Chapter 2: Early Astronomy

A lunar eclipse.

For thousands of years, people have observed a phenomenon that is rarer and apparently less predictable than the phases of the Moon or the annual cycles of constellations and seasons. Every generation or so, they saw the Sun blotted out by a solar eclipse, and every year or so they saw the Moon blotted out by a lunar eclipse. The sudden disappearance of the Sun and darkening of the sky was a terrifying spectacle. Ancient Greek warriors were frightened into interrupting their battle by a solar eclipse, and Christopher Columbus used prior knowledge of an eclipse in 1504 to gain control over the inhabitants of the island of Hispaniola. Mark Twain borrowed this idea in his book "A Connecticut Yankee in King Arthur's Court." A lunar eclipse is less spectacular, but can also be a fearful sight when the cause is not understood.

Total solar eclipse, revealing the Sun's corona.

We know today that a solar eclipse occurs when the Moon passes between the Earth and the Sun, such that the Moon's shadow falls on the Earth. A lunar eclipse occurs when the Earth passes between the Moon and the Sun, such that the Earth's shadow falls on the Moon.

Comparison of angular diameter of the Sun, Moon, and planets. To get a true representation of the sizes, view the image at a distance of 102.6 = [1 / tan(33.5/60*pi/180)] times the width of the largest circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.26 m away. Planetary angular diameters are from factsheets at https://nssdc.gsfc.nasa.gov/planetary/factsheet/ and Sun/Moon ones are from https://education.gsfc.nasa.gov/eclipse/pages/faq.html .

Total solar eclipses occur due to an amazing coincidence: the Sun and the Moon are the same angular size in the sky! Angular size alone is no indicator of true physical size — two celestial objects with very different sizes just happen to be at distances where they both subtend a ½° angle. So when the Moon passes between the Sun and the Earth, the lunar disk can completely cover the Sun.

Geometry of a Lunar eclipse.

If you watch a lunar eclipse closely, first you'll see the Moon go into partial shadow (the penumbra), and then into full shadow (the umbra).  The penumbra is caused by the finite size of the Sun. It's an extended disk of light, rather than a point source, so there are areas where only part of the light is blocked.  Partial eclipses occur when an object passes through the penumbra, and total eclipses occur when the object is completely within the umbra. Partial solar eclipses sometimes also occur due to slight variations in the Earth-Sun distance. Sometimes the Moon appears to be slightly smaller than the disk of the Sun.  When this happens, the Moon does not completely cover the Sun, and a ring of light shows around the edge of the Moon.  This is called an annular eclipse.

Geometry of a total Solar eclipse.

Why are lunar eclipses so much more common than solar eclipses? The Greeks observed the Earth's curved shadow during a lunar eclipse. By measuring the amount of time the Moon spends in shadow and the shape of the shadow, they could tell the Earth's shadow is much bigger than the Moon. This suggests that the Earth itself is bigger than the Moon. Therefore, it's more likely for the Moon to pass through the Earth's large shadow than it is for a given point on the Earth to pass through the Moon's small shadow.  Also, the Moon's shadow is small and tapers nearly to a point, so that only a small area on Earth a few kilometers wide is fully darkened by a total eclipse of the Sun. A solar eclipse only lasts for a few minutes as the Earth's rotation sweeps the shadow across the surface at close to 1,000 miles per hour. But a lunar eclipse can last longer than an hour and is visible to inhabitants on the entire night side of Earth.