The Sun, moon, and earth as a system Section 22C

The sun’s motions Apparent motion Actual motion The sun’s motion across the sky as we see it Caused by the earth’s rotation Ecliptic – the line marking the apparent path of the sun against the background of the stars as the sun moves around the earth. Actual motion Rotates on it’s axis every 25-36 days Orbits the center of the Milky Way galaxy 300 km/second Moving toward the constellation Hercules The sun is moving toward a point on the eastern side of the constellation Hercules.

The Earth’s motion Revolution Rotation Around the sun once a year Elliptical orbit Elliptical plane Counterclockwise around the sun Rotation On its axis (tilted 23½ degrees) Every 24 hours Counterclockwise as viewed from the north pole The sun and earth are the only two bodies in the solar system that lie exactly on the elliptical plane. Revolution and orbit can be used interchangeably. The earth revolves counterclockwise to the sun as viewed from above the elliptical plane. The earth rotates from east to west as from above.

The Moon’s Motions Revolution Rotation Elliptical orbit Around the earth On its axis (tilted 5.4 degrees) Counterclockwise as viewed from above Average distance from the earth is 384,400 km Perigee – 362,600 km Apogee – 405,400 km Rotation The moon’s orbital period is equal to its rotational period. Libration – the slight wobbling of the moon as it orbits All of the objects in our solar system have an elliptical orbit. Average distance – 238,900 mi Perigee – 225,400 mi Apogee – 252,000 mi Moon looks slightly larger at its perigee. Sometimes we can see “beyond the edge” of the moon’s face at times because of its libration.

The moon’s phases

terms Lunar phase – the shape and change of the lighted face of the moon Near side – always faces Earth Far side – always faces away from Earth Light side – surface of the moon facing the sun Dark side – surface of the moon in shadow Terminator – line between dark and light sides Waxing – gradually increasing Waning – gradually decreasing

Bell Ringers Learning Target: I can describe a solar and a lunar eclipse. Bell Ringer: Draw and label the phases of the lunar cycle. Selenology – study of the moon’s geology

Times and seasons Two reference points for marking days, months, and years sidereal – distant star (coming back to the same place on ecliptic plane) solar or synodic – sun

Sidereal month – 27.3 days Synodic month – 29.53 days

The year Julian year – 365.25 days Calendar year – 365 days Leap year – includes one extra day Every four years

The month Time it takes the moon to make one complete orbit of the earth Synodic month – 29.53 days Lunar month

The day One full rotation of the earth The time from one local noon to the next

The seasons Repeated, natural changes in temperature, precipitation, and the amount of daylight Caused by the revolution and tilt of the earth

The Seasons Summer solstice – north pole is pointed directly at the sun The sun’s rays hit the Tropic of Cancer at 90 degrees Longest day of the year June 21 Winter solstice – south pole is pointed directly at the sun The sun’s rays hit the Tropic of Capricorn at 90 degrees Shortest day of the year December 21

The seasons Equinox – the poles are not pointing toward or away from the sun Equal light and dark hours Spring equinox – March 20 Fall equinox – September 20

eclipse One astronomical body blocks the sun’s light to another body

Solar eclipse The moon blocks the sun’s light The moon must be in one of the two points in its orbit where it crosses the ecliptic plane. The moon must come between the sun and the earth. Total solar eclipse – the moon completely blocks the sun Umbra – area of total darkness Only lasts a few minutes Partial solar eclipse – only part of the sun is blocked Penumbra – partial darkness Umbra is only about 100 miles wide

Image provided by: Maximilian Reuter; Maximilian's website Summary Author: Maximilian Reuter; Susanne Pfeifer; Jim Foster August 2011 Earth Science Picture of the Day Viewer's Choice The image above shows the total solar eclipse of March 29, 2006 as observed from the MSG satellite, in geostationary orbit 22,369 mi (36,000 km) above the equator. Note that the eclipsed area, where the shadow of the full Moon reached the Earth's surface, lies over the cloudless, east central Sahara Desert. The region that experienced a total solar eclipse at the time this image was acquired (10:00 UTC) is located at the center of the deeply shadowed region (umbra). This region has a diameter of about 112 mi (180 km). The dark region (penumbra) just outside the deepest shadow experienced a partial solar eclipse.

Annular eclipse – moon is at its apogee and doesn’t completely block the sun

Lunar eclipse The earth blocks the sun’s light from the moon Only happens at a full moon Moon’s orbit must be at the ecliptic

Tidal Forces The gravitational pull of the sun and moon can create ocean and earth tides Earth tides – the earth’s crust is pulled – up to 55cm can affect earthquake and volcanic activity