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Celestial Motions and Time

Celestial Motions and Time. Solar Time. Time measured with respect to the Sun. Apparent solar day is the interval between two successive returns of the Sun to the local meridian

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Celestial Motions and Time

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  1. Celestial Motions and Time

  2. Solar Time Time measured with respect to the Sun. • Apparent solar day is the interval between two successive returns of the Sun to the local meridian • Apparent solar time: based on the daily motion of the true sun, the actual position of the Sun in the sky (sundials) • Apparent solar time runs unevenly

  3. Solar Time • Mean solar time is used for accurate timekeeping • Mean solar time is a mathematical construction only and cannot be physically observed • Mean solar day = 24 hours

  4. Solar Time • A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons • Length = 365 days 5h 48m

  5. Sidereal Time • Sidereal time is a measure of the position of the Earth in its rotation around its axis • Very close to, but not identical to, the motion of stars • Sidereal day = 23h56m; 4 minutes shorter than a solar day • Sidereal year = 365 days 6h 9m

  6. Precession • The axis of the earth “wobbles” like a top as it slows down • One complete “wobble” takes about 26,000 years • Result: precession of the equinoxes, changing of north celestial pole

  7. http://calgary.rasc.ca/images/precession.gif

  8. Synodic Time • The synodic month is the time it takes for the Moon to complete one cycle of phases, the time between successive new moons. • The synodic month is measured with respect to the Sun and is approximately 29.5 days. • A sidereal month is 27.3 days

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