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