An event that has happened just twice in the past four hundred years, is happening again Tuesday.

All you have to do to see it is step out your front door. It's known as a tetrad, you would probably recognize it as a total lunar eclipse.  The moon will pass completely into the shadow of the Earth.

An eclipse is not rare, but this type of eclipse is.  Not only is it a full lunar eclipse, but it's one of four that will happen about every six months starting Tuesday and running through next year.  It's also called a blood moon, because after it passes into the Earth's shadow it turns red.

"It would be dark if it weren't for the Earth's atmosphere," said UW-L Planetarium Director Bob Allen, "but the atmosphere bends some sunlight into there, refracting or bending effect, and that is the longer wavelength red because the blue is scattered out by the dust particles in the Earth's atmosphere."

This is only the third time since 1900 there has been a tetrad, but there were none in the 300 years prior to 1900 which makes them even more rare. We are lucky here in the U.S., all four will be visible from anywhere in the country, so some parts of the world are missing out.

Seeing a tetrad is rare, but just seeing a regular eclipse is hard enough. "Half of the time you're going to be on the Earth's daytime side when they happen so you're not going to see them," said Allen, "number two between the three types of lunar eclipses, penumbral, partial and total that if it's just a penumbral it's not very exciting, even a partial it's ok, but it's not like a total, so to get a total lunar eclipse you're really talking about once a decade."

The next tetrad won't be until 2032.  Having binoculars and a telescope help, but it can be seen with the eye and they are safe to look at.  The eclipse gets under way just before midnight Monday, but it'll get better at 1 a.m. Tuesday and will be the best from 2 a.m. until 3:30 a.m.

The U-W-La Crosse astronomy club is setting up in the field north of Emerson Elementary School Tuesday morning with some telescopes and binoculars.  They will be there from 1 a.m. until 4 a.m. and the public is free to join them.