Astronomy

Let's get the basics straight first.

We live on earth which is the 3rd planet from the sun. The sun is a star, just like all the others you see in the sky, it's just a hell of a lot closer to us. It's pretty big too, and we orbit around it.

We live on the one called 'Earth' by the way

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This solar system is one of 100 billion others in our galaxy, called the Milky Way. No, a Milky Way isn't just a chocolate bar! Unfortunately we can't get a picture of it, but it would look something like this:

This Milky Way is in the universe (get help if you don't already know what that is). The universe is expected to contain somewhere between 200 billion - 2 trillion galaxies. So there are somewhere between 200 billion and 2 trillion galaxies in the universe, of which each contain about 100 billion stars. That's a lot! Don't worry about memorising these numbers, if it comes up on the exam, it'll be multi choice and quite obvious.

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So we humans quite like exploring the solar system and we put things in orbit around the earth quite often, satellites. A satellite, as you may be surprised to know, is anything that orbits the earth really, the moon can actually be described as a natural satellite.

Wait, so why do things orbit the earth?

Because the earth has gravity (that's the thing that keeps you on the ground by the way). This gravity doesn't just apply to us here on earth, it applies to space too. The moon orbits us because of the earth's gravitational field strength (a fancier way of saying how strong gravity is). Speaking of gravitational field strength, you need to know that it's different on each planet. Here on earth it's 10 (9.8 actually, but GCSE says 10), and on the moon it's only 1.6. Gravitational field strength is measured in metres / second squared (the same unit as acceleration). Why? Because because it's  measure of how fast objects will accelerate if you drop it while on that planet. If you drop an apple (for example) here on earth, it will accelerate towards the ground at , you guessed it, 10 (9.8) metres / second squared. This gravitational field strength is stronger if the planet is more massive (heavier).

Some orbits you need to know

Low Earth orbit

Like it's name, a low earth orbit is just an orbit that is quite close to the earth. Satellites in this orbit whiz around the earth pretty fast. The International Space Station (ISS) is in a low earth orbit, because that means that when they need more food up there, you don't need to go so far to give it to them.

geostationary orbit

If you ever forget anything in science, you can usually dissect the word into to parts to figure it out and the same is true of this word, geostationary. Stationary, because it is stationary ralative to the earth. Basically what that means is that if a satellite is in a geostationary orbit above South Africa, Then it stays above South africa. It moves around the earth at the same speed as the earth rotates so it stays in the same position above the earth. This is particularly handy when you want to use your GPS or watch TV, These kinds of satellites are usually put up here.

Other space things

 There are some huge chunks of ice and other crap floating around in space too, these guys are called comets, they're another thing you need to know. They orbit the sun in huge elliptical orbits (below), which is something you may be asked to draw in an exam

One Final Thing

There's a nice little formula you need to know, and lucky for you, it's given to you in the exam!

This formula isn't actually that hard to do, because all you need to know is orbital radius and the time. The thing is, is that examiners love to mess around with these questions to mess with you. Here's an exam past paper question that I'll guide you through.

For now I'm just going to go through question 10d because the rest are quite straightforward. 
Step 1: Collect all the data you need to put into the formula. The table says that the distance from the sun is 150 * 10^6 km, and the time period is 365 days.
Step 2: Look at the question. What units does it want the final answer in? In this question it wants it in km/s, so we need to convert the time from days to seconds
Step 3: It's time to convert the days into seconds. Start of simple, turn days into hours. There are 24 hours in a day so multiply 365 by 24 to get 8,760 hours. In each hour there are 60 minutes, so multiply 8,760 by 60 to get 525,600. There are 60 seconds in each minute so you need to again multiply 525,600 by 60 to get 31,536,000. Now just sub it into the formula.

2 x pi x (150 x 10^6) all divided by 31,536,000 gives us 29.9 km/s

That's it from me for now

I hope you found this useful! Please let me know if there are any other topics you'd like me to do and contact me if you don't understand!