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Seasons and Calendar

Seasons and Calendar . Lecture 4. 2-5 What causes the seasons 2-6 The effect of changes in the direction of Earth’s axis of rotation 2-7 The role of astronomy in measuring time 2-8 How the modern calendar developed You do not need to study about the celestial coordinates (Box 2-1).

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Seasons and Calendar

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  1. Seasons and Calendar Lecture 4

  2. 2-5 What causes the seasons 2-6 The effect of changes in the direction of Earth’s axis of rotation 2-7 The role of astronomy in measuring time 2-8 How the modern calendar developed You do not need to study about the celestial coordinates (Box 2-1).

  3. Misconception about seasons Common misconception is that the season is due to the changing distance between the Sun and Earth Summer Winter Wrong Idea! The truth is opposite! Earth is about 3% closer to the Sun during the Northern hemisphere winter.

  4. Seasons (a) The Sun in winter Why are there seasons? Northern and southern hemisphere seasons are opposite, why?  all because of the tilt of the Earth spin axis The Sun is high in the midday summer sky… … so a shaft of sunlight is concentrated onto a small area, which heats the ground effectively and makes the days warm.

  5. Seasons (b) The Sun in winter Why are there seasons? Northern and southern seasons are opposite, why?  all because of the tilt of the Earth spin axis The Sun is low in the midday winter sky… … so the same shaft of sunlight is spread out over a larger area and less heating of the ground takes place.

  6. Seasons summer days are longer AND Sun stayshigher in the sky than winter days. shorter days weaker sunlight longer days stronger sunlight

  7. Orbital motion of Earth seen from North Pole, rotation and orbital direction of Earth are both counter-clockwise

  8. Figure 2-12 Spring in the northern hemisphere; autumn in the southern hemisphere Winter in the northern hemisphere; summer in the southern hemisphere Summer in the northern hemisphere; winter in the southern hemisphere Autumn in the northern hemisphere; spring in the southern hemisphere North pole North pole in continuous darkness North pole in continuous daylight North pole South pole 23 1/2° South pole in continuous darkness South pole in continuous daylight Earth’s orbit South pole

  9. Sun’s yearly path in the celestial sphere Ecliptic  something to do with eclipses (detailed in the next chapter).

  10. Equinoxes and solstices equinox : in Latin, “equal night” Spring (vernal) equinox Autumnal equinox day and night have the same length (12hr) solstice : “stand still”  stops moving northward or southward… Summer solstice Winter solstice However, these seasonal prefixes are for the northern hemisphere only!

  11. Figure 2-16 During summer in the northern hemisphere, the Sun rises in the northeast and sets in the northwest. In winter in the northern hemisphere, the Sun rises in the southeast and sets in the southwest. On the first day of spring and the first day of fall, the Sun rises precisely in the east. Zenith Celestial equator North celestial pole W S N Horizon E Dec. 21 Mar. 21 June 21 Sept. 22

  12. Earth at winder solstice …the Sun is directly overhead at noon on the Tropic of Capricorn… The Sun does not rise north of the Arctic Circle… N Arctic Circle Tropic of Cancer Sun’s rays Equator Tropic of Capricorn S Antarctic Circle …and the Sun does not set south of the Antarctic Circle.

  13. Earth at summer solstice The Sun does not set north of the Arctic Circle… …the Sun is directly overhead at noon on the Tropic of Cancer… …and the Sun does not rise south of the Antarctic Circle. Arctic Circle N N Tropic of Cancer Arctic Circle Equator Sun’s rays Tropic of Capricorn Tropic of Cancer Antarctic Circle Equator Tropic of Capricorn S S Antarctic Circle

  14. Moon • Zodiac : band around Ecliptic where all planets, Sun, Moon are located. • There are 12 constellations in Zodiac

  15. Precession Change in the orientation of the rotation axis of a rotating object http://www.youtube.com/watch?v=TbJ5JNn77UM

  16. Precession • Earth spin axis precesses with a period of 26,000 years.

  17. Changing zodiac • Zodiac signs need to be changed every ~2000 years!

  18. Time, Meridian, and Transit Korean sundial 600 years ago Sundial : tracking time using the location of the Sun apparent solar time Meridian : a circle paths through NP, SP, and Zenith. A celestial object crosses Meridian twice a day, once above the horizon and the other below the horizon. Transit : Crossing of the upper meridian by an object. Apparent solar day : time between two successive transits by the Sun

  19. Sun as a Timekeeper • Not a good one!

  20. Mean Solar Time To avoid the irregular move of the apparent Sun, astronomers invented an imaginary Sun called “Mean Sun”. Mean Sun moves along the equator at a uniform rate. The difference b/w mean and apparent solar times can be as large as ~30 minutes. Time Zones : rough interval of 15° longitude for the convenience. At noon at the center of a time zone, mean Sun transits.

  21. Sidereal Time Earth moves about 1° around its orbit in one day… …so Earth must make a complete rotation plus 1° to bring this location to local solar noon on March 22. To vernal equinox star as a timekeeper  used by astronomers Sun Local solar noon on March 21 is at this location on Earth. 1° 1° Earth on March 21 Earth on March 22

  22. Calendar : Leap Year system. Length of a year ~ 365 ¼ days. Sidereal year : time required for the Sun to return to the same position w.r.t. stars 365.2564 days Tropical year : time required for the Sun to return to the vernal equinox. Because of the precession, it is shorter than the sidereal year. 365.2422 days In leap years, we add one extra day in February (Feb 29). • Calendar gets complicated because of fractional days (0.2422 days). • Roughly, in every four years, there will be one extra day.  add one extra day in every 4th year. Leap Year (if the year number is a multiple of 4, then the year is a leap year). • Then, in every four year, we are adding little too much (4×0.2422 days = 0.9688 days, but we added 1.0 day). The difference is 0.0312 days over 4 years or 3.12 days in 400 years.

  23. Current Leap Year system = Gregorian Calendar 1. If year is dividable by 400 a leap year 2. If year is dividable by 100  not a leap year 3. If year is dividable by 4  leap year Ex) Year 2000 = leap year 2001 = no 2004 = yes 2100 = no 2400 = yes … • 3.12 days extra over 400 years  If the leap year is a multiple of 100, then the year is no longer a leap year. However, if a year is a multiple of 400, then it is a leap year. • In 1582, Pope Gregory XIII introduced a new calendar system. - 1 year = 365.2425 days - year dividable by 4  leap year - year dividable by 100  not a leap year - year dividable by 400  leap year • Over 400 years, we will have 0.03 days too short.  to adjust, we add leap seconds occasionally.

  24. In summary… Important Concepts Important Terms Ecliptic precession equinox solstice meridian transit tropical year leap year, leap second • Apparent solar time • Mean solar time • sidereal time • precession • Calendar • Chapter/sections covered in this lecture : sections 2-5 through 2-8

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