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Astronomy Study of the moon, stars, planets and other objects in space Vocabulary • • • • • • • • Geocentrism Heliocentrism Inertia Gravity Ellipse Rotation Revolution Axis • Solstice • Equinox “The Earth is Flat” • Early Europeans believed that the earth was flat-if one sailed far enough, they would fall off the edge of the earth • Exploration by Columbus disproved the myth as did calculations by early scientists A Brief History • Ancient Greeks -observed the sky and the constellations • Named the planets “wandering stars” because they seemed to move into different positions in the sky • Believed that earth is surrounded by rotating dome called the celestial sphere • Believed the earth was the center of the universe-geocentricism A Brief History, cont. • Copernicus-had the idea that the sun was the center of the universe- heliocentricism • The earth and the other planets revolved around the sun • Galileo-first scientist to view space through a telescope-found further evidence to support the heliocentric model of the universe Planetary Orbits • Tycho Brahe-made observations about the positions of the planets • Johannes Kepler-figured out that the orbits of planets around the sun are not circular, but ellipses • Ellipse-an elongated circle or oval shape Inertia and Gravity • Issac Newton-discovered that the forces of inertia and gravity keep the planets in their positions in space • Inertia-a moving object will stay in motion; a stationary object will remain in place • Gravity-attractive force between two objects-force depends on the mass and the distance between them Gravity • Everything on earth is affected by gravity • The earth has gravity due to its mass • Earth’s gravity pulls everything on earth towards its center • Every living thing also has gravity because every living thing has mass-so they are pressing up and away from the earth Gravity, cont. • Gravity holds all the planets in orbit around the sun • Planets, sun, moons all have mass and gravity • Without gravity, the planets would move through space through their inertia Earth’s Rotation • Axis-imaginary line that passes through the earth at the North and South Pole • Rotation-earth’s spin on its axis causes day and night • Earth rotates counter-clockwise so the sun appears to move from east to west • Rotation of the earth on its axis takes approximately 24 hours or 1 day Earth’s Revolution • Revolution-movement of one object around another object • Orbit-the path an object travels in its revolution • Earth’s orbit is not a circle, but more oval shaped • The earth takes about one year to travel around the sun Leap Year • Earth’s orbit around the sun takes 365¼ days • Our calendar year is based on 365 days/year • To take into account the ¼ fraction of a day, one day is added to the month of February every four years-called Leap Year • Leap Year is 366 days long Seasons • Sun’s rays hit the earth more directly at the equator and are more spread out near the poles • Earth’s Tilt-the earth is not straight up and down, but is tilted 23.5° • The earth has seasons because it is tilted on its axis Seasons, cont. • Earth is tilted toward the sun for part of the year and away from the sun for the other part • Hemisphere tilted toward the sun-Summer • Hemisphere tilted away from the sun-Winter • Neither hemisphere tilted toward or away from the sun-Spring and Fall • Distance from the sun has nothing to do with the seasons-when the earth is at its greatest distance from the sun it is summer in the Northern Hemisphere Sun’s angle at different times of the year Solstice and Equinox • Both occur twice a year • Summer Solstice (June 21) and Winter Solstice (December 21) Shortest and Longest daylight hours of the year • Autumnal (Fall) Equinox (September 23) and Vernal (Spring) Equinox (March 21) Light and dark hours are equal