Seasons

2. Why do we have seasons?
 Are the Seasons caused by the Earth being closer to or farther
away from the sun?
NO
Closer to the sun in
January, and farther
away in July.
 Perihelion
is the point
along
Earth’s orbit
when Earth
is closest to
the Sun.
 January
 Aphelion is
the point
along Earth’s
orbit when
Earth is
farthest
from the
Sun.
 July
Distance A
Distance B
 Distance B (the tilt)
is so small that it
really does not make
us any closer to the
sun.

3. Why do we have seasons?
Seasons are caused by different amounts of
sunlight reaching earth because of a
combination of
1. The tilt of Earth and
2. It’s revolution around the sun

4. Sun’s Path Across Sky

5. Sun’s Path Across Sky

6. Flashlight Activity

7. Flashlight Activity

8. NO – The amount of energy leaving the flashlight is the same at all times.
Likewise, the average amount of energy leaving the Earth receives is the same.
The energy is more concentrated in “A” because the energy is in a smaller area, therefor the
temperature would be warmer. In B, the energy is spread out over a larger surface,
decreasing the concentration and temperature.
In summer, the light hits us more direct, like in part
“A”, the concentration is greater, we get a lot of heat
therefore temperatures are warmer on average.
In winter, the light hits us indirectly, like in part “B”,
the light is less concentrated and spread out, we get a
little bit of heat therefore temperatures are cooler on
average.

9. To Student 1:
0 Earth’s orbit is only slightly elliptical; we are
about the same distance from the Sun
throughout the year.
0 For the Northern Hemisphere, we are
actually closest to the Sun in the winter!
0 Distance from the Sun can’t explain seasons.
To Student 2:
0 You are confusing day and night with the reasons for
the seasons.
0 Earth turns on its axis (rotates) once every day (24
hours).
0 When we are turned toward the Sun, it is daytime
(not summer).
0 When we are turned away from the Sun, it is
nighttime (not winter).
To Student 3:
0 First, the tilt of Earth’s axis points toward the
North Star, so the December 21 Earth
diagram needs to be tilted the other way.
0 Second, you seem to have forgotten that
Earth rotates once every 24 hours, not once
every 365 days (one year). The revolution of
Earth around the sun takes 365.25 days.

10. TILT
 The tilt of earth effects the angle at which the sun’s rays
strikes the surface.
When rays strike at
high direct angles,
they are more
intense,
concentrated, and
warm.
When rays strike at
low indirect angles,
they are spread out,
less concentrated,
and cool.

11. TILT
 The tilt causes different location to get different amounts of solar
radiation…just like our flashlights.
 Areas where the rays strike at high, direct angles get more intense
radiation and are warmer.
 Areas where the rays strike at low, indirect angles get less intense
light and are cooler.

14. TILT
• If the tilt where the only factor, then every location on earth
would have one season and never change.
• One location would
always receive the
most direct rays
and would always
be hot. (On this
graphic the
Southern
Hemisphere).
• Another location
would always
receive less intense
rays and would
always be cool.

18. Revolution
• In addition to tilt, earth’s revolution is also a factor that
causes seasons.
• Earth’s revolution causes the suns most direct rays to
strike at different locations as earth orbits the sun.
• When Northern hemisphere is
facing the sun:
• Most direct rays over Northern
Hemisphere.
• Striking at Tropic of Cancer
• Summer Solstice (June 21 or 22)
• When Northern hemisphere is facing away
from the sun:
• Most direct rays over Southern
Hemisphere.
• Striking at Tropic of Capricorn
• Winter Solstice (Dec.21 or 22)
• Neither Northern or Southern
Hemisphere faces sun.
• Direct rays over equator.
• Autumnal Equinox (Sept.21 or 22)
• Neither Northern or Southern Hemisphere faces
sun.
• Direct rays over equator.
• Vernal Equinox (March 20 or 21)

19. Seasons

20. Length of Daylight
• You probably noticed with seasons comes longer and
shorter days. Now you can understand why this is
so…
• The main reason for longer and shorter daylight is
earth’s tilt

22. Length of Daylight – Summer Solstice
• When the Northern Hemisphere is tilted toward the sun =
Summer Solstice.
• More Sunlight covering the northern hemisphere than
darkness.
• LONGER DAYLIGHT!
• June 21 or 22 – Longest Day of the
Year
• Light = MORE (15 hours)
• Dark = LESS (9 hours)

23. Length of Daylight – Winter Solstice
• When the Northern Hemisphere is tilted away from the sun =
Winter Solstice.
• More darkness covering the northern hemisphere than sunlight.
• SHORTER DAYLIGHT!
• December 21 or 22 –
Shortest Day of the Year
• Light = LESS (9 hours)
• Dark = MORE (15 hours)

24. Length of Daylight - Equinox
• When the Northern Hemisphere is not tilted away from or
toward the sun = Equinox (Vernal and Autumnal).
• Equal amounts of daylight and darkness everywhere.
• EQUAL DAYLIGHT!
• September 22 or 23 / March 22 or 23
• Light = 12 hours
• Dark = 12 hours

26. The Sun’s Path
• From our perspective here on earth, the sun
“appears” to take a path across the sky as
seen on the graphic.

27. The Sun’s Path
• Notice that from March to June, the sun angle travels
high in the sky = warm, summer weather.
• It takes its longest path across the sky at this time, and
thus, we have the longest amount of daylight (15 hours).

29. The Sun’s Path
• From June to September, the sun angle gets lower in
the sky again. The lower angle means cooler weather
and the start of fall.
• Notice how the path is shorter, and thus, the daylight
hours start to get shorter.

31. The Sun’s Path
• From September to December, the sun angle reaches its lowest
point. This means very little solar radiation and cold, winter
weather.
• Notice that how the sun’s path is the shortest at this time, and
thus the amount of daylight is also the shortest at only 9 hours.

33. The Sun’s Path
• From December to March, the sun angle begins to rise
again, the days start to get warmer, and spring arrives.
• Spring is marked not only by warmer weather from the
higher sun angle, but also longer days.