We might ask ourselves why the moon moves around, around, around the whole time? Why is the moon left in the orbit around the Earth? Why doesn’t the moon fly away? Why doesn’t the moon fall on our heads?
I will try to explain the questions above and at the same time I am going to explain why all the planets move around and which forces that affect the movement of the planets and why the solar system doesn’t break down.

It isn’t just I that wonder about those questions, it was a man in the 17th century whose name was Isaac Newton. He was wondering about the same questions that I. When he saw a falling apple, he got the idea that the apple is pulling to the Earth. He also wondered over if it was the same force that holds Earth in its orbit.

It looks like the moon is “falling” to the centre of the Earth, the whole time, but the speed makes it hold it into an orbit. If the Earth doesn’t pull the moon, it should move away from the Earth in a straight line and would not come back. Newton shows that it is that same force, gravitation that holds the moon in the orbit around the Earth. Newton also knew that the movement of the planets was an impact of the Sun’s gravity.

The movement of bodies (e.g. a ball or a planet), continues unchanged on a straight line if it is not impacted by another force (Newtons 1st law). The force can e.g. be the gravitation from another body e.g. the Sun or frictional resistance that is a contact force. Out in the space there is no frictional resistance, no force that stops. It is the reason to why things like a ball or a planet or a spaceship can continue travelling in infinity, when it has come in motion, if nothing has stopped it.

In order that the planets can move around a star, they need  that same amount of gravitation to the star that outwards, otherwise  the planet would crash with the sun or fly away out in the space. The force outwards (the movement) are as big as the gravitation inwards (Newtons 3rd law) which is very good for us. The closer the planet is to the sun, the faster it moves around the sun. This because  the gravity is stronger the closer the planet is to the sun.  Therefore the planets motion must be faster, in other case the planet would enter in the sun (and be hotter than the pizzas in the oven at “Pizza Planeten”).

All bodies with mass affect each other with a force, so called attraction force. The bigger mass a body has the bigger gravity it has. Everywhere in universe the gravity is the unifying force. It is the gravity that makes that you and I and the air around us is left on the Earth. The gravitation force keeps together our solar system and makes that the Sun’s billions of stars neighbors together make a galaxy, the Milky Way.

The gravity is one of the four natural forces and the weakest, but acting at very long distance! Tidewater is a good example of that the gravitation acts on long distance. The moon “drags” the Earths water towards itself, so that it becomes flood on the side that the moon is on. The tidewater is important for the oceans, the water gets a circulation. Imagine  an aquarium without a pump, the water should be bad.

The planets and the galaxies affect each other so the orbits aren’t completely round. The planets have elliptical (oval) orbits round the Sun. The Earth has an almost circular orbit, while Mercury has a very oval orbit. Planets that have an oval orbit have an adverse climate. It is very warm when the planet is near the Sun and very cold when it is far away. It means that life can’t exist on a planet that has a very elliptic orbit. Luckily we are living on Earth that has an almost circular orbit around the Sun and furthermore,  the Earth has a perfect distance to the Sun. The Earth is in “The Goldilocks zone” i.e. it is either too warm or too cold.

Sources: Naturkunskap AB, Natur och Kultur s.71, 91>
Fysikens Värld, Illusterad Vetenskaps Bibliotek s.21>
Illustrerad vetenskap, Människan i rymden s.33
Illustrerad vetenskap, Världens vilda väder s.10-11
Den stora planeten, S Hawking s.192>
Universum – Illustrerat upplagsverk s.23