Transcript Earth`s Orbit and the Seasons

Earth’s Orbit and the Seasons
Seasons on the Earth
Receives only half
the heat and light
1 KW/m2
1 KW/m2
1 m2
2 m2
In winter a bundle of light is spread over a bigger area than in summer
owing to the inclination away from the Sun
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
Phases of Moon and Time
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)
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
Total Solar Eclipse: Total obscuration of the
Sun by the Moon, possible because the
angular size of Moon equals Sun’s
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
Annular Solar Eclipse:
(when the tip of moon’s umbra doesn’t quite
reach the earth)
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
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 ?
Orbital and angular motion of the Earth
Rotation period of Earth
must be less than one
solar day
Solar & Sidereal
Days
T=24h
(Solar day)
T=23h 56m 04s
(Sidereal Day)
Noon
Not to Scale
T=0h
• 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
Solar and Lunar Calendar
•
•
•
•
Rotation of the earth = 1 solar day
Revolution around the sun = 365.2422 d
Orbital period of moon = 1 month = 29.5306 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 29.5306 = 354.3672 days, not one year
• Solar calendar is the one most widely used
• Roman Julian Calendar year = 365 d + leap yr
Gregorian-Julian Calendar
• Problem with Julian Calendar  average yr
is 365.25 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)
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
Precession and the location of Polaris:
Period 26,000 Yrs