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 hoursSidereal 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 SunTropical year: Time for Earth to complete one cycle of seasonsTropical 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 skyA sundial gives apparent solar time
9 Mean Solar TimeLength 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 meridianIt is a local definition of timeAn analemma gives the position of the Sun in the sky at noon through the year.
11 Universal TimeUniversal 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, EnglandIt is the standard time used for astronomy and navigation around the worldIndiana 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 standardizationTime zones suggested by Canadian Sanford Fleming in 1878Adopted by U.S. railroads on Nov. 18, 1883International Prime Meridian Conference in 1884 formalized Greenwich as the Prime Meridian and set up the time zonesAdopted officially in the US by the Standard Time Act in 1918
15 Time Zones in IndianaIndiana is on the western side of the eastern time zoneGeographically, 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 summerKids go to school in the dark most of the year.