As the Earth orbits the Sun, different parts of the planet face towards or away from it, receiving varying amounts of heat. The Earth is tilted at an angle and always tilts the same way. This means that when the Earth is on one side of the Sun, the Northern Hemisphere leans towards the Sun and experiences summer. At the same time, the Southern Hemisphere is leaning away from the Sun and is having winter weather. Six months later, the Earth is on the other side of the Sun and the situation is reversed. Spring begins in a hemisphere at the moment at which it starts to lean towards the Sun; Autumn starts when it begins to lean away from it.
The seasons have nothing to do with how far the Earth is from the Sun. If this were the case, it would be hotter in the northern hemisphere during January as opposed to July. Instead, the seasons are caused by the Earth being tilted on its axis by an average of 23.5 degrees (Earth’s tilt on its axis actually varies from near 22 degrees to 24.5 degrees). Here’s how it works:
The Earth has an elliptical orbit around our Sun. This being said, the Earth is at its closest point distance wise to the Sun in January (called the Perihelion) and the furthest in July (the Aphelion). But this distance change is not great enough to cause any substantial difference in our climate. This is why the Earth’s 23.5 degree tilt is all important in changing our seasons. Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude. This situates the northern hemisphere in a more direct path of the Sun’s energy. What this means is less sunlight gets scattered before reaching the ground because it has less distance to travel through the atmosphere. In addition, the high sun angle produces long days. The opposite is true in the southern hemisphere, where the low sun angle produces short days. Furthermore, a large amount of the Sun’s energy is scattered before reaching the ground because the energy has to travel through more of the atmosphere. Therefore near June 21st, the southern hemisphere is having its winter solstice because it “leans” away from the Sun.
Advancing 90 days, the Earth is at the autumnal equinox on or about September 21st. As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator. Basically, the Sun’s energy is in balance between the northern and southern hemispheres. The same holds true on the spring equinox near March 21st, as the Sun is once again directly over the equator.
Lastly, on the winter solstice near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude. The southern hemisphere is therefore receiving the direct sunlight, with little scattering of the sun’s rays and a high sun angle producing long days. The northern hemisphere is tipped away from the Sun, producing short days and a low sun angle.
Picture Credit : Google