1. The Seasons

2. Why do we have seasons?
Earth’s distance from the sun varies throughout the
year – doesn’t that cause the seasons?
This was the most commonly given answer during a poll taken at a recent Harvard graduation
Tilt!
Because of the tilt of Earth’s axis, the amount of Insolation (incoming solar radiation) received by Northern and Southern Hemispheres varies seasonally
A. Northern Hemisphere has summer when it tilts toward the sun, winter when it tilts away
When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and the other way around.

3. Insolation

4. What Causes the Seasons?
The angle of the Sun matters: when the Sun is at a low angle, it does not heat as efficiently as it does when it is high angle.

7. For this reason, it’s warmer near the equator than at the poles.
Seasons on Earth
Angle of incidence… equator vs. poles
North Pole
Equator
Earth
South Pole
Thus, Insolation is more intense near the equator compared to the poles.
For this reason, it’s warmer near the equator than at the poles.

8. Earth North Pole South Pole Earth’s Seasons
Tilt of the Earth’s axis towards or away from the sun creates the seasons
When the north pole tilts toward the sun, it gets more radiation – more warmth
during the summer
SUMMER (Northern Hemisphere)
North Pole
Earth
When the north pole tilts toward the sun, the south pole tilts away
So when it’s summer in the north,
it’s winter in the south
South Pole
WINTER (Southern Hemisphere)

9. Earth North Pole Equator South Pole Earth’s Seasons
Tilt of the Earth’s axis towards or away from the sun creates the seasons
WINTER (Northern Hemisphere)
When the north pole tilts away
from the sun, it gets less Insolation –
So it’s colder during the winter
North Pole
Equator
Earth
South Pole
When the north pole tilts away from the sun, the south pole tilts toward it…
When it’s winter in the north,
it’s summer in the south
SUMMER (Southern Hemisphere)

11. What Causes the Seasons?

14. Because of the tilt of the Earth’s axis, the altitude of the Sun changes during the year: It gets higher above the horizon during the summer than it does during the winter.
Summer: The combination of a long daytime and a Sun that is relatively high above the horizon leads to much more heating in the day, plus a shorter period of cooling at night. Overall, it is warmer

17. SEPTEMBER AND MARCH 22 Equinox

18. JUNE 21

19. DECEMBER 21