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