Explaining the seasons
Physics Narrative for 11-14
A better explanation
There must be another explanation for the seasonal changes. Here is a loop showing the variation of the solar radiation arriving over a year (repeated a few times, so you can see the pattern).
The explanation depends on the amount each square metre of Earth is warmed by the Sun; most in the summer, least in the winter.
The annual variation in warming
This annual variation in warming is the result of a combination of two factors:
- The annual movement of the Earth around the Sun.
- The tilt of the Earth.
These factors combine so that in the winter:
- The days are shorter, so that the Sun warms the ground for less time each day.
- The Sun is lower in the sky in the winter, so the warming effect of it is spread over more ground.
The angle of incidence of the Sun's rays
Even without movement and tilt, the fact that the Earth is a sphere means that the angle of incidence of the Sun's beams on the Earth's surface will not be the same everywhere, although the length of day would be. The tilted axis of rotation affects both the angles and the lengths of the days. Both of these affect the warming effect of the Sun.
It is the annual movement of the Earth around the Sun, together with the tilt of the axis of rotation, that changes the angle at which the light from the Sun hits the ground. As the Earth moves around the Sun, the axis of rotation maintains its tilt such that the Northern Hemisphere leans away from the Sun in the winter, and towards the Sun in summer.
The axis of rotation of Earth is tilted by 23.5 degrees with respect to its plane of orbit. Over a year this tilt affects the angles at which the Sun's beams strike the ground, so how large an area they warm.
Bringing it home to Birmingham
Birmingham, in England, is 52.5 degrees north. So at noon in the middle of winter, the Sun's rays strike Birmingham at 29 degrees (52.5°-23.5°) to the horizontal. At noon in the summer they strike Birmingham at 77 degrees (52.5°+23.5°) to the horizontal.
Maximum warming would happen if the beam hit at 90 degrees to the horizontal. At the autumn and the spring equinoxes, when the axis of rotation is not tilted towards or away from the Sun, the beams just hit the ground in Birmingham at 52.5 degrees from the horizontal, again measured at noon.
The effect of this reduction in the angle during winter is that the Sun's rays are now spread out over an area that is 2.7 times larger than the point where the Sun is directly overhead. The consequence is that the ground and the air will be a lot cooler so it will feel like winter.
In summary, the seasonal changes are due to the movement of the Earth around the Sun over a year and the tilt of the Earth's axis. These in turn affect the angle at which the Sun's rays strike the surface of the Earth and the length of each day.
- It is hotter in the summer because our part of the Earth is tilted towards the Sun, so the Sun's beams are spread out over a smaller area and strike the ground for more time each day.
- It is colder in the winter because our part of the Earth is tilted away from the Sun, so the Sun's rays are spread out over a bigger area and strike the ground for less time each day.