The Sun-Earth System: The Sun-Earth System October 2, 2007


Total Solar Eclipse : Total Solar Eclipse When the moon passes between the sun and the Earth The moon blocks out the sun completely Moon has to be in a perfect spot in its orbit to block the sun (think perspective) The path of totality is 167 miles in diameter The area on Earth where the moon TOTALLY eclipses (covers) the sun without any of the sun peaking out From anywhere in the grey penumbra, you will see some part of the sun shining from behind the moon. The penumbra is the area of partial eclipse. Only from within the tiny area where the dark umbra touches the earth will you see the sun completely covered and witness a total eclipse


Annular Eclipse: Annular Eclipse Moon orbit distance isn’t the same all the time Distance varies from 221,000 to 252,000 miles 13% variation in the Moon's distance makes the Moon's apparent size in our sky vary by the same amount Nearside of moon orbit  larger than the sun  total solar eclipse Far side of moon orbit  smaller than the Sun  can’t completely cover the sun Annular eclipse So bright—difficult to see corona


Annular Eclipse: Annular Eclipse


Slide5: Geographic abreviations: n = north, s = south, e = east, w = west, c = central


Revolution: A History: Revolution: A History Ptolemy Copernicus Geocentric model: everything revolves (goes around) the Earth The Earth was believed to be the center of the solar system Stars moved around the Earth through the celestial sphere in a fixed pattern It was believed that the stars were all holes in sphere around Earth and the other planets A huge light was behind the sphere showing through the holes to create stars


Retrograde Motion: Retrograde Motion Planets wander across the night sky changing positions Moving eastward and then westward Retrograde motion Early astronomers did not know how to explain this motion Present day model for retrograde motion


Revolution: Ptolemy: Revolution: Ptolemy Ptolemy was a Greek astronomer living in Egypt in the 100s—no telescope Didn’t understand retrograde motion Developed a system/model for where planets would be in the night sky Planets were on small circular orbits (epicycles) The center of each orbit was the Earth Earth moved on a larger circular orbit (deferent) Retrograde motion occurred when the planet moved along the part of the epicycle that an observer on Earth could see Did not work perfectly; however, was used until the 1500s!


Revolution: Copernicus: Revolution: Copernicus Polish astronomer (1473-1543) who proposed that the sun was the center of the solar system—no telescope Heliocentric model Earth was a planet; all planets orbited around the sun Retrograde occurs because all of the planets orbit at different speeds and distances around the sun


Revolution: Tycho: Revolution: Tycho Tycho Brahe (1500s)—recreational astronomer—no telescope Studied the planets throughout their whole entire orbits Not just at certain points Noticed weird occurrences in Mars’ orbit (retrograde motion) Did not survive past 1601 to apply the data he collected His assistant, Johannes Kepler, continued his work


Revolution: Kepler: Revolution: Kepler Kepler discovered that the reasons why the planets weren’t moving according previous thought because The planets’ orbits were elliptical rather than round Kepler then developed three laws


Kepler’s First Law: Kepler’s First Law Planets travel in elliptical orbits with the sun at one focus Ellipses have two centers (foci) just to the left and right of where a center would be on a circular orbit Thus a planet’s orbit changes in its distance to the sun throughout its orbit


Kepler’s First Law: Kepler’s First Law


Kepler’s Second Law: Kepler’s Second Law The Equal Area Law Each planet moves around the sun such that if an imaginary line were drawn joining the planet to the sun that over an equal amount of time, it would cover an equal area of space This means that the speed at which the planet travels is not equal around the elliptical orbit Planets travel faster when they are closer to the sun Kepler did not know why


Kepler’s Second Law: Kepler’s Second Law


Kepler’s Third Law: Kepler’s Third Law The Harmonic Law The period (P) of a planet (the amount of time it takes a planet to travel one orbit around the sun) squared is equal to the cube of its average distance (D) from the sun P2 = D3 The farther a planet is from the sun, the longer its period of revolution Orbit is larger Moves more slowly than other planets closer to the sun Earth’s orbit = 30 km\s Mars’ = 49 km\s Believed the sun and its rotation was the force that kept all the planets in motion around the sun


Isaac Newton and the Laws of Motion: Isaac Newton and the Laws of Motion Law One: An object will move forever in a straight line at the same speed unless some external force changes its speed or direction The force of gravity keeps the planets orbiting the sun


Newton’s Law of Gravitation : Newton’s Law of Gravitation Law of gravitation: every mass exerts a force of attraction on every other mass The strength is proportional to the masses and inversely proportional to the distance between them Large mass = large gravitational force Small distance = large gravitational force Explains why the planets move at different speeds within their orbits


The Earth’s Axis: The Earth’s Axis The Earth is tilted on its axis at 23.5º (from a perpendicular) pointing toward Polaris Causes seasons One hemisphere is pointed toward the sun Warmer temperatures and longer days Summer At middle latitudes The other hemisphere is pointed away from the sun Cooler temperatures and shorter days Winter At middle latitudes


The Earth’s Axis: The Earth’s Axis Seasons occur because the same amount of energy is spread over different amounts of surface areas Differences in heating Large area = cooler seasons Smaller area = warmer seasons


The Earth’s Revolution: The Earth’s Revolution We know Earth revolves because the night sky changes Moves counterclockwise when viewed from over head in its elliptical orbit Average distance from the sun = 150 million kilometers Closest (perihelion) January 2nd 147.6 mil. km Farthest (aphelion) July 4th 152.4 mil. km