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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
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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
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 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
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)
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
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
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)
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
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
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)
