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AST 208 Topics

AST 208 Topics. Time and celestial coordinates. Telescopes. And instruments. The Solar System. The Moon. Celestial Mechanics. Time and the Seasons. Celestial Sphere. What is a day?. A day is defined as the time between two successive upper transits of a given celestial reference point

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AST 208 Topics

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  1. AST 208 Topics • Time and celestial coordinates

  2. Telescopes

  3. And instruments

  4. The Solar System

  5. The Moon

  6. Celestial Mechanics

  7. Time and the Seasons

  8. Celestial Sphere

  9. What is a day? • A day is defined as the time between two successive upper transits of a given celestial reference point • An upper transit occurs when the reference point crosses the meridian moving westward

  10. Apparent solar time • One can use the Sun to measure the length of a day. However, compared to a constant rate clock, the length of the day measured in this fashion changes during the course of the year • Earth’s orbit is not a circle • Earth does not orbit in the plane of the equator, but the plane of the ecliptic

  11. A sun dial can measure apparent solar time

  12. Mean Solar Time • Imagine a fictitious point (the mean sun) that moves at a constant rate along the celestial equator at the average rate of the true sun • Equation of time

  13. The analemma

  14. Time Zones • 24 hours in 360 degrees • Each 1 hour time zone is 15 degrees wide

  15. Greenwich Mean Time = 5 hours later than Eastern Standard Time • Universal Coordinated Time (UT) • Based on atomic clocks • Leap seconds added when the difference between atomic clock time and earth rotation time becomes too big • Close to GMT

  16. Solar Calendars • A sidereal year is the time the earth takes to orbit the sun with respect to a stellar reference point = 365.2564 mean solar days

  17. Precession of the equinoxes

  18. Tropical Year • Year of the seasons: orbital period with respect to the vernal equinox, that precesses about 50 seconds of arc per year = 365.2422 mean solar days

  19. Julian Calendar • Cycle of 3 years of 365 days followed by one year of 366 days • Gradually gets out of sync with the seasons because the tropical year is not exactly 365.25 days long

  20. Gregorian calendar • Modified Julian system. Only those century years divisible by 400 are leap years, except century years divisible by 4000 are not leap years • Builds an error of 1 day per 20,000 years

  21. Change from Julian to Gregorian • 1582 for much of Catholic Europe • 1700 Protestant German countries • 1752 Great Britain and its colonies • Sept 2, 1752 was followed by Sept. 14, 1752 • Early colonial dates may be given as “old style” or “new style”

  22. Calendar Riots

  23. Lunar Calendars • Based on cycle of the lunar phases rather than the apparent motion of the sun in the sky • From one full moon to the next takes about 29.5 days (one synodic period) • This does not go evenly into 365 days

  24. Cathedrals as Observatories

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