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Space Topics: Moon

Eclipses

Solar and lunar eclipses occur when the orbital motions of Earth and the Moon bring them into one line with the Sun.  If the Moon’s orbit around Earth were in the same plane as Earth’s orbit around the Sun, there would be one solar and one lunar eclipse every month, with the solar eclipse at the Moon’s new phase, and the lunar eclipse at the Moon’s full phase.  But the Moon’s orbit is tilted with respect to Earth’s by about 5 degrees.  So most of the time, when the Moon is in its new phase, it does not travel across the Sun’s disk, nor does it enter Earth’s shadow when it is full.  The lining up of Earth, Moon, and Sun only happens a handful of times per year.

The annular solar eclipse of February 16, 1999 This seven-image sequence covers the entire 2.5 hour long annular eclipse of Feb 16, 1999 as seen from Greenough, Australia. Credit: ©1999 by Fred Espenak

Solar Eclipses

When the Moon crosses the Sun as seen from Earth, causing a partial or total solar eclipse.  An eclipse reveals one of the more astonishing coincidences in the natural world: seen from Earth, the Moon’s disk almost perfectly matches the size of the Sun’s disk.  During a total solar eclipse, the Moon just blots out the sphere of the Sun, allowing us to view the Sun’s corona.  Looking at that coincidence in detail:

• The Sun is about 1,400,000 kilometers (900,000 miles) in diameter, and it lies, on average, about 150,000,000 kilometers (93,000,000 miles) away from Earth
• The Moon is about 3,500 kilometers (2,200 miles) in diameter; on average, it lies 380,000 kilometers (240,000 miles) away from Earth.
• The ratio of the Sun’s distance to its size is about 110 to 1 -- and so is the ratio of the Moon’s distance to its size.
• Thus, the Sun and Moon happen to appear to be the same size in our sky. 

Hundreds of millions of years ago, when the Moon was closer to Earth, it would have appeared larger than the Sun.  In the future, when it is farther away, it will fail to blot out the whole disk of the Sun during eclipses.  We just happen to be living at the right few-hundred-million-year period to see this coincidence in the sky.  Actually, because the Moon’s orbit is not perfectly circular, it is sometimes farther from or closer to Earth.  When it is far enough from Earth, the apparent size of its disk actually is smaller than the disk of the Sun.  A solar eclipse at this time is called “annular.”  In an annular eclipse, a ring of the Sun is visible around the edges of the disk of the Moon.

> Tables of Solar Eclipses from 2001 to 2010 and 2011 to 2020

Lunar Eclipses

Just as the Moon sometimes eclipses the Sun as seen from Earth, Earth eclipses the Sun as seen from the Moon.  When Earth eclipses the Sun as seen from the Moon -- in other words, when the Moon enters Earth’s shadow -- we see a lunar eclipse. Although solar and lunar eclipses happen at about the same frequency, two to three times per year, it is much likelier that a human observer will see a lunar eclipse than a solar eclipse.  That’s because solar eclipses are only visible within the narrow path that the Moon’s shadow traces on the surface of Earth.  For a total solar eclipse, that path is about 15,000 kilometers (9,000 miles) long, but only about 150 kilometers (90 miles) wide.  By contrast, when a lunar eclipse happens, every observer on the nighttime hemisphere of Earth can see it.

> Tables of Lunar Eclipses from 2001 to 2010 and 2011 to 2020