Chapter 22 Origin of Modern Astronomy Section 2 The Earth-Moon-Sun System Notes 22-2

Vocabulary Rotation Revolution Precession Perihelion Aphelion Perigee Apogee Phases of the moon Solar eclipse Lunar eclipse

Motions of Earth Rotation: the turning or spinning of a body on its axis o Main result is day and night o Standard for telling time (24 hours in one rotation) o Sidereal day: time it takes for Earth to make one complete rotation (360 degrees) with respect to a star other than our Sun Measured by the time it takes a star to reappear in the same position it was the day before Occurs 4 minutes earlier each day o This would cause noon(12pm) to be midnight(12am) after 6 months of time had gone by

Motions of Earth Revolution: motion of a body on its orbit around a point in space Earth revolves around 107,000 km/hr o Avg distance from the Sun is 150 million km o Perihelion: closest to sun 147 million km January 3 o Aphelion: farthest from sun 152 million km July 4 Also appears to move the constellations in the night sky o Ecliptic: the apparent path of the sun against the celestial sphere

Motions of Earth Ecliptic: the imaginary plane that connects the Earth’s orbital path with the celestial sphere o Placement of the Equator is the celestial equator Earth is tilted 23.5 degrees o This gives the earth its seasons o Spring Equinox (March 20 or 21) – Sun appears at Equator o Autumnal Equinox (Sept 22 or 23) – Sun appears at Equator o Summer Solstice (June 21 or 22) – Sun appears at Tropic of Cancer o Winter Solstice (Dec ) – Sun appears at Tropic of Capricorn

Motions of Earth Precession: wobble of Earth on its axis every 26,000 years o Varies in tilt between 21.5 and 24.5 degrees o Affects climate changes; can cause ice ages and warm ups o North Star is currently Polaris In 13,000 years, precession will cause Vega to the North Star Earth-Sun motion o The whole solar system is moving around the center of our galaxy (Milky Way) at 250 km/s o The galaxy is approaching our neighboring galaxy, Andromeda

Motions of Earth-Moon System Moon orbits the Earth in a counterclockwise direction when observed from the North Pole. The path of the moon is elliptical o Perigee: the moon is closest to the Earth o Apogee: the moon is farthest from the Earth Phases of Moon o Changes in the amount of the moon that appears lit o Caused by how much of the sun lit side is facing Earth o Waxing phases: causes more and more of the moon to appear o Waning phases: causes less and less of the moon to appear es_calendar.phtml es_calendar.phtml

Motions of Earth-Moon System Lunar Motions o Synodic Month: (29.5 days) cycle of the moon through its phases Only an apparent time period of the moon around the Earth o Sidereal Month: (27 1/3 days) true time period for the moon to revolve around the Earth o Moon’s rotation and revolution around Earth are the same Same side of the moon always faces the Earth This is where the dark side of the moon comes from o Only satellites and astronauts have seen the other side of the moon o More cratered than the side facing the Earth Same side of the moon faces the sun for about two weeks o Causes a very high temp on the sunny side of the moon – 127°C o Dark side temp – (-173°C)

Motions of Earth-Moon System Eclipses o Solar Eclipse: when the moon moves in a line directly between the Earth and the sun…this casts a dark shadow on the Earth o Lunar Eclipse: when the Earth is between the sun and the moon…this casts a shadow on the moon o - NASA’s Official Eclipse Website o - Space.com

Eclipse Why isn’t there a solar eclipse every new moon and a lunar eclipse every full moon? Moon’s orbit is inclined about 5 degrees to the plane of the Earth and the sun’s orbit Most of the time the shadow created during a new moon and full moon miss the Earth. The moon’s orbit must cross the plane of the ecliptic for an eclipse to take place

Eclipse Lunar Eclipse: o Can see the shadow of the Earth moving across the moon surface o At completion, the moon is visible as a copper colored disk This is because the Earth’s atmosphere bends and transmits long- wavelength light (red) into the shadow o Can last up to 4 hrs and is visible to anyone on the side of the Earth facing the moon Solar Eclipse: o Moon moves between the sun and Earth. o Can see the sky darken while in the shadow of the moon Only the outer edge of the sun (corona and chromosphere) can been seen o Total eclipse happens when the viewer is in the umbra Partial eclipse happens when the viewer is in the penumbra o Next Total eclipse in the US is August 21, 2017 From Oregon to South Carolina

Lunar Eclipse

Solar Eclipse