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Earth’s Orbit and the Seasons

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Presentation on theme: "Earth’s Orbit and the Seasons"— Presentation transcript:

1 Earth’s Orbit and the Seasons

2 Seasons on the Earth

3 1 KW/m2 1 KW/m2 1 m2 2 m2 Receives only half the heat and light
In winter a bundle of light is spread over a bigger area than in summer owing to the inclination away from the Sun

4 Phases of Moon: Earth-Sun Positions
What time of day does the new moon rise and set ? What time of day does the full moon rise and set ? ‘Horns’ of a crescent moon always point away from the Sun

5 Full Moon rises at dusk and sets at dawn
The New Moon rises and sets with the Sun (lies in the same direction as the Sun)

6 Lunar Eclipse (around Full Moon) Umbra and Penumbra- total or partial obscuration
Umbra – dark part of the shadow; Penumbra – lighter part of the shadow Earth’s maximum umbra at Moon’s distance is 9200 Kms; the penumbra is 16000 Kms across

7 Total Solar Eclipse: Total obscuration of the Sun by the Moon, possible because the angular size of Moon equals Sun’s

8 Solar Eclipse (at New Moon): Visible from a narrow ‘band’ across the Earth
Diameter (Moon) = 3476 Km = ¼ Diameter (Earth) Moon’s umbra reaches only a small area < 270 Km where solar eclipse is visible

9 Annular Solar Eclipse: (when the tip of moon’s umbra doesn’t quite reach the earth)

10 Eclipses occur on Line of Nodes: Earth-Moon-Sun must be in line
Why do eclipses NOT occur each new and full moon? The E-M-S line can may deviate by up to 5 degrees

11 Solar and Sidereal Day How long is one day ?
From noon (Sun directly overhead) to noon ? How long does it take for the Earth to complete one rotation on its axis ?

12 Orbital and angular motion of the Earth
Rotation period of Earth must be less than one solar day

13 Solar & Sidereal Days T=24h (Solar day) T=23h 56m 04s (Sidereal Day)
Noon T=23h 56m 04s (Sidereal Day) T=0h Noon Not to Scale

14 The earth moves each day by 1 degree in its orbit around the Sun
The earth moves each day by 1 degree in its orbit around the Sun. So each day the earth has to rotate a bit more to reach the noon position To rotate one extra degree requires 24 x 60 = 4 minutes 360 Solar day (noon-to-noon = 24 hours) is rotation period with respect to the Sun  4 minutes longer than the true rotation period of the Earth with respect to the stars called the Sidereal day = 23h 56 m 366 sidereal days per year

15 Solar and Lunar Calendar
Rotation of the earth = 1 solar day Revolution around the sun = d Orbital period of moon = 1 month = d Lunar OR solar calendars are possible, but not both since a month is not (i) exact no. of days, and (ii) year is not exact no. of lunar months 12 x = days, not one year Solar calendar is the one most widely used Roman Julian Calendar year = 365 d + leap yr

16 Gregorian-Julian Calendar
Problem with Julian Calendar  average yr is d, 11 minutes too long ! By the 1500’s, the time of Pope Gregory, the calendar was ahead of astronomical time keeping by more than two weeks Gregorian reform: Century years not divisible by 400 are ordinary years, not leap years (Example: 1700 AD was not a leap year, but 2000 AD was)

17 Motions of the Earth: Rotation on its axis (day), Revolution or orbit around the Sun (year), and Precession of the Polar N-S axis Position of the north star changes due to the slow precession of the Earth’s axis due to the gravity of the Sun – just like the wobbling axis of a spinning top

18 Precession and the location of Polaris: Period 26,000 Yrs

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