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Clockwork of the Universe

Clockwork of the Universe. Topics. Basic Motions Celestial Sphere Seasons The Calendar. Basic Motions of Earth. Rotation about an axis 24 Hours Revolution about the Sun 365.242199 Days (vernal equinox to vernal equinox) 365.256366 Days (one revolution about the Sun)

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Clockwork of the Universe

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  1. Clockwork of the Universe

  2. Topics • Basic Motions • Celestial Sphere • Seasons • The Calendar

  3. Basic Motions of Earth • Rotation about an axis • 24 Hours • Revolution about the Sun • 365.242199 Days (vernal equinox to vernal equinox) • 365.256366 Days (one revolution about the Sun) • Revolution about the galactic center • 240 million years • Motion towards Andromeda • A few billion years

  4. p.54

  5. Celestial Sphere

  6. Fig. 4-17, p.67

  7. Fig. 4-18, p.67

  8. Fig. 4-22, p.69

  9. Seasons • The Earth’s axis • Tilted by 23.5owith respect to orbital plane. So, at any given place on Earth, the flux of sunlight varies from day to day. Flux is greater in summer than in winter. • Flux • Energy received per square meter per second

  10. Lunar Motions

  11. Fig. 4-2b, p.56

  12. Fig. 4-3a, p.56

  13. The Calendar • Calendar Year • A normal calendar year contains 365 days. • Tropical Year • But, the actual time between successive first days of spring, a period called a tropical year, is 365.242199 days, just under 365 ¼ days. • After 4 Years • After 4 years, the calendar year is behind by 4 x 0.242199 = 0.968796 days

  14. The Calendar – II • Leap Year • Since the calendar year is behind by almost a day (0.968796 days) we add a whole extra day (February 29th), every 4 years, to make a leap year with 366 days. • The leap year causes the calendar to be ahead by only 1 – 0.968796 = 0.0312 days.

  15. The Calendar – III • 400 Year Correction • So after 100 x (4 years) the calendar would be ahead by 100 x 0.0312 days, that is, about 3 days; therefore, 3 days must be dropped every 400 years! • This is done by making century years exactly 365 days long, except for those divisible by 400, such as the year 2000.

  16. The Calendar – IV • Modern Gregorian Calendar • With one more revision the modern Gregorian calendar (of 1582) slips by only 1 day in 20,000 years

  17. Summary • Earth’s Motion • A combination of spin about its axis, revolution about the Sun, revolution about the galactic center and a general driftin space. • The spin and axial tilt have a profound influence on the planet’s climate. • The Calendar • The calendar is complicated because the tropical year is not exactly 365 days long. Consequently, the length of the calendar year has to be corrected by one day from time to time to keep the two kinds of “year” in sync over a long period of time.

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