atmosphere) to produce the reddening of the Moon in eclipse.
According to John Keller, director of the Fiske Planetarium at the University of Colorado:
"It’s going to be really pretty. Having the horizon will accentuate the supermoon and the eclipse. It’s a combination of an optical illusion and the actual size of the moon,”
Keller went on:
“The supermoon can be up to 10% bigger than a non-supermoon. That is a real effect, the Moon is ever so slightly larger. But when the Moon is directly overhead, there is nothing else around it except for stars. It’s having that perspective of the Moon on the horizon, with stuff in the foreground that we know is big, making the Moon look bigger.”
He added that he illusion isn’t happening in your eye. It’s happening in your brain:
“It’s cognitive. It’s not happening at your retina. If you took the size of the moon on your retina at moonset and the size of the moon on your retina when it is at zenith, it would hit the same number of rods and cones on your retina. But your brain takes that signal and changes that into an understanding of what it is seeing.”