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AST 208 Topics

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- Time and celestial coordinates

Time and the Seasons

- 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

- 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

- 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

- 24 hours in 360 degrees
- Each 1 hour time zone is 15 degrees wide

- 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

- 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

- 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

- 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

- 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

- 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”

- 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