The Earth-Moon-Sun System Chapter 22, Section 2

Motions of the Earth The two main motions of Earth are rotation and revolution Rotation – the turning, or spinning, of a body on its axis Revolution – the motion of a body, such as a planet or moon, along a path around some point in space Precession – the slight movement, over a period of 26,000 years, of Earth’s axis

Rotation The main results of the earth’s rotation are night and day There are two kinds of days! Mean Solar day – time interval from one noon to the next (~ 24 hours) Sidereal day – the time it takes for the Earth to make one complete rotation with respect to a star other than our sun (23 hours, 26 minutes, and 4 seconds) We use the mean solar day, and most astronomers use the sidereal day

Solar Day vs. Sidereal Day

Revolution Earth revolves around the sun in an elliptical orbit at an average speed of 107,000 km per hour At perihelion, Earth is closest to the sun (~147 million km away), this occurs ~ January 3rd each year At aphelion, Earth is farthest from the sun (~152 million km away), this occurs ~ July 4th each year The apparent annual path of the sun against the celestial sphere (stars) is called the ecliptic The paths of the planets travel near the ecliptic

Earth’s Orbital Motion

Earth’s Axis and Seasons From the reference angle of the ecliptic, the Earth’s axis of rotation is tilted 23.5 degrees Because of this tilt, we have our yearly seasons The equinoxes mark when the sun has reached the intersection of the celestial equator, and the solstices are when the sun has reached 23.5 degrees with respect to the celestial equator

Earth’s Axis and Seasons

Concept Check What phenomena result from Earth’s rotation and revolution? Rotation: night and day Revolution: seasons

Precession The Earth’s axis maintains around the same angle, but the direction it points is always changing The result is our axis “drawing” a circle in the sky Think of a spinning top! It has a period of 26,000 years, meaning in the year 14,000 the axis won’t be pointing at the North Star (Polaris), but instead at Vega (which then becomes our North Star)

Precession

Concept Check In the year 14,000, what star will be the new “North Star”? Vega

Earth-Sun Motion In addition to its own movements, the Earth accompanies the sun as it speeds toward the star Vega, at ~ 20km/s The sun, like other stars around it, revolve around the galaxy, this trip takes 230 million years at speeds approaching 250 kilometers per second Earth is presently approaching the Great Galaxy in Andromeda

Earth-Sun Motion

Concept Check In what ways does Earth move? Earth revolves around the sun, rotates on its axis, and moves slightly on its axis. It also revolves around the Milky Way.

Motions of the Earth-Moon System Earth has one natural satellite, the moon Our moon orbits Earth within a period of about one month Because the moon’s orbit is elliptical, its distance to Earth varies averaging at 384,401 kilometers Perigee – the moon’s closest approach to Earth Apogee – the moon’s farthest approach

Earth-Moon System

Phases of the Moon Phases of the Moon – a change in the amount of the moon that appears lit, happens on a monthly basis Lunar phases are a result of the motion of the moon and the sunlight that is reflected from its surface The percentage that is visible depends on the location of the moon with respect to Earth and the sun When the moon lies between the sun and Earth, none of its bright side faces Earth (New Moon) When Earth is between the moon and the sun, all its lighted side face Earth (Full Moon)

Phases of the Moon

Lunar Motions Synodic Month – the cycle of the moon through its phases (~ 29 ½ days) Sidereal Month – the true period of the moon (amount of time it takes to revolve around Earth) is ~ 27 1/3 days The moon’s period of rotation about its axis is the same as its revolution around Earth, resulting in us only ever seeing one side of the moon Only satellites and astronauts have ever seen the “far” side of the moon

Lunar Motions

Concept Check Why does the same side of the moon always face Earth? The moon’s period of rotation about its axis and its revolution around Earth are the same.

Eclipses Solar Eclipse – when the moon moves in a line directly between Earth and the sun it casts a dark shadow on Earth (occurs during new moon phases) Lunar Eclipse – the moon is eclipsed when it moves into Earth’s shadow (occurs during full moon phases) The moon’s orbit is inclined about 5 degrees to the plane that contains Earth and the sun, so its shadow will often miss Earth During a new-moon or full-moon phase, the moon’s orbit must cross the plane of the ecliptic for an eclipse to take place This only occurs four times a year (in pairs, a solar and a lunar) at the equinoxes

Solar Eclipse

Lunar Eclipse

Concept Check Why don’t eclipses happen during every full-moon or new-moon phase? The moon’s orbit must cross the plane of the ecliptic for an eclipse to happen.

Assignment Read Chapter 22, Section 2 (pg. 622-629) Do Chapter 22 Assessment #1-34 (pg. 639-640)