It's the first of four consecutive full lunar eclipses to occur in roughly six-month intervals over the next year and a half -- the April 15 eclipse to be followed by another on Oct. 8, another on April 4, 2015, and the last one on Sept. 28, 2015. The run of four is known as a lunar eclipse "tetrad."
Lunar eclipses arrive in three varieties: penumbral, partial and total. A penumbral eclipse is the weakest and most subtle of the three, with the moon only barely passing along the edge of the Earth’s shadow. Often, a partial eclipse isn't even noticeable to the untrained eye. A partial eclipse is when a fraction of the moon is darkened by Earth’s shadow. While a full eclipse happens when the moon, Earth, and sun are perfectly aligned -- the corona of the sun beaming around all edges of the Earth, casting the moon a dramatic dark red color.
Usually, eclipses happen in random orders -- two consecutive partials followed by a full, followed by a partial, followed by a penumbral, and so on. That's why a tetrad of eclipses is considered a special occurrence.
“During the 21st century, there are 9 sets of tetrads, so I would describe tetrads as a frequent occurrence in the current pattern of lunar eclipses,” explained Fred Espenak, who studies lunar eclipse patterns at NASA. “But this has not always been the case. During the three hundred year interval from 1600 to 1900, for instance, there were no tetrads at all.”
"The most unique thing about the 2014-2015 tetrad is that all of (eclipses) are visible for all or parts of the USA," Espenak added.
That is if the weather cooperates.
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