Why do we have seasons on Earth? (Geography/Science)

It is perhaps something we all have taken for granted, and perhaps some of us think it has got something to do with the distance of the sun from the Earth at different times of the year.

Well, actually, the reason why we have seasons is because the Earth is tilted. What this basically means is this: imagine a stick stuck right through the Earth from the North Pole to the South Pole, and another through the Sun. This is called the axis. If you put the Earth beside the Sun, these two sticks are not parallel. As it revolves around the Sun, the Earth's stick is at an angle of 23.5º to that of the Sun.

Why is the Earth tilted? When our solar system was formed about 5 billion years ago, there were lots of collisions between large celestial bodies. Due to these collisions, all the planets tilt at different angles.

If the Earth's axis were parallel to the Sun and not tilted, the Sun would remain exactly halfway between the North and South Poles, and there would be no seasons on Earth. Each area on Earth would have the same relative climate and the same amount of daylight throughout the entire year.

These images from NASA demonstrate this tilt:

 

This excellent video below explains how the Earth's seasons are a result of its tilt. Even though the narrator speaks very clearly and quite slowly, some of you may have difficulty understanding everything, so I have transcribed it. Try to understand the explanation just by watching the video first. Then, follow the transcript while playing the video again.

Transcript:

The axis of the Earth is an imaginary line on which the Earth rotates. It links up the two poles. Both the axis and the Earth are tilted at an angle of twenty-three and a half degrees during a revolution.

The tilting of the axis results in direct sunlight falling on different places during different seasons. This causes variations in the duration of days, nights and seasons.

Relationship between the location of the overhead sun and the seasons

Similarly, the revolution of the Earth and the tilting of the axis result in different angles of the sun during different periods. When the sun is directly overhead, we call this the overhead sun. At this time the Earth's surface and the midday sun form a ninety-degree angle. Different locations of the overhead sun results in variations in the amount of solar radiation received in different areas and at different periods.

Spring Equinox

On 21st or 22nd March, the overhead sun is over the equator. The equator receives the largest amount of solar radiation. At this time, the northern hemisphere is in the spring equinox while the southern hemisphere is in the autumn equinox. The angle of the sun decreases towards the poles. On this day, the two hemispheres receive a similar amount of solar radiation, and the length of day and night is the same at all places on the Earth.

After this day, it is spring in the northern hemisphere where the day is longer than the night. In the southern hemisphere, it becomes autumn, when the day is shorter than the night.

Summer Solstice

On 21st or 22nd June, the overhead sun is over the Tropic of Cancer. It receives the largest amount of solar radiation. At this time, the northern hemisphere is in the summer solstice while the southern hemisphere is in the winter solstice. The angle of the sun decreases towards the poles. On this day, the length of daytime of the northern hemisphere is the longest in the year while that of the southern hemisphere is the shortest in the year.

Besides, there are twenty-four hours of daylight at the Arctic Circle and twenty-four hours of darkness at the Antarctic Circle.

Autumn Equinox

On 22nd or 23rd September, the overhead sun is over the equator again. The equator receives the largest amount of solar radiation. On this day, the northern hemisphere is in the autumn equinox while the southern hemisphere is in the spring equinox. The angle of the sun decreases towards the poles. On this day, the two hemispheres receive a similar amount of solar radiation, and the length of day and night is the same at all places on the Earth.

After this day, it is autumn in the northern hemisphere where the day is shorter than the night. In the southern hemisphere, it becomes spring, when the day is longer than the night.

Winter Solstice

On 21st and 22nd December, the overhead sun is over the Tropic of Capricorn. It receives the largest amount of solar radiation. On this day, the northern hemisphere is in the winter solstice while the southern hemisphere is in the summer solstice. The angle of the sun decreases towards the poles.

On this day, the length of daytime in the northern hemisphere is the shortest in the year while that of the southern hemisphere is the longest in the year.

There are twenty-four hours of darkness at the Arctic Circle and twenty-four hours of daylight at the Antarctic Circle. (Note that at this point in the video (5:38), the Poles are labelled the wrong way around).

Now, explore space with this excellent interactive explanation from BBC Bitesize.

It too explains, among other things, how seasons and the phases of the moon are formed. For a more detailed explanation of the phases of the moon, click here. As with the previous video, try watching it without the aid of my transcript first. Then, while following my transcript, listen to it again. At the end of the video, there is an interactive quiz.

Transcript:

When you're ready for space exploration, press start.

The Universe contains billions and billions of galaxies, and this one, called The Milky Way, contains 300 billion stars. Sounds a lot, but it's kind of average for a galaxy. One of those 300 billion stars is our Sun; yes, the one you see in the sky in the daytime.

Eight planets move in orbit around the Sun. Click on the one that you're sitting on.

Yea, that's planet Earth. Moving out from the Sun, we pass Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All these planets are kept in orbit by powerful gravitational forces, otherwise, they'd just fly off into space.

Here is tiny Pluto; it used to be considered the ninth planet in our solar system, but in 2006, scientists decided it was too small. Now, it's known as the dwarf planet instead.

This is the Earth seen from space. As the planets move along, they spin around, too. Let's take a look from above the North Pole. From here you can see the Earth spinning anti-clockwise. Each rotation takes 24 hours, which is why each day is 24 hours long.

When you're on the light side, the side lit by the Sun, it's daytime, and when you're on the dark side, it's night. Looking at the Earth from the side now, we can see that it spins towards the East. See Britain down there, it's about to move from the shadow into light; that's from night into day. Let's look at it in a different way.

So, we're on the ground, in the dark, looking South, and waiting for the sunrise as our part of the planet moves into light.

Click where you think the Sun will appear.

Well done!

That's spot on. Because we're spinning towards the East, the Sun first appears in the Eastern sky. It's the same at night with the moon and the stars - they seem to move from East to West as the Earth spins, but don't forget that as well as spinning, we're also moving through space. It takes 365 days for the Earth to go once all the way around the Sun, and that is why a year is 365 days long, except when it is a leap year. Because the Earth takes slightly more than 365 days to orbit the Sun, we need to make a correction to the calendar. So, every four years we add an extra day to February. If you are born on Feb 29th, it's a four-year wait between birthdays.

Another thing about our planet is that it's a little bit wonky. Earth's axis, that's the line from the North Pole to the South Pole, is tilted. That gives us the seasons. How? Let's see.

Watch what happens as the Earth orbits the Sun. The UK is in the northern half of the northern hemisphere of Earth. When our half of the planet is tilted towards the Sun, it feels warmer. Days are longer and nights are shorter. People wear less - it's summertime!

Six months later, we're on the other side of the Sun, and now we're tilted away from it. Days are shorter and nights are longer. You're in your warm clothes - and it's winter! Brrr!

Here's a question. When it's winter in the UK, where on Earth is it summer?

Exactly! In Australia, they have summer in December - Christmas dinner on the beach, anyone?

We've seen that the Earth orbits the Sun. Ok, but we also have our very own satellite orbiting Earth: the Moon. The Moon is made of rock, and unlike the stars, it can't produce any light of its own. All that silvery moonlight is just reflected from the Sun. The Moon orbits the Earth, held neatly in place by gravity and it takes 28 days to go around once. You can see here how, sometimes, the Moon is in the same part of the sky as the Sun, and sometimes it's on the opposite side, and sometimes, it's halfway between.

That's why when you look up at the Moon, it looks different day by day. We call these different appearances the phases of the Moon, like the full Moon and the new Moon.

Pull the Moon around its orbit to watch it change phase.

Once in every 28 days, you can't see the Moon at all. Many people think that's because the Earth is shading the Moon, but that's not right. You can see here that's because the Moon has a shadow side facing the Earth - this is known as the New Moon. Of course, the Moon isn't the only satellite orbiting the Earth. There are thousands of human-made satellites in orbit, too. They're shot up there by rockets to do many different things: observe weather patterns, send TV and communication signals, or just to spy on other countries.

Other satellites collect information about space itself like space telescopes. Scientists use them to study things we can see in space but can't yet travel to, things in our solar system, our galaxy, and the furthest regions of deep space.

Ready for a recap?

1. Gravitational forces keep planets in orbit around the Sun and satellites in orbit around planets.

2. The Earth takes 24 hours (1 day) to complete one rotation on its axis. The Earth takes 1 year to orbit once around the sun.

3. The Earth spins at a slight angle on its orbital path. Its tilt towards or away from the Sun is what gives us seasons.

4- The Moon takes 28 days to orbit the Earth. During this time we see different phases of the Moon.

Ok, I think that's all for now. Time for a test!