A100 Oct. 13 Keeping Time READ Essay 2 – Keeping Time in the text, pp. 177-186 Review on Oct. 15 2nd Exam Friday, Oct. 17 Today’s APOD The Sun Today

Celestial Timekeeping

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?

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.

The difference between the solar day and the sidereal day

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.

When and why do we have leap years? The length of a tropical year is about 365.25 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).

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

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.

Mean Solar Time

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!)

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

Standard Time & Time Zones

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

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

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.

Sunset Time, 2008

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

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

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

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

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

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

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

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

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

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