Elizabeth Weise

USATODAY

Sky watchers are getting ready for an evening of special viewing when a total lunar eclipse arrives just after midnight on April 15.

What's more, this begins a rare sequence of four total lunar eclipses expected over the next two years.

Some Christians see this series of so-called blood moons as linked to a biblical prophecy of the End Times.

A lunar eclipse occurs when the sun, Earth and moon line up so the Earth's shadow falls on the moon, darkening it.

The one on April 15 will begin at 1:20 a.m. on the East Coast, according to Sky and Telescope magazine.

"Eclipses are one of the few astronomical events that can easily be enjoyed with the naked eye," though a pair of binoculars brings it into even greater focus, said astronomy writer Gary Kronk.

As it begins, "the Earth's shadow will make a slow crawl across the moon's face, appearing as if there is an increasingly large 'bite' taken out of the moon," said Deborah Byrd with EarthSky.org, an online science magazine.

At first, the full moon will just appear to be a little darker than normal, "but eventually people will notice a much darker arc moving across the moon, with a distinct rusty reddish-brown color," said astronomer Gerald McKeegan at the Chabot Space and Science Center in Oakland, Calif.

The bloody red color the moon takes on during an eclipse is caused by refraction of sunlight by the Earth's atmosphere.

"It is the same effect that you see when the sun turns reddish-orange at sunset, only in this case the refracted sunlight projects all the way to the moon," McKeegan said.

This eclipse also features an extra astronomical quirk. Mars will appear "as a fiery red 'star' next to the moon. Together red Mars and the red shadow on the moon's face should be a spectacular sight and an incredible photo opportunity," said Byrd.

The only downside to this month's lunar eclipse is that it comes very late at night for most of America.

However, many observatories and science museums will have special events during the eclipse. For those where it's cloudy, the Griffith Observatory in Los Angeles will be streaming the event live.

The April 15 eclipse marks the start of a lunar tetrad. That event occurs when there are four successive total lunar eclipses, with no partial lunar eclipses in between, each of which is separated from the other by six full moons.

A book out last year, Four Blood Moons: Something is about to change, suggests the event could be a fulfillment of biblical prophecy.

In it, John Hagee writes that the red disks of the moon during the full eclipses are referred to in the book of Joel 2:31: "The sun will be turned into darkness, and the moon into blood, before the great and terrible day of the Lord come."

The other three eclipses of this lunar tetrad are on Oct. 8 of this year and then April 8 and Sept. 28 of 2015.

Tetrads are by no means unknown, says Byrd. There will be a total of eight lunar tetrads between 2001 and 2100.