Written by: Gareth Dorrian, Post Doctoral Research Associate in Space Science, Nottingham Trent University
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Why do we have different seasons at specific times of the year? – Shrey, age nine, Mumbai, India
Over the course of a year, the Earth goes on a journey around the Sun. The reason we have seasons is because, during its journey around the Sun, the Earth is tilted. The Earth’s tilt affects the amount of daylight each hemisphere gets, which in turn makes the temperature hotter or colder.
For example, if you live in the northern hemisphere – that’s north of the equator, like in Europe, USA, or India – then winter happens in December, January and February. That’s when the northern hemisphere is tilted away from the Sun, and the days are shorter.
For anywhere south of the equator, such as Australia or Latin America, it’s summer during these months. That’s because the southern hemisphere is tilted toward the Sun, and the days are longer.
Solstices and equinoxes
Every season has a middle point. In summer and winter, these midpoints are called solstices. The summer solstice is the longest day, and shortest night, of the year. The winter solstice is the shortest day of the year, and the longest night.
In spring and autumn, the midpoints are called the equinoxes. At the spring and autumn equinoxes, day and night are the same length.
For thousands and thousands of years – right back to the Stone Age – people have known how to work out when the solstices and equinoxes happen throughout the year.
Indeed, they built hundreds of amazing stone circles – like the famous Stonehenge – all over Europe, which marked certain times of the seasons across the year.
These days, we even know how to calculate the seasons on other planets. For example, the next Spring equinox on Mars is on the 23rd March.
Journey around the Sun
To understand how this works, imagine a small ball (representing the Earth) moving around a lightbulb (the Sun) in a circle. Let’s say the ball has a line drawn around the middle, representing the equator. If you have these things at home, you can try this yourself.
As the ball moves around the lightbulb, the half closest to the light will be lit, while the other half will be in darkness. One full circle around the lightbulb represents one full year on Earth.
As you move the ball around the lightbulb, try spinning it between your fingertips, so that the light always shines directly onto the equator.
If the Earth span like this, day and night would be the same length all year round, and there would be no seasons.
Now, take that small ball and tilt it at an angle, so that the light from the bulb no longer shines directly on the equator. If you are doing this at home, it might help to colour in either the top or bottom half of the ball.
The Earth’s tilt
Now the hemispheres of the ball will get different amounts of light at any one time. The hemisphere tilted away from the bulb gets less light, and the hemisphere tilted towards the bulb gets more.
That means it’s “summer” in the hemisphere tilted towards the lightbulb, and “winter” in the hemisphere tilted away.
Keeping the ball at the same angle, move it to the other side of the light bulb. The hemisphere that was tilted away from the bulb is now tilted towards it. So, the hemisphere that was in “winter” when you started moving the ball, is now in “summer”, and the hemisphere that was in “summer” is now in “winter”.
The same thing happens as the Earth moves around the Sun, which is what gives us different seasons at specific times of the year.
Remember, the decrease in sunlight and colder temperatures you get during winter is not because the hemisphere is further away, but because the sun is above the horizon for a much shorter time.
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