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15 degrees 46 seconds 48 minutes American Astronomical Society apparent motion ASSEMBLING THE SUNDIAL base Big Dipper clock bright called the tropical Celestial Equator centuries complete one wobble constellations day of summer day-night cycle daylight days long diagram difference directly overhead divisible by 400 Earth's axis EARTH'S ORBIT equals 365 days equatorial ring fictitious sun frame full moon gnomon Gregorian Calendar Hipparchus inches interval Julian Calendar Julius Caesar Jupiter lunar Mars mean sun measure the length midnight Moon's phase move westward natural time units Naval Observatory noon mark North Pole North Star Northern Hemisphere number of days Orion planets pointers Polaris Pope Gregory XIII precession pull the Earth's revolves seasons seconds of mean set their clocks shadow sidereal day slit Space travelers speed standard star clock tilted Tropic of Cancer Tropic of Capricorn twelve o'clock position United States Naval Vernal Equinox weeks winter York zone