Solar vs. Sidereal Day
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Transcript of Solar vs. Sidereal Day
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Solar vs. Sidereal Day
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An Earth Day
• Sidereal Day: 23 hr 56 min 4 secMotion relative to background stars
• Mean Solar Day: 24 hoursThe average time between meridian crossings of the Sun
• Apparent Solar Day: variesThe actual time between the meridian crossings
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Synodic vs. Sidereal Month
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Lunar Month
• Sidereal month: 27.321662 days. Defined relative to the stars.
• Tropical month: 27.321582 days. Defined relative to the celestial equator.
• Anomalistic month: 27.554550 days. Defined as perigee to perigee.
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An Earth Year
• Based on the motion of the Sun on the sky or the motion of Earth around the Sun
• Julian Year: 365.25 mean solar days. Used in calendars before 1582.
• Gregorian Year: 365.2425 mean solar days. Used in calendars today.
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An Earth Year
• Sidereal Year: 365.256363 mean solar days. Defined by the motion relative to stars.
• Anomalistic Year: 365.259635 mean solar days. Defined as perihelion to perihelion.
• Tropical year: 365.242190 mean solar days. Defined from equinox to equinox. Thus, related to the seasons.
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Definitions Zenith: The point on the sky that is directly
overhead of the observer. Horizon: The great circle on the celestial
sphere that is 90 degrees from the zenith Hour circle: The great circle through the
position of a celestial body and the celestial poles
Meridian: The great circle passes through the zenith and both celestial poles
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Even more definitions
Altitude: The minimum angular distance between the position of a celestial body and the horizon
Azimuth: The angular bearing of an object, measured from North (0 degrees) through East (90 degrees), South (180 degrees), West (270 degrees), and back to North (360 degrees)
Hour angle: The angle between the meridian and an object’s hour circle (west is positive)
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Even more definitions
• Declination: The minimum angular distance from the position of a celestial body and the celestial equator
• Right Ascension: The eastward angle from the vernal equinox to the intersection of an object’s hour circle with the equator
• An hour of angle = 15 degrees
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Motion on the Sky
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Time Zones
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Motion Depends on Declination
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The Sky at the North Pole
• At the North Pole, the North Celestial Pole is at the zenith
• Stars never rise or set• Planets, Moon, and
Sun do rise and set…Why?
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Stars Rise and Set at the Equator
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The Sky at Our Latitude
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The Sky at Southern Latitudes
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Sun Changes Declination
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The Arctic and Tropical Regions
• How are the arctic and tropical regions defined?
• Can you answer this without looking at your books?
• Have you ever been to these regions?
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Back to the North Pole
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Back to the Equator
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Cold Arctic Regions
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Tropical Heat
• In the Tropics, Sun is always close to zenith
• In the Tropics, the sun reaches the Zenith at some point in the year
• Lots of heating due to sunlight
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Land of the Midnight Sun
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The Sky for Navigation