Saturday's Eclipse Features An Impossible Trick of the Moon: The Selenelion

Earth will be treated to a third Blood Moon eclipse in a year on Saturday, and it'll be historic in more than just one way.

The eclipse will only be around for five minutes in the early morning of April 4. But just as interesting, it's smack in the middle of this year's tetrad (four eclipses in a row) and even better, we may even get to witness the even rarer – and pretty much impossible – selenelion.

EarthSky pointed out this celestial wonder: an impossibility that lets us Earthlings witness the eclipsed moon setting as the sun rises, or the converse – the eclipsed moon rising as the sun sets.

Why is this impossible? It's a matter of simple geometry: Saturday's eclipse is only happening because the sun and moon are 180 degrees apart in the sky. With the celestial bodies perfectly in place for the big show, it would seem against nature for the human eye to catch them together in the sky.

But it is possible, thanks to something called atmospheric refraction – EarthSky explained that it's the same thing that makes a spoon dipped in a glass of water look like it's in two pieces.

"You might actually see images of the sun and totally eclipsed moon, both above your horizon at once, lifted up by the effect of refraction."
But you have to be as perfectly positioned as the sun and moon to glimpse a selenelion on Saturday; here in the U.S., only people in Louisiana, Arkansas, Missouri, Iowa, and Minnesota can catch it.

But, like everyone else trying to catch Saturday's eclipse, you'll have to be very quick. The event will be extremely short – less than five minutes, actually, making the selenelion even rarer.

As for the tetrad, Universe Today noted that this century will witness eight of them, which is all the more remarkable when you consider that there were none from 1600 to 1900. Saturday's eclipse is the third in this year's tetrad, which began in April last year and will finish this September.

People in the western U.S., Hawaii, Japan, New Zealand, and Australia will be able to watch the entire eclipse in all its five-minute glory on Saturday. From the Midwest east, people will only see the partial phase, which is when it creeps into Earth's shadow. This begins at 5:16 a.m. CDT; totality follows at 6:58 a.m. Saturday morning.

The moon will also be much lower in the western sky that it was during the last event – which also makes it pretty unique, Sky & Telescope added. And because it will sneak into Earth's shadow so slightly on Saturday, the eclipse will create a range of red shades – from bright red at its Northeastern corner, to deep red across the rest.

The depth of that red will depend on how much volcanic ash and dust is in the atmosphere; if a volcano recently exploded, it'll be dark red, grey or black. Only Saturday's historic eclipse will tell.

[Photo Courtesy Uriel Sinai/Getty Images]