'Blood Moons' Explained: What Causes a Lunar Eclipse Tetrad? (Infographic)

Diagrams explain how eclipses work.

Editor’s note: To find out more about the rare supermoon lunar eclipse of Sept. 27-28 and how to see it, visit: Supermoon Lunar Eclipse 2015: Full ‘Blood Moon’ Coverage . Such an event won’t happen again until 2033.

The word “eclipse” means to obscure. When the moon passes between the Earth and the sun, it’s called a solar eclipse . When the Earth passes between the sun and the moon, it is a lunar eclipse.

The Earth casts its shadow far out into space, beyond the orbit of the moon. Once in a while the moon passes through the shadow , and an eclipse occurs. [Amazing Total Lunar Eclipse Photos ]

Four Blood Moons: Total Lunar Eclipse Series Not a Sign of Apocalypse

Lunar eclipses occur in pairs with solar eclipses, two weeks apart. This is because the Earth, sun and moon must be aligned for an eclipse to occur, and the alignment can only happen twice during the moon’s month-long orbit.

A lunar eclipse lasts for hours as the moon slowly orbits through the Earth’s shadow.

If the moon passes through the edge of Earth’s shadow, it’s called a partial eclipse. Passage through the dense center of the shadow is a total lunar eclipse .

Four Blood Moons: A Tetrad Of Lunar Eclipses Explained | Video

A sequence of four total lunar eclipses including no partial eclipses is called a tetrad. In 2014-2015, four lunar eclipses will occur. This sequence features eclipses on April 15, 2014; Oct. 8, 2014; April 4, 2015 and Sept. 27, 2015.

Earth’s shadow is red at the edges for the same reason a sunset is red: When sunlight is scattered by passing through Earth’s atmosphere , the other colors of the spectrum are removed.

In a solar eclipse, the moon casts its shadow on Earth . The darkest part of the moon’s shadow – the umbra — is no more than 166 miles wide (267 kilometers) on the surface of the Earth.

Because the orbit of the moon is tilted relative to the orbit of Earth, eclipses can’t happen at every new and full moon. The shadow cones pass “above” or “below” the moon and Earth most of the time. Only at two points during the year do the sun, Earth and moon line up properly to allow for eclipses.

A lunar eclipse is visible to anyone who can see the moon , which usually means half of the Earth at a time. In a total solar eclipse, the moon’s shadow passes directly over only a narrow strip on the Earth’s surface. Observers in a larger area can see the sun partially eclipsed.

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