If you live away from the equator and the North and South Poles, you will probably notice that the weather changes throughout the year. Each period of a certain weather condition is known as a season and there are four seasons each year. Summer is hot and the days are long. Autumn gradually gets cooler and sees plants and flowers dying, leaves falling off trees and animals beginning to emigrate to somewhere warmer or go into hibernation. Winter is cold and the days are short, trees are bare and most plants are dormant. Spring sees the days gradually getting longer and warmer, animals coming out of hibernation or returning from their winter holiday and new life being born. The changing seasons are caused by the fact that Earth is tilted. Earth's tilt is 23.4 degrees which means that as the planet travels around the Sun, depending on the time of the year, it is either going to be tilted towards the Sun, away from it or somewhere in between. The diagram below shows Earth's journey around the Sun.

As the image shows, the rays received from the Sun vary at different times of the year. In June, the Northern Hemisphere (containing places like North America, Europe, Russia and China) is tilted towards the Sun. This means that the Sun shines more directly on this part of the planet in June, meaning that the Sun's rays are more intense and the temperature is hotter. The days are also long because more of the Northern Hemisphere is pointing towards the Sun so the Sun is able to cover a large area. At the same time, the Southern Hemisphere (containing countries like Australia, New Zealand, Argentina and South Africa) is pointing away from the Sun. This causes the Sun's rays to be less direct, resulting in colder and shorter days. This means that in June, it is summer in the Northern Hemisphere but winter in the Southern Hemisphere. The complete opposite happens in December when the Northern Hemisphere is tilted away from the Sun and the Southern Hemisphere is pointed towards it. This causes it to be winter in the Northern Hemisphere and summer in the Southern Hemisphere. 

Earth isn't the only planet to be tilted as it orbits the Sun. In fact, just about all of the planets are tilted. Uranus is so tilted that it appears to have toppled over and orbits the Sun on its side! As we have seen, Earth's tilt causes its seasons. But, as it isn't the only planet to be tilted, it isn't the only planet to have seasons. Mars, the Red Planet, also has changing weather conditions throughout the year. Mars tilts at an angle of 25 degrees, similar to Earth's tilt of 23.4 degrees, so has its own version of spring, summer, autumn and winter. We know of no life on Mars so we are unable to see the effects that the change of seasons has on living objects on the planet, but we can see clear changes at Mars' North and South Poles. Mars, like Earth, has polar caps. During Martian summer, the caps shrink as they melt in the Sun's heat and in winter they increase in size as the temperatures drop. As Mars takes almost twice as long as Earth to orbit the Sun, the seasons on Mars also last twice as long.

Another thing worth mentioning is that Earth doesn't orbit the Sun in an exact circle - its orbit is actually elliptical meaning that its distance from the Sun varies at different times of the year. Earth is at its closest to the Sun, or perihelion as scientists like to call it, just after the start of January each year. At this point, it is about 147 million kilometres (91 million miles) from the Sun. Earth reaches aphelion, or its most distant point from the Sun, just after the start of July each year when it is about 152 million kilometres (94.5 million miles) away from the Sun. Although there is a difference of about 5 million kilometres (3.5 million miles) between Earth's closest and most distant points in its orbit of the Sun, it doesn't really affect the seasons greatly. When Earth is closest to the Sun in January, it is Summer in the Southern Hemisphere seeing as the southern part of the world is pointing towards the Sun. However, this doesn't really cause Southern Hemisphere summers to be any warmer than summers in the Northern Hemisphere (when Earth is at its most distant from the Sun). This is because there is more water in the Southern Hemisphere which absorbs some of the heat received from the Sun. It does however have an effect on winters. Winters in the Northern Hemisphere, when the northern half of the world is tilted away from the Sun but the planet itself is at perihelion (closest to the Sun), are more moderate than winters in the Southern Hemisphere when Earth is at aphelion (furthest from the Sun) and can be more extreme.

One final point is that summers in the Southern Hemisphere are shorter than summers in the Northern Hemisphere. This is also caused by the fact that Earth is closer to the Sun during a Southern Hemisphere summer. It therefore has a shorter distance to travel around the Sun to get to its next season, and the pull of gravity from the Sun also causes the planet to travel slightly faster. Here is a table giving information about the seasons - when they occur and how long they last.

During my tour of the Solar System, I took the photograph below while visiting a village in England in winter. It shows what a typical winter's day would be like. I will return in my spaceship to the same location to take pictures of the same square in spring, summer and autumn to show how one place can change so much due to the fact that Earth is tilted. 

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