IGCSE Physics Lesson: Astronomy

What is Astronomy?

One way to describe Astronomy is to say it is the study of objects (often referred to as bodies) and occurrences in space. You could say that means EVERYTHING and in a sense, it does. However, generally it is more to do with the movement and interaction of very large objects like planets, stars, moons, comets and other satellites over very large distances and often huge time spans.

So where do we start?

How about, the Earth, the Sun and the Moon? We live on the Earth and can see the Sun and Moon nearly every day.

In the sky, the Sun and Moon look quite similar in size, however, they are greatly different.

Even though they are not perfect, we think of celestial bodies as spherical. Their orbits are not circular either so at this point we will just write an average distance:

The Sun has a radius of 695,700km and for now we are going to imagine it as stationary in the middle of the Solar System.

The Earth has a radius of 6370km and orbits the sun at an average distance of 150 million km – that’s 150,000,000km.

The Moon has a radius of 1737km and orbits the Earth at an average distance of 385,000km.

We can see that the earth is a satellite of the Sun and that the Moon is a satellite of the earth. For this example, we are imagining the Sun as stationary, which it is not. The Sun is at the centre of our solar system and is orbited by all the planets, including Earth.  It is also part of a galaxy, which is a collection of stars, which is also moving through the universe.

The Earth is a planet, because it orbits the Sun. There are other planets in the solar system. Earth is the third closest one after Mercury and Venus, then after Earth there are Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Some people call Pluto a Dwarf-Planet or Minor Planet. It has been classified and reclassified numerous times and may well have been again by the time you read this. It is not particularly important to this lesson!

The Sun is at the centre of the solar system and the planets travel round the sun in an elliptical orbit:

Elliptical orbits are more like an oval if you look at them from above, not a circle.

Now, what makes all these huge objects spin round each other?

Gravity. All objects pull towards each other due to gravity. The Moon and Earth are in a little system of their own and the Sun and the Earth are in another. The Sun will also affect the moon. However, it is so far away that it is not the object that the moon is orbiting. Earth has the stronger pull as gravity fades over distance.

For any two objects in proximity to each other, there will be a specific speed or velocity where one object will orbit the other without any collision. This is the orbital velocity. There are also other effects due to the gravitational pull.

Tides & Moons

The gravitational pull of both the Moon and Sun are responsible for producing tides. As they pull against the Earth and Earth pulls back, the great bodies of water are dragged around and make a bulge in the middle. When both the Sun and the Moon are pulling from the same point larger tides are created which are called spring tides:

Compared to the moons of other planets in our solar system, Earth’s moon is large. It is large enough to have a significant impact on the tides. Other planets, some of which are much larger than earth, have numerous, much smaller moons, which have much less effect on them than ours.

The definition of a moon is that it is a satellite that orbits a planet. It can also have other properties and still be a moon. Earth’s moon is a pretty plain piece of rock. However, other moons, like Jupiter’s moon Europa, is so much more interesting. In fact, scientists are currently considering it to have one of the highest potentials for life in the solar system.

Videos 20 mins

The Origin of the Moon:

Jupiter's Galilean Moons:

Gravitational Field Strength

In the Forces section of this course, we looked at the force of gravity on Earth. It is approximately 10 N/kg and we refer to it by the symbol g.

The value g varies from planet to planet and moon to moon, depending on their mass and volume and therefore density. You have probably seen the footage of the astronauts jumping high on the moon and taking a long time to come down. This is because the gravity on the moon is less then earth.

Celestial Body

Value of g (N/kg)









The Moon (Earth’s)




The four outer planets are much less dense than Earth, being comprised mainly of gas, among other things. Therefore, even though, Saturn and Jupiter are much bigger, their gravitational strength is only a few times stronger than on Earth.

Remember, it is the Sun’s gravity which keeps all the planets in elliptical orbits around it. Newton’s Laws of motion state that, they also pull at the Sun in much the same way as the Moon causes tides on the earth. As these orbits are not circular, they have points at which they are closer and further away. The closet point is the Perihelion and furthest the Aphelion.

Other Objects

There are other objects in our solar system orbiting the sun, such as asteroids and comets. Their orbits are even more elliptical than planets. Also, the planets are almost all in one plane, if you can imagine them as balls on the same pool table. However, comets are at all sorts of angles.

A comet’s orbit would be more like this, with the Sun nowhere near the centre and being extremely elliptical, unlike the planets:

Halley’s Comet

Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 74–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.

Video 15 mins

Halley’s Comet Orbital Path:

Human-made satellites

Gravity is also the force that keeps our satellites in orbit round the earth. These can be used for many things such as telecommunications and photography (like those we see on Google Earth).

Orbital Speed

Orbital speed quite often comes up on the exams. They are quite straight forward questions based on:

Speed or Velocity = Distance/Time

The orbits’ satellites above earth are circular so the distance can be worked out as:

Distance space equals space 2 πr space space
straight r space equals space left parenthesis radius space of space planet right parenthesis space plus space left parenthesis height space of space satellite right parenthesis space divided by space time space period

A satellite takes 2 hours to orbit the earth at a height 100km. Calculate the orbital speed in km/s.

Speed space equals space distance divided by time space
Distance space equals space 2 πr space space
Distance space equals space 2 space straight x space straight pi space straight x space left parenthesis 6370 space plus space 100 right parenthesis space space
equals space 40652 km space space
Time space equals space 2 hours space space
2 space straight x space 60 space straight x space 60 space equals space 7200 space seconds space space
40652 divided by 7200 space equals space 5.65 space km divided by straight s
Orbital space Speed space equals space 5.65 km divided by straight s

A satellite is orbiting earth at a height of 150km. It takes 3.6 hours to orbit the earth. What is its orbital speed in km/s?

The Solar System

Unfortunately, it is very difficult to draw a diagram of the solar system accurately as it looks nothing like the diagram above. If we drew it to scale, we would have to draw it so small that everything within it would practically be dots! The solar system and distances in astronomy in general are so massive that they are difficult to represent or even to imagine.

Diagrams like this are more accurate:

You can see us in the centre there. No human has ever travelled further than the Moon, which is about 385000km away. You can see the Sun not far from the Earth. It takes the light from the sun approximately 8.3 minutes to reach us.

Light space travels space at space 300 comma 000 space km divided by straight s space space
If space straight a space jet space could space travel space at space 1000 space km divided by straight h space all space the space way space it space would space still space take colon space space
8.3 space straight x space 60 space equals space 149 comma 400 comma 000 space km space space
149 comma 400 comma 000 space divided by space 1000 space equals space 149 comma 400 space hours space space
149 comma 400 space divided by space 24 space left parenthesis hours right parenthesis space equals space 6225 space days space space
6225 space divided by space 365 space left parenthesis days right parenthesis space equals space 17 space years

It would take, therefore, 17 years to reach the Sun travelling at 1000km/h all the way. Where would we get the fuel from? On the diagram, reaching the Sun looks relatively easy.

Video 15 mins

Watch This Video:


The Solar System is based around the Sun. The Sun is one of billions of stars that make up a galaxy. Galaxies are huge collections of stars. The galaxy that our Sun belongs to is called the Milky Way.

The Milky Way is made up of billions of stars in a spiral formation. Our Sun is midway down one of the arms and would appear as a tiny speck in this picture, if at all.

It is estimated that there are 200-400 billion stars in the Milky Way. Our nearest neighbour is Alpha Centauri. Remember, it takes light about 8m 20s to reach us from the Sun but the light from Alpha Centauri takes approximately 4.5 years to arrive. How long  would that be in a 1000km/jet?

Nearly 5 million years. (I hope you brought enough food for the journey!) How long would it take for light to travel from one side of our galaxy to the other?

About 100,000 years. In space, we use the term light years to measure distances not time. So, our galaxy is approximately 100,000 light years across.

The nearest galaxy to us is the Andromeda galaxy which is approximately 2 million light years away.

Even if we humans could invent a space ship that travelled at the speed of light (which we are nowhere near yet) it would take 2 million years to get there!

Videos 10 mins

Milky Way in 3D:

How Small is the Earth?

Star Size Comparison:

The Universe

Yes, you guessed it, there are billions of galaxies too. Some estimate there are between 200 and 500 billion galaxies in the universe. Some are like the Milky Way with billions of stars. Some stars are like the Sun with planets. However, if there are alien species out there it wouldn’t be surprising if we never met, because we are so very far apart.

Video 15 mins

The Known Universe:

Answer to Activty:

3.16 km/s

Last modified: Thursday, 12 May 2022, 2:32 PM