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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
AST 208 Topics

  • Time and celestial coordinates









What is a day
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


Apparent solar time
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



Mean solar time
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



Time zones
Time Zones

  • 24 hours in 360 degrees

  • Each 1 hour time zone is 15 degrees wide



Solar calendars
Solar Calendars Time

  • 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



Tropical year
Tropical Year Time

  • 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


Julian calendar
Julian Calendar Time

  • 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


Gregorian calendar
Gregorian calendar Time

  • 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


Change from julian to gregorian
Change from Julian to Gregorian Time

  • 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”



Lunar calendars
Lunar Calendars Time

  • 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



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