What causes the Earth’s Seasons?
Seasons
NOT
NOT the varying distance
of the Earth from the Sun.
A) The Earth’s orbit is nearly circular:
distance from Sun doesn’t vary greatly.
B) The Earth’s closest approach
to the Sun is in JANUARY.
January
C) Summer in Northern Hemisphere
= Winter in Southern Hemisphere
Alice Springs,
Australia
24° South
JF MAM JJASO ND
Monterrey,
Mexico
July
What causes the Earth’s Seasons?
The tilt of the Earth’s rotation
axis relative to the Earth’s
orbit around the Sun.
25° North
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Sun’s path on celestial sphere = ecliptic
Observed properties of Summer:
Warmer temperatures
Constellations along ecliptic = zodiac
Longer daylight
Sun high in the sky at noon
Observed properties of Winter:
The ecliptic is
NOT the same
as the celestial
equator!!
Cooler temperatures
Shorter daylight
Sun low in the sky at noon
The Sun appears to move
west to east relative to stars
(1 year cycle)
Observation: Sun moves eastward
along the ecliptic, relative to stars
(about 1 degree per day).
What causes this annual motion??
NCP
Vernal (spring) equinox: Sun crosses
celestial equator, northbound (March 21)
Autumnal
Equinox
Summer
Solstice
CEq
Winter
Solstice
Vernal
Equinox
Summer solstice: Sun is furthest
north of the celestial equator (June 21)
Autumnal equinox: Sun crosses
celestial equator, southbound (Sept 22)
Winter solstice: Sun is furthest south
of the celestial equator (Dec 21)
SCP
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Two reasons why summer is
warmer than winter:
The view from mid latitudes…
Zenith
Summer Solstice
NCP
Vernal & Autumnal
Equinoxes
1. Sun is above the horizon longer in
summer.
2. Sun rises higher in sky in summer.
CEq
W
Winter Solstice
S
N
E
What’s this got to do with the
tilt of the Earth’s axis?
1 KW/m2
1 KW/m2
The ecliptic is tilted by 23.5° relative
to the celestial equator
BECAUSE
1 m2
2 m2
to Polaris
the Earth’s axis of rotation is tilted by
23.5° relative to its orbit around the Sun.
Another view:
(Earth’s size greatly exaggerated.)
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In June (here in Ohio) what direction would you
look to see the Sun rise?
a) due North
b) North of East
c) due East
d) South of East
December 21
June 21
2) The day is based on the time
between one noon and the next.
e) due South
Local noon = instant when Sun crosses
your upper meridian (above the horizon).
A.M.= “ante meridiem” (before
Sun crosses meridian)
P.M.= “post meridiem” (after
Sun crosses meridian)
Meridian = great circle on celestial sphere
running north to south, through your zenith.
Apparent solar day = time between
one local noon and the next.
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