Trees in autumn

If the Earth was perfectly “upright” while orbiting the Sun, we would receive the same amount of sunlight every day of the year. However, the Earth’s axis is tilted by 23.5 degrees. As a result, the Northern Hemisphere faces towards the Sun in one part of the orbit. This means longer days and greater exposure to the Sun’s rays. At the same time, the Southern Hemisphere faces away from the Sun, and experiences the opposite effect. Half a year later, it’s the other way around. The tilted axis is the main cause of the seasons — the effect of the variation in the Sun–Earth distance during the solar year is much, much smaller.