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Orbits of Earth and Moon. Looking at a two-dimensional diagram, you might wonder why there isn't an eclipse every month. The answer is that the Moon's orbit is slightly tilted with respect to the Earth's orbit. The two points where its orbit crosses Earth orbit are called nodes. Only if the Moon is full when it's near a node do the Sun, Earth and Moon line up for a lunar eclipse. ©Mona Evans, “Lunar Eclipses”

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An Earth and a Lunar Eclipse Seen From Space

Sun eclipsed by Earth (left) and Moon (right). (Image credit: NASA/SDO) The Solar Dynamics Observatory orbits Earth in such a way that very occasionally it lines up with Earth & Sun (or Moon & Sun) to cause an eclipse. The Moon's shadow is better-defined & more curved than Earth's, because the Moon has no atmosphere, and is smaller and farther away from the Satellite. ©Mona Evans, “Lunar Eclipses”

Passing the Moon: The first body New Horizons passed after launch was our own Moon, just eight hours and thirty five minutes after liftoff on Jan. 19, 2006. New Horizons reached the closest distance to the Moon before crossing lunar orbit.

Solar Eclipses: An Observer's Guide (Infographic) - When the moon covers up the sun, skywatchers delight in the opportunity to see a rare spectacle. Mona Evans, “Solar Eclipses”

If you were on the Moon during a total lunar eclipse, you'd see a total solar eclipse. It might look something like this, with the Earth's nightside faintly shining. (Credit: Hana Gartstein) Mona Evans, "Lunar Eclipses"

Lunar Eclipse infographic. (Credit: Karl Tate) Mona Evans, “Lunar Eclipses”

Partial lunar eclipse from Hudson, Florida. June 4, 2012. (Photo: David Dickinson) I love the way he's caught the eclipsed Moon near the horizon through the trees. ©Mona Evans, “Lunar Eclipses”

Phases of the Moon. A solar eclipse can only occur at the new Moon because that's the only time that Sun, Moon and Earth could line up with the Moon blocking the Sun from our view. ©Mona Evans, “Solar Eclipses”

Total lunar eclipse. The green light beam is a laser being used to measure the Earth-Moon distance. The laser's target is a retroreflector left on the Moon by Apollo 15 astronauts. Timing the return of the laser pulse allows a highly accurate determination of the distance traveled. Performance is improved during the eclipse because direct sunlight is blocked. (Credit & copyright: Dan Long) Mona Evans, "Blood Moons and Lunar Tetrads"

A total lunar eclipse occurs during a Full Moon when the Sun, Earth and Moon line up exactly in that order. Light from the Sun (white lines) skirts the Earth’s atmosphere, which bends and reddens it. It reaches and reflects off the Moon back toward the Earth and we see a beautifully colored disk during totality. Credit: NASA with additions by Bob King.

Phases of the Moon. A lunar eclipse only occurs at the full moon because that's the only time the Moon could pass through the Earth's shadow. (Diagram: ©Mona Evans, “Lunar Eclipses”