Chapter 3Chapter 3Eclipses and the Motion of the MoonEclipses and the Motion of the Moon

The phases of the Moon were introduced in Section 1, but without discussion of eclipses of the Sun and Moon.

1. Because of the Moon’s 5°.16 orbital tilt relative to Earth’s orbital plane (the ecliptic), eclipses can only occur at specific times ― eclipse seasons, roughly every 5½ months apart.

2. The observable phenomena during eclipses are quite specific, and prove useful for dating events in ancient history.

When eclipses can occur.

Syzygy occurs when the Sun, Moon, and Earth are aligned.

Geometry for solar eclipses.

Phases for a solar eclipse: partial phases, totality or annularity, etc.

Solar eclipse to scale.

Situations for different eclipse types/

Annular solar eclipses occur when the Moon’s shadow cone does not reach Earth.

Diamond ring effect during a solar eclipse.

Eclipsed Sun.

Time sequence of a solar eclipse.

Solar chromosphere and corona during totality.

Solar corona and surface activity.

Where solar eclipses are seen.

Solar eclipse sites in the present era.

Situation for a lunar eclipse.

Eclipse of the Moon.

Moon moves through Earth’s shadow, which is 2½ times larger.

Moon during totality.

Geometry of a lunar eclipse.

Geometry to scale.

Solar and Lunar Eclipse LimitsEclipses of the Sun or Moon can only occur when either body is within a specific distance of a node of the Moon’s orbit − where it crosses the ecliptic. Such intervals are known as eclipse seasons. They are when eclipses can occur.

The Earth’s shadow cone has a smaller dimension on the antisolar side of the Moon’s orbit than on the sunward side, so lunar eclipse seasons are only 1924 days long, whereas solar eclipse seasons are 3137 days long. Since the synodic period of the Moon is ~29½ days long, a lunar eclipse may not occur during an eclipse season, but a solar eclipse must occur. If 2 solar eclipses occur during an eclipse season, a lunar eclipse must occur halfway between them.

Similar eclipses occur in a recurrent pattern every 18 years, 11⅓ days (6585.32 days), called the saros interval, so take place from almost the same place on Earth every 3 saros cycles (to make a complete day).

Solar eclipses are important for providing a natural means of viewing the solar corona and chromosphere. Lunar eclipses are important for establishing the physical properties of rocks on the lunar surface.

Because the Moon’s orbit regresses in space, the nodes precess backwards along its orbit. An eclipse year is only 346.62 days long, implying that eclipse seasons begin about 3 weeks earlier each year than the year previous.

Eclipse seasons inferredfrom the Moon’s dailypositions during January2014.

Note that eclipses canonly occur in late Apriland early November.

The Moon’s orbit is inclined to the eclipticby ~5½°.