1. Announcements Homework Set 1 is due today
Homework set 2: Chapter 2 # 45, 46, 50, 52, 53 & 54
Use Exam Formula Sheet as you do the homework to familiarize yourself with where things are on it.

2. Hint for homework problems # 52, 53 & 54

3. Terrestrial Coordinates
Longitude is measured CCW (+) or CW (-) around from Greenwich England
Latitude is measured North or South of the equator
Both are measured in degrees, minutes and seconds

4. Celestial Coordinates
The angle between the celestial equator and the ecliptic is 23.5°
Right Ascension (RA) is measured CCW from the Vernal Equinox and is in hours, minutes and seconds
Declination (Dec) is measured above (+) or below (-) the celestial equator and is in degrees, minutes and seconds
See Appendix A6 for more on celestial coordinates

5. Finding the CE and NCP at your latitude
Altitude of NCP above due north horizon along the meridian is just f, your latitude (+ for north, - for south)
Altitude of the celestial equator above due south horizon along the meridian is 90°-f

6. Example
Chapter 2 problem # 43: The Moon’s orbit is tilted by about 5° relative to the Earth’s orbit around the Sun. What is the highest altitude in the sky that the Moon can reach, as seen in Philadelphia (latitude 40° North)?

7. Example Solution
What is being asked?...Maximum altitude of the Moon from 40° North latitude.
What information is given?...latitude = 40° N
Tilt angle of Moon from ecliptic = 5°
Tilt angle of ecliptic from celestial equator = 23.5°

8. Example Solution 2
Equation(s) to use: Refer to diagram two slides back. The altitude of the celestial equator above the local horizon is 90° - Latitude
CE = 90° - 40° = 50°
Maximum altitude of Ecliptic = CE °
= 50° ° = 73.5°
Maximum altitude of Moon = EclipticMax + 5°
= 73.5° + 5° = 78.5°

9. Time and Astronomy The 24 Hour Day?
One rotation of Earth = 1 sidereal day 23 hours 56 minutes seconds This is the time required for the Earth to complete one rotation with respect to the fixed stars

10. As the Earth rotates it also moves around the Sun.
So, for the Sun to return to the same place in the sky the Earth must rotate a little more than one complete rotation

11. Noon–to–noon isn’t always 24 hours

12. The Mean Solar Day is exactly 24 hours
The Mean Solar Day is exactly 24 hours. It is the time between meridian transits of the Sun averaged over four years

13. The Year 1 orbit around the Sun = 365.2564 days The sidereal year
1 Tropical Year = mean solar days The time from Vernal equinox to Vernal equinox

14. Pope Gregory XIII’s Calendar
The Gregorian Calendar (1582)
Most years have 365 days
Years evenly divisible by 4 have 366 days except century years.
Only century years evenly divisible by 400 are leap years

15. Precession of the Equinox
Like a spinning gyroscope, the Earth precesses. The period of the precession is 25,920 years

16. The Precession of the Equinox leads to a shift of the celestial pole

17. It also shifts the constellations of the zodiac

18. The Lunar Cycle Sidereal versus Synodic

19. The sidereal period is the orbital period with respect to the “fixed” stars: 27.32166 days

20. The Synodic period is the length of the cycle of phases
days

21. Since the angular size of the Sun and Moon are almost the same, eclipses occur

22. The Saros Cycle is an eclipse cycle
223 Synodic Months or 18 years days

23. The Saros Cycle is the repetition cycle of eclipse