Today's useless fact - Why are there no seasons on Mercury?

First, it's important to understand why most planets - e.g. Earth - have seasons . Unfortunately, most people don't understand why this is. It is a common misconception that it is because of the elliptical nature of Earth's orbit that causes the season. This is not the case. The difference in distance from the Sun over the course of a year is not sufficient to cause the seasons. Additionally, this would not explain why the souther hemisphere's seasons are opposite of the northern hemisphere's.

What causes the seasons is the fact that Earth's axis is tilted 23.5 degrees from perpendicular to Earth's plain of revolution. Earth's axis points toward the same point in the sky all year round. This means that its orientation to the Sun changes as Earth orbits the Sun. So, at some times of the year, the North Pole is tilted toward the Sun and at other times of the year, it is pointed away. At those times that the north pole is pointed toward the sun, the northern hemisphere gets more direct sunlight and longer days. The more direct sunlight more strongly heats Earth's surface. The longer days mean more time to heat the surface. These two factors combine to cause warmer temperatures. When the north pole is tilted away from the Sun, then the northern hemisphere gets less direct sunlight and shorter days. This leads to cooler weather.

This also explains why the southern hemisphere has it season opposite of the northern hemisphere. When the north pole is tilted toward the Sun, then the southern hemisphere is tilted away from the sun and vice versa.

Now, why doesn't Mercury have seasons? It's because Mercury's axis is only tilted 0.1 degrees off perpendicular, to the plane of it's orbit. In other words, there is no significant till to its axis. So, other than rising and setting as a result of its rotation, there is no significant change the way the light comes into Mercury's surface. Therefore, there are no seasons. Check out the links in our Seasons category for more.