The Four Seasons

PSLE Science textbooks explain that the Earth’s revolution round the Sun causes the 4 seasons. However, many of these books do not appear to explain that the tilt of the Earth’s axis is also a contributory factor.

In Figure 1 below, note that the Earth’s axis is not vertical but tilted at an angle away from the vertical line. The sun’s rays will cause half of the Earth to experience day, while the other half to experience night.

Figure 1



Imagine a place on Earth marked “A” in the Fig 1 above. "A" will experience night. As the Earth rotates, A will move towards A1. The dotted line between A and A1 is the path taken by the place marked “A”, as the Earth rotates.

Note that the dotted line between A and A1 has a longer period of day, as compared to the night. This causes the parts of the Earth marked between A to A1 to experience summer.

At the same time, the place on the Earth marked “B”, will move towards B1 as the Earth rotates. Also note that the dotted line between B and B1 has a longer period of night, as compared to the day. This causes the part of Earth marked between B to B1 to experience winter.

The Earth revolves round the sun once every year. This means that in 6 months, the sunlight will fall onto the Earth from the opposite direction.

Figure 2


In Figure 2, we have a different diagram showing the Earth 6 months later. Since the earth takes 12 months to revolve around the Sun, in 6 months, it makes only half a revolution. This causes the sunlight to fall onto the Earth from the opposite direction.

It can be seen that the parts of the Earth marked between A to A1 will now experience a longer period of night, as compared to the day. This now means that the parts of the Earth marked A to A1 will experience winter.

Again, at the same time, the parts between B and B1 will experience a longer period of day, as compared to the night and hence, will experience summer.

From these 2 diagrams, we can see that when the Northern hemisphere experiences summer, the Southern hemisphere will experience winter and vice-versa.

The 4 seasons is thus caused by the Earth's revolution around the Sun and the tilt of the Earth's axis.