22.2 The Earth-Moon-Sun System

MOTIONS OF THE EARTH Rotation The motions of Earth are: Rotation – turning, or spinning, of a body on its axis Revolution – motion of a body along its orbit around some point in space Precession – slight movement of Earth’s axis. Rotation The Earth is moving in three different ways. It spins on it’s axis, orbits the sun, and wobbles like a top on it’s axis. Results are Day and Night how long does it take for Earth to make one full rotation?

Revolution The Earth moves in an Elliptical orbit Perihelion - Earth closest to sun 147x106 km (~January 2) Aphelion – Earth furthest from sun 152x106 km (~July 2) The earth’s orbit around the sun is not a perfect circle. It is an ellipse as show here. When the earth is closest to the sun, in January, it is call perihelion. Peri meaning close and helion meaning sun. when the earth is farthest away from the sun in July it is called aphelion. The moon orbits the Earth in much the same way. When the moon is closest we call that perigee and farthest we call that apogee.

Precession Precession – the motion of Earth’s axis as it traces out a circle Period for the axis to complete one full circle is 26,000 years The earth’s axis wobbles between 21.5 and 24.5 degrees every 41,000 years. The moves as if the axis is drawing a circle in space as shown on in the picture on the top left. This movement impacts the global climate. The smaller the angle of the earth’s tilt the less variation in global climate zone, the greater the angle the more extremes there are between the poles and the equator. Earth’s axis tilt varies between 21.5° and 24.5° Presently axis is pointing at North Star and is 23.5°

Solar system in motion Solar System speeds in the direction of the star Vega at Solar System revolves around the Milky Way Galaxy at speeds will take 230 million years to revolve around Milky Way Galaxy Milky Way Galaxy is approaching the Andromeda Galaxy The earth is part of the solar system which is only one of 300 billion solar systems with in the milky way galaxy, which is only one of billions of galaxies in the universe. Our solar system is spinning through our galaxy which is also moving through space.

MOTIONS OF THE EARTH-MOON SYSTEM Moon orbits Earth in about one month, counterclockwise Perigee – Moon is closest to Earth Apogee – Moon is furthest from Earth Moon’s distance to Earth varies, average of 384,401 km Why does the distance from Earth to the Moon vary?

Phases of the Moon Phases of the moon - changes in how much of the sunlit side of the moon faces Earth Half of the moon is lit at all times We see a portion of the moon depending where the moon is in relation to the sun and Earth Take a close look at this picture. When the moon is between the earth and the sun the side facing away from the earth is lit by the sun,so we do not see the moon. This phase is called New moon. As the moon revolves earth from new moon to full moon it is said to be waxing. When the moon is less than half full it is called a crescent, when it is more than half full it is called gibbous. When the Earth is between the moon and the sun the lit side of the moon faces the earth and is a full moon. As the moon moves from back towards the sun and less and less is visible from earth it is called a waning moon.

Lunar Motions Rotates on it’s axis every 27 1/3 days Revolves around Earth every 27 1/3 days Same side faces Earth because period of rotation and revolution is the same Try this, stand up and walk around your chair facing you computer the entire time. You have made one full revolution, but you did not rotate. Now walk around your chair again, this time keep your right shoulder facing your chair the entire time. This second time you revolved and rotated at the same time. And you did it at the same rate. Now image the chair as the earth and you as the moon. If the moon moved as you did the 1st time we would be able to see all sides of the moon each month. Yet the moon does not, it moves as you did the second time, so we earthlings only see one side of the moon.

ECLIPSES Solar Eclipse – Moon moves between Earth and Sun, during new-moon phases Lunar Eclipse – Moon moves into Earth’s shadow, during full-moon phases An eclipse can only take places if the sun, Earth and moon are perfectly aligned A lunar eclipse is when during the full moon phase the earth is directly between the sun and the moon so moon is no longer visible because it is not reflecting the sun’s light. During a solar eclipse all but the outer most portion of the sun is blocked by the sun, so only the corona of the sun is visible from earth. Only part of the world can see a solar eclipse because it only happens during the day, when it’s day time in one part of the world its night time on the other side. During the year, how often can an eclipse happen? Why?

Summary review use the following questions to write your summary What are the two main motions of the Earth? What is a precession? Why does the same side of the moon always face Earth? What kind of eclipse happens when the Earth comes between the sun and the moon?