Earth is quite special among its rocky terrestrial friends: Mercury, Venus and Mars. It is the biggest and heaviest of this group. If you think about it, Earth is quite big – about 12,700 kilometres across. But then, there is Jupiter, which is way bigger and heavier than Earth. It is like the big brother of the Solar System.
What is interesting about Earth is its shape. You might think it is just a sphere, like the rest of the planets. But Earth or any other planet in your Solar System is not a perfect sphere.
Yes. You read it right. Earth is more like a squashed ball, known as an oblate spheroid. This means Earth is slightly stretched at the equator and flattened at the poles. Now, you might wonder, why isn’t Earth perfectly round? The answer lies in something called centrifugal force.
Centrifugal force is the outward force felt by objects that are spinning. Imagine spinning around in a chair or on your feet. You will feel a force pulling you away from the centre and your arms or legs might stretch out. Or picture yourself on a merry-go-round – you feel a bit of extra force acting on you, tugging you to the side. This is the centrifugal force in action.
Because Earth and other planets and moons spin, this centrifugal force causes them to bulge at their equators. The effect can be subtle, but it is there.
The circumference of the Earth at the equator is approximately 40,075 kilometres, while at the poles, it measures about 40,008 kilometres. So, the difference in circumference is about sixtyseven kilometres. Although this might seem minimal, consider this: The tallest mountain peak on your planet, Mt Everest, is less than nine kilometres high while the aeroplanes typically cruise at an altitude of about eleven kilometres. This means the difference in circumference is over seven times the height of Mt Everest and six times the height at which aeroplanes fly.
Consider Jupiter and Saturn for better examples. These gas giants, when viewed globally, show a noticeable squishing and bulging in the middle. Their squished shape is more evident because they spin the fastest in your Solar System. The faster an object spins, the more it is affected by centrifugal force. An extreme case of centrifugal force’s impact can be seen with Haumea, a dwarf planet in the Kuiper Belt, beyond Neptune. Haumea, roughly the size of Pluto, spins so rapidly (a full rotation every four hours) that it has an almost egg-like shape.
Earth also has a friend in space – the Moon. This is pretty special because Earth is the only planet in the Solar System with just one Moon. Other planets, such as Venus and Mercury, do not have moons, while big planets such as Jupiter and Saturn have over 100 moons.
And the Moon does a lot more than just hang in the sky and look pretty. First off, the Moon is super important for the ocean’s tides. Its gravity pulls on the Earth’s oceans, creating high and low tides. These tides are really important for animals and plants living in and near the sea.
The Moon plays a big role in keeping Earth steady. It does this by holding the Earth at just the right tilt. But what does “tilt” mean? Let’s break it down in simple terms.
First, let us talk about an axis. An axis is like an invisible stick that goes through the middle of something that spins. Everything that spins has an axis – from tiny particles to huge stars. The axis runs through the centre of whatever is spinning. This centre point is called the centre of mass. It is like the perfect balance point of an object. For planets such as Earth, this centre is usually right in the middle. Now, Earth’s centre of mass changes a little bit because of ocean tides, but not enough to make the planet wobble off its axis.
Every planet in your Solar System spins around its axis. This is why planets have a north and south pole – these are the places where the axis meets the surface of the planet. The time it takes for a planet to spin all the way around once is called its rotation period. This is why there are days and nights. For Earth, it takes about twenty-four hours to spin around once, which is one Earth day.
Some planets, such as Mercury, Venus, and Jupiter, spin almost straight up and down. But Earth does not spin straight up and down. It is tilted on its axis. This tilt is called axial tilt or obliquity. It is the angle between how a planet spins and the line that goes straight up and down from its path around the Sun. Earth’s axis is tilted about 23.5 degrees. Because of this tilt, the Sun shines on different parts of Earth at different angles during the year. This is what causes the seasons.
Uranus has the craziest tilt in your Solar System. Its tilt is about ninety-eight degrees. That means its north pole is almost lying down, close to where its equator is. Scientists think a long time ago, something as big as Earth might have crashed into Uranus and knocked it sideways.
The Moon plays a vital role in keeping Earth’s tilt just right. Without the Moon, Earth might tilt excessively or insufficiently, drastically altering the weather and seasons. Thus, the Moon’s influence is crucial in maintaining stability and comfort on Earth. However, Earth’s rotation isn’t uniform. This is quite intuitive.
Excerpted with permission from I Am the Universe: The History and Future of the Cosmos, Srijan Pal Singh, Penguin India.
