1. A100 Oct. 13 Keeping Time
READ Essay 2 – Keeping Time in the text, pp
Review on Oct. 15
2nd Exam Friday, Oct. 17
Today’s APOD
The Sun Today

2. Celestial Timekeeping

3. Astronomical Time Periods
How do we define the day, month, year, and planetary time periods?
How do we tell the time of day?
When and why do we have leap years?

4. The “day” has two lengths
Solar day: The Sun makes one circuit around the sky in 24 hours
Sidereal day: Earth rotates once on its axis in 23 hrs, 56 min, and 4.07 sec.

5. The difference between the solar day and the sidereal day

6. The “year” is also complicated to define
Sidereal year: Time for Earth to complete one orbit of Sun
Tropical year: Time for Earth to complete one cycle of seasons
Tropical year is about 20 minutes (1/26,000) shorter than a sidereal year because of Earth’s precession.

7. When and why do we have leap years?
The length of a tropical year is about days.
In order to keep the calendar year synchronized with the seasons, we must add one day every four years (February 29).
For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years unless they are divisible by 400 (e.g., 2000).

8. How do we tell the time of day?
Apparent solar time depends on the position of the Sun in the local sky
A sundial gives apparent solar time

9. Mean Solar Time
Length of an apparent solar day changes during the year because Earth’s orbit is slightly elliptical.
Mean solar time is based on the average length of a day.
Noon is average time at which Sun crosses meridian
It is a local definition of time
An analemma gives the position of the Sun in the sky at noon through the year.

10. Mean Solar Time

11. Universal Time
Universal time (UT) is defined to be the mean solar time at 0° longitude.
It is also known as Greenwich Mean Time (GMT) because 0° longitude is defined to pass through Greenwich, England
It is the standard time used for astronomy and navigation around the world
Indiana is 5 time zones west of the Greenwich meridian (actually… it’s 6!)

12. Origin of Time Zones Originally each town kept its own time
Arrival of railroads required standardization
Time zones suggested by Canadian Sanford Fleming in 1878
Adopted by U.S. railroads on Nov. 18, 1883
International Prime Meridian Conference in 1884 formalized Greenwich as the Prime Meridian and set up the time zones
Adopted officially in the US by the Standard Time Act in 1918

13. Standard Time & Time Zones

14. US Time Zones
The official U.S. time - clock

15. Time Zones in Indiana
Indiana is on the western side of the eastern time zone
Geographically, Indiana’s longitude should put it in the central time zone.
On eastern time, the sun rises and sets late (effectively, we are on double daylight savings time in the summer

16. Daylight Savings Time for Indiana
In the U.S., daylight savings time is observed from the second Sunday in March until the first Sunday in November (NOTE ERROR IN TEXT!)
Sunset is late in the summer
Kids go to school in the dark most of the year.

17. Sunset Time, 2008

18. Sun’s Path in the Local Sky
Special Latitudes:
the poles
the equator
23.5 degrees N
23.5 degrees S
66.5 degrees N
66.5 degrees S
The official U.S. time - clock

19. Special Latitudes
Arctic Circle (66.5°N): Sun never sets on summer solstice
Tropic of Cancer (23.5°N): Sun directly overhead at noon on summer solstice

20. Special Latitudes
Antarctic Circle (66.5°S): Sun never sets on winter solstice
Tropic of Capricorn (23.5°S): Sun directly overhead at noon on winter solstice

21. Sun’s Path at North Pole
Sun remains above horizon from spring equinox to fall equinox
Altitude barely changes during a day

22. Sun’s Path at Equator Sun rises straight up and sets straight down
North of celestial equator during spring and summer
South of celestial equator during winter and fall

23. Sun’s Path at Tropic of Cancer
Sun passes through zenith at noon on summer solstice

24. Sun’s Path at Arctic Circle
Sun grazes horizon at midnight on summer solstice

25. Planetary Periods
Planetary periods can be measured with respect to stars (sidereal) or to apparent position of Sun (synodic).

26. Planetary Periods
Difference between a planet’s orbital (sidereal) and synodic period depends on how far planet moves in one Earth year

27. Dates to Remember Read Essay 2 in the Text Review on Wednesday
2nd exam on Friday, Oct. 17