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### 6 Cards in this Set

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 How does the sky move as Earth moves? -The celestial sphere is a model of the sky, carrying the celestial objects around Earth. Because Earth rotates eastward, the celestial sphere appears to rotate westward on its axis. The northern and southern celestial poles are the pivots on which the sky appears to rotate. -The celestial equator, an imaginary line around the sky above Earth's equator, divides the sky in half. -Astronomers often refer to angles "on" the sky as if the stars, sun, moon, and planets were equivalent to spots painted on a plaster ceiling. These angular distances are unrelated to the true distance between the objects in light-years. -What you see of the celestial sphere depends on your latitude. Much of the southern hemisphere of the sky is not visible from northern latitudes. To see that part of the sky, you would have to travel southward over Earth's surface. -The angular distance from the horizon to the north celestial pole always equals your latitude. This is the basis for celestial navigation. -The gravitational forces of the moon and sun act on the spinning Earth and cause it to precess like a top. Earth's axis of rotation sweeps around in a conical motion with a period of 26,000 years, and consequently the celestial poles and celestial equator move slowly against the background of the stars. How do astronomers refer to stars? -Astronomers divide the sky into 88 constellations. Although the constellations originated in Greek and Middle Eastern mythology, the names are Latin. Even the modern constellations, added to fill in the spaces between the ancient figures, have Latin names. -The names of stars usually come from ancient Arabic, although modern astronomers often refer to a star by its constellation and a Greek letter assigned according to its brightness within the constellation. How can you compare the brightness of the stars? -The magnitude system is the astronomer's brightness scale. First-magnitude stars are brighter than second-magnitude stars, and so on. The magnitude you see when you look at a star in the sky is its apparent visual magnitude, whichd oes not take into account its distance from Earth. -Apparent visual magnitude, mv, includes only the light that human eyes can see. What causes the seasons? -Because Earth orbits the sun, the sun appears to move eastward along the ecliptic throught the constellations. It circles the entire zodiac in a year. -Because the ecliptic is tipped 23.5 degrees to the celestial equator, the sun spends half the year in the northern celestial hemisphere and half in the southern celestial hemisphere. -In the summer, the sun is above the horizon longer and shines more directly down on the ground. Both effects cause warmer weather. How do the planets move? -In addition to the sun, the visible planets also move along the ecliptic, and their positions give rise to the ancient superstition called astrology. -Mercury and Venus follow orbits inside Earth's orbit and never move far from the sun. They are visible in the east before dawn or in the west after sunset. How do astronomical cycles affect Earth's climate? Changes in the shape of Earth's orbit, in its precession, and in its axial tilt can alter the planet's heat balance and seem to be at least partly responsible for the ice ages and glacial periods.