Why the seasons change

In Astana in June the Sun stands high at noon, the day lasts about 16.5 hours, and it is warm outside. In December the Sun barely rises above the horizon, the day lasts only about 8 hours, and it is cold.

Three things change together: the height of the Sun at noon, the length of the day, and the temperature. That is no coincidence. There is one cause, and it explains all three changes.

The question of this lesson: why does the Sun climb higher and the day last longer in summer? To answer it we need two ideas: the tilt of Earth's axis and the angle of sunlight. We will take them in order.

Earth rotates on its axis: one turn takes about 24 hours, which gives us day and night. At the same time Earth revolves around the Sun: one full circle takes 365.25 days, that is, one year.

Earth's axis does not stand straight up relative to the orbit. It is tilted by 23.5°. This tilt is what creates the seasons.

The most important point: the axis does not change direction. Its north end points at Polaris all year. While Earth moves along its orbit, the axis stays “parallel to itself”.

That is why in June the Northern Hemisphere leans toward the Sun, and in December away from it. The axis does not flip: Earth's position on the orbit changes. Test this with the model below.

Shine a torch straight down at a table: you get a small bright spot. Tilt the torch: the spot stretches out and grows dimmer. The amount of light is the same, but it is spread over more area.

The Sun works the same way. When the Sun is high, its rays hit the ground steeply and the energy is concentrated on a small area. When the Sun is low, the same beam is stretched over a larger area, and each square metre receives less energy. The closer a place is to the equator, the higher its noon Sun.

There is a second factor too: the length of the day. In June the Sun shines in Astana for about 16.5 hours, in December for only about 8. The longer the Sun shines, the longer the ground is heated.

In summer both factors work together: the Sun is high and the day is long. In winter they work together the other way: the Sun is low and the day is short. That is why the difference between summer and winter is so large.

There are four special days in the year.

• 20-21 March, the spring equinox: day and night each last about 12 hours everywhere on Earth.
• 20-21 June, the summer solstice: the longest day in the Northern Hemisphere (about 16.5 hours in Astana).
• 22-23 September, the autumn equinox: day equals night again.
• 21-22 December, the winter solstice: the shortest day in the Northern Hemisphere (about 8 hours in Astana).

The astronomical seasons are counted from these days.

When it is summer in Kazakhstan, it is winter in Australia. In December, students in Sydney start their summer holidays and go to the beach, while Astana is frozen. Same date, opposite seasons, because in December the Southern Hemisphere leans toward the Sun.

Earth's orbit is almost a circle. Earth comes closest to the Sun around 3 January: about 147 million kilometres. This point is called perihelion. Earth is farthest around 4 July: about 152 million kilometres, aphelion. The difference is only about 3 percent.

Notice: Earth is closest to the Sun in the middle of the northern winter.

There are two strong pieces of evidence against the “it is the distance” idea.

1. Earth is closest to the Sun in January, when Kazakhstan has its hardest frosts. If closeness warmed us, January would be the warmest month.
2. If the seasons depended on distance, the whole planet would have the same season at the same time. But the hemispheres have opposite seasons.

Conclusion: the cause of the seasons is the tilt of Earth's axis, not the distance to the Sun.

The seasons are created by the tilt of Earth's axis. The axis is tilted by 23.5° and always points at the same spot in the sky, so through the year first the Northern and then the Southern Hemisphere leans toward the Sun. That changes the angle of sunlight and the length of the day, and with them the temperature.

1. The axis is tilted by 23.5° and does not change direction.
2. A high Sun and a long day deliver more energy: that is summer. A low Sun and a short day: winter.
3. Four key days: the equinoxes on 20-21 March and 22-23 September, the solstices on 20-21 June and 21-22 December. The hemispheres have opposite seasons.
4. The distance to the Sun is not the reason: Earth is closest to the Sun in January.