The Earth, Sun, and Moon

Diurnal Motion Geocentrially speaking, the Earth sits inside a celestial sphere. Fixed on the sphere are stars. Each day, the Earth rotates once (west-to-east) on its axis. This causes us to face different directions and see different stars. The stars’ daily (diurnal) motion reflects the Earth’s spin.

The Yearly Motion In addition to rotating, the Earth also revolves about the Sun. As the Earth revolves, the Sun is projected in front of different constellations at different times of year. The path the Sun takes across heavens is called the ecliptic. The constellations which the Sun passes through are zodiac constellations. Because the Sun is bright, we can only see some constellations at certain times of year.

The Seasons Since the plane of the ecliptic is tilted 23.5° with respect to the celestial equator, we have seasons.

The Motion of the Sun From the Geocentric point of view, the Sun moves from the northern part to the southern part of the sky with the seasons. When the Sun is furthest north (south), it’s the summer (winter) solstice. When the Sun crosses the celestial equator, it’s the vernal (or autumnal) equinox.

The Day Because the Earth revolves about the Sun, the word “day” has two definitions. A sidereal day is one rotation with respect to the stars. A solar day is one rotation with respect to the Sun. Sidereal and solar days differ by about 4 minutes.

Precession In addition to its rotation and revolution, the Earth’s axis also precesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about 26,000 years.

Because of precession: Polaris won’t always be the North Star A sidereal year is not quite a Julian year. Orion won’t always be a winter constellation Horoscopes are all wrong!

The Lunar Period The Moon moves west-to-east in the sky (like the Sun), and takes about a month to circle the Earth (hence the word month). But once again, there’s a difference between the Moon’s sidereal period with respect to the stars (27 days), and the synodic period with respect to the Sun (29 days).

Phases of the Moon The Moon reflects light from the Sun, so its phase tells you the relative position of the Sun. You can therefore tell time from the Moon!

Phases of the Moon Full Moon occurs when the Moon is opposite in the sky from the Sun. This is called opposition. New Moon occurs when the Moon is in the same direction as the Sun, i.e., in conjunction.

The Moon’s Shadow The Moon’s orbital plane is tilted by 5.2° from the ecliptic plane. Hence, ½ the time, the Moon is slightly north of the ecliptic (and ½ the time, it is south of the ecliptic). The shadow of one body very rarely falls on the other.

Lunar Eclipses If the Moon crosses the ecliptic plane while exactly opposite the Sun, it will fall in the Earth’s shadow. This is a lunar eclipse. This happens roughly twice a year.

Solar Eclipses If the Moon crosses the ecliptic plane while exactly in front of the Sun, a tiny piece of the Earth will fall in the Moon’s shadow. This is a solar eclipse. Since the Moon and Sun appear to be the same size in the sky, the alignment of a total solar eclipse must be perfect.

Total Solar Eclipse You only get to see this outer atmosphere (corona) of the Sun if the Sun’s entire body is blocked out.

Partial Solar Eclipse If you’re on the edge of an eclipse path and only a slice of the Sun is blocked out, it’s called a partial eclipse. These are not very exciting, since the uneclipsed part of the Sun is still extremely bright.

Annular Eclipses Because the Moon’s orbit about the Earth is not perfectly circular, it is sometimes too far away to block out the whole Sun, even when perfectly aligned. When this happens, you get an annular eclipse. It is rarer than a total solar eclipse, but, like other partial eclipses, they are, in general, pretty boring.

Solar Eclipse Paths through 2025

Next time -- The Problem with Planets