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).

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.

Seasons The Sun is high in the midday summer sky… (a) The Sun in winter 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. Why are there seasons? Northern and southern hemisphere seasons are opposite, why?  all because of the tilt of the Earth spin axis

Seasons The Sun is low in the midday winter sky… (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.

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

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

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

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

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!

Figure 2-16 Zenith Celestial equator North celestial pole W S N Horizon E For an observer at 35 deg north. Dec. 21 Mar. 21 June 21 During summer in the northern hemisphere, the Sun rises in the northeast and sets in the northwest. Sept. 22 On the first day of spring and the first day of fall, the Sun rises precisely in the east. In winter in the northern hemisphere, the Sun rises in the southeast and sets in the southwest.

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

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

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

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

Precession Earth spin axis precesses with a period of 26,000 years. Figure 2-19 Precession Because the Earth’s rotation axis is tilted, the gravitational pull of the Moon and the Sun on the Earth’s equatorial bulge together cause the Earth to precess.

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

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

Sun as a Timekeeper Not a good one!

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.

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

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.

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