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The Sky as Clock and Calendar

The Sky as Clock and Calendar. Noon. Sunset. Sunrise. Midnight. Solar Day. Based on Earth’s rotation. Day is split into 24 hours. A Noticeable Exception. Land of the Midnight Sun (and “Noon Moon”) Northern Norway in Summer. But a Problem….

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The Sky as Clock and Calendar

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  1. The Sky as Clock and Calendar

  2. Noon Sunset Sunrise Midnight Solar Day • Based on Earth’s rotation Day is split into 24 hours

  3. A Noticeable Exception Land of the Midnight Sun (and “Noon Moon”) Northern Norway in Summer

  4. But a Problem… As people traveled and communicated faster, it became aware that it couldn’t be Noon in 2 places at once. Solution: Time Zones!

  5. Carving the Earth into 24 Slices The 360o of the spherical Earth is split into 24 slices. Each slice is 360/24 = 15o wide. On the surface, the 25,000 miles of circumference is split into 24 bands (zones). Each zone is about 25,000/24 = 1,040 miles wide (on average)

  6. Timezones of the Lower 48

  7. When is Noon not Noon? Answer: When you’re not in the center of your time zone. The sun rises and sets at different times in different parts of the same time zone. Example: Central Time Zone on Sept. 8th Info from http://aa.usno.navy.mil/data/docs/RS_OneDay.php

  8. Fun Facts About Time Zones • British railroads created world’s first time zone in 1847 • American railroads were using time zones by 1883 • Canadian Sir Sanford Fleming is credited with first to think of 24 time zones world-wide. He did so in 1879. • The US Congress approved of our current 4 time zones in 1918.

  9. More Fun Facts • Some parts of the world use half-hour time zones • China uses only a single time zone (even though it should have at least 4) • Time Zones start at Greenwich, England (GMT = Greenwich Mean Time) • International Date Line: Go one way, have two birthdays. Go the other way, skip a day!

  10. Time Zones of the World

  11. Daylight Savings Time • Shifts clocks back 1 hour and say that sun rises and sets earlier • Example: Sun rises at 5:30 am and sets at 6:16 pm (instead of 6:30 am and 7:16 pm) • No change in number of daylight hours, only which hours are daylight • Saves energy, easier to wake up, safer for morning commuters, etc.

  12. The Solar Year • After 365 ¼ days, stars appear in exact same spots at exact same times • Difficulty splitting this time up: • 12 Lunar months (lunar cycles) = 12 x 29.5 days = 354 days • 13 Lunar months = 13 x 29.5 = 383.5 days • Only solution: uneven months based loosely on lunar months

  13. A History of our Calendar 46 BC – Calendar seriously out of whack. • Julius Caesar declares the year to have 445 days instead of 365 days • Thereafter, each year = 365 days, BUT… • Every 4th year = 366 days (leap year) • System known as Julian calendar

  14. But a problem… • Each year is slightly less than 365 1/4th days. • Result: By 1582, calendar was 10 days off (again…) • Pope Gregory XIII modifies calendar: • Drop 10 days from 1582 (only 355 days) • Not every 4th year is a leap year: If year divisible by 100, NOT a Leap Year (1700, 1800, 1900 were NOT leap years)…UNLESS… • 4th year is divisible by 400, then it IS a Leap Year (1600 and 2000 WERE Leap Years) • System known as Gregorian Calendar

  15. Not a Smash Hit with Everyone • People were freaked out about “losing” 10 days of their lives • Protestant countries reluctant to agree to Pope’s new calendar • Britain and her colonies didn’t adopt until 1752 (when they had to drop 11 days instead of 10) • Will be correct for 3000 years before it will be off a day

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