The Earth, Sun, and Moon

The Earth, Sun, and Moon

STONEHENGE

Stonehenge • Built 2800-1800 B.C. • It’s the oldest timepiece in existence • Designed for astronomical observation • It’s a monument in S. England • Helps in predicting eclipses & cycles of • moon • Aligns at the solstices

Map of Stonehenge

Stonehenge Astronomy

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 alsorevolvesabout 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 arezodiac constellations. Because the Sun is bright, we can only see some constellations at certain times of year.

Motions of Earth • Revolution is the motion of a body, such as a planet or moon, along a path around some point in space. • Perihelion is the time in January when Earth is closest to the sun. • Aphelion is the time in July when Earth is farthest from the sun.

Motions of Earth • Earth’s Axis and Seasons • The plane of the ecliptic is an imaginary plane that connects Earth’s orbit with the celestial sphere. • Because of the inclination of Earth’s axis to the plane of the ecliptic, Earth has its yearly cycle of seasons.

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 thevernal(orautumnal) equinox.

The Day Because the Earth revolves about the Sun, the word “day” has two definitions. A sidereal dayis one rotation with respect to the stars. A solar dayis 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 alsoprecesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of thisprecession of the equinoxesis about 26,000 years.

Because of precession: • Polaris won’t always be the North Star • The solar system speeds in the direction of the star Vega. • Earth is presently approaching one of its nearest galactic neighbors, the Great Galaxy in Andromeda

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 theMoon’s siderealperiod with respect to the stars (27 days), and thesynodicperiod with respect to the Sun (29 days).

Months are defined on the basis of the moon’s revolution around the earth • Perigee is the point at which the moon is closest to Earth. • Apogee is the point at which the moon is farthest from Earth

Synodic Month: One lunar revolution from full moon to full moon. Defined relative to the positions of the earth and the moon. Sidereal Month: One lunar revolution measured with respect to the fixed stars. Months and Years are not composed of even numbers of 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 calledopposition. New Moon occurs when the Moon is in the same direction as the Sun, i.e., inconjunction.

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 alunar eclipse. This happens roughly twice a year.

This is an animation showing a total lunar eclipse from start to finish.

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

Here is an animation showing a total solar eclipse

https://vimeo.com/122795016

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 apartial 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.

The Earth, Sun, and Moon