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A Year on Earth

Earth's Tilt and The Seasons

Earth, like all of the planets in the Solar System, travels around the Sun. One complete orbit of the Sun is known as a year and it takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to complete an orbit. As this is actually almost 5 hours and 49 minutes longer than a year on a calendar, an extra day is added every four years to compensate. The year in which this happens is known as a leap year and the extra day is added to February, giving the month 29 days instead of its usual 28 days. 

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.

Diagram showing Earth's journey around the Sun and how its tilt causes the seasons

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. 

Penguins huddling together
Something else worth noticing is the positions of the poles in June and December. In June the North Pole is in constant sunlight whereas the South Pole is in continuous darkness. This means that the Sun never sets during summer at the North Pole and in countries close to the Arctic circle. At the same time, it doesn't rise in Antarctica (the South Pole). Arctic daytime actually lasts from around 21st March to 23rd September with nighttime in Antarctica for the same period of time. Around 23rd September, day and night gradually switches at the two poles.

During the months of darkness in the Antarctic, male penguins huddle together to keep themselves, and the eggs of their unborn babies, warm. Their female partners meanwhile go to sea to find food.

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.

Dates Season in Northern Hemisphere Season in Southern Hemisphere Length of Season
21st March - 20th June Spring Autumn/Fall 92 days
21st June - 22nd September Summer Winter 94 days
23rd September - 20th December Autumn/Fall Spring 89 days
21st December - 20th March Winter Summer 90 days (91 in a Leap Year)

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