ASTRONOMY - our Solar System, Satellites, Stars and Galaxies

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 See also Cosmology - the Big Bang Theory of the Universe

 and the Life Cycle of Stars for more detailed notes

 How has the explanation, theory and model of the Solar System evolved through time?

 Explain the usefulness of telescopes and photography - observing the Solar system and the cosmos in general with visible light.

How has astronomy changed over the centuries?



Models of the Solar System

The ideas about the structure of the Solar System have changed over time, including the change from the geocentric to the heliocentric models and the discovery of new planets.

The geocentric model theory:

The sun, moon, planets, and stars ie everything, all orbited the Earth in a series of concentric circles.

This model originates from ancient Greek civilisation 2000-2500 years ago and lasted for 1500-2000 years.

Ancient astronomers didn't have telescopes to make accurate observations to test out any other model.

What they observed was the Sun and moon moving across the sky in the same way every day and every night.

They did observe the apparently wandering of the planets - the word planet comes from an ancient Greek meaning 'wanderer', but this did not lead to a questioning of the geocentric model

Prior to ~1500s this was the accepted model of the Universe from the time of the ancient Greeks.

The heliocentric model:

From the 1500s onwards and with the help of telescopes, evidence was mounting up to indicate that the planets were orbiting the Sun in perfect circles - the heliocentric model.

We now recognise that the Earth and other planets are moving around our Sun in elliptical orbits.

Initially the Sun was considered as the centre of the Universe, but of course we now know it is now only the centre of our solar system.

Astronomers such as Copernicus working in the mid-16th century, were making observations and calculations to explain the movement of the planets without the geocentric model and that a heliocentric model fitted the data better.

Copernicus published his heliocentric theory and calculations in 1543, just in time, two months before his death!

Galileo’s observations of Jupiter, using the telescope, provided evidence for the heliocentric model of the Solar System - initially it provided evidence that not everything revolved around the Earth.

Galileo, in the early 17th century, working with the newly invented telescope, found his view of the 'universe' in conflict with that of the Catholic Church, especially after discovering moons orbiting around the planet Jupiter, which meant not everything orbited the Earth and the geocentric model was flawed.

One piece of Galileo's evidence came from observing the moons moving around Jupiter with his newly invented improved telescope of 1609.

Initially Galileo thought four stars were surrounding Jupiter, but he figured out they were moons of Jupiter because of their movement - they moved along with Jupiter and were observed in different positions around it - therefore showing that all 'heavenly' bodies were not moving around the Earth.

The Catholic Church was not too impressed by a scientific model challenging the religious view of how our 'universe' works.

Over the following centuries more evidence for the heliocentric model increased, helped by the technological advances in astronomy eg improved telescope designs.

From the early 20th century onwards, it was recognised that our solar system is just one of billions of systems with a central star in a galaxy and that there are billions of galaxies that make up everything - the Universe!

 


Our contemporary model of our Solar System and beyond!

The 8 major planets, minor planets and asteroids orbit the Sun in slightly elliptical orbits (our 'Solar System'), but our Sun is just one of millions-billions of stars in our galaxy (we see part of it as the 'Milky Way') and in turn the observable universe itself contains billions of other galaxies.

(a) Our Solar System - the Sun (a main sequence star) and orbiting planets and asteroids

The movement of the planets and asteroids has been observed from visible light (reflected sunlight) for thousands of years, initially with the naked eye and from the early 16th century onwards, with telescopes.

With modern techniques, the Sun can be observed by detecting emissions in various regions of the electromagnetic spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray emissions.

(b) Our Milky Way - is our view of looking through our own galaxy

Our solar system is just one small part of a galaxy - which is a massive collection of billions of stars held together by gravity.

Until relatively recently, the Milky Way galaxy, has been observed with the naked eye and then telescopes on Earth, but now it can be viewed through powerful telescopes on satellites eg the Hubble Space Telescope.

Our galaxy, and for that matter distant galaxies, can be continually observed using everything from giant radio telescopes, huge optical\visible light telescopes to gamma ray burst detectors.

(c) The Universe is everything - see separate page Cosmology - the Big Bang Theory of the Universe


Methods of observation

Be able to compare methods of observing the Universe using visible light, including the naked eye, photography and telescopes.

In observing the night sky, the naked eye, apart from aesthetic appreciation, has been largely replaced by photography, usually coupled to a telescope.

However, historically, stars, planes, comets, our Moon have all been successfully discovered, observed, mapped and plotted via naked eye observations and astronomical tables of data assembled.

Distant stars can be seen because they are so hot and powerful emitters of electromagnetic radiation eg visible light.

Optical telescopes have much better light gathering power than the naked eye and the lens and lens-reflecting mirror systems can produced greatly magnified images and can peer into deep space totally inaccessible to the naked eye.

Optical telescopes were the earliest types used to examine the 'universe'.

Optical telescopes detect visible light and convex refracting lenses and concave reflecting mirrors are used in their construction.

To improve the quality of the image you can increase the diameter of the objective lens - the primary light gathering lens at the front end of the telescope.

You also get better resolution the greater the optical quality of the lenses.

For mirror based reflecting telescopes, you can increase the diameter of the concave mirror to gather more light and improve the quality and resolution of the image.

Optical telescopes are limited to visible light observations, so you need other types of telescopes.

For more on detecting different EM radiations see the first section of my Life Cycle of Stars page.

Photographing the same patch of sky and comparing images from one night to another can show up whether an object is moving eg asteroid or comet or some new star appearing or an old star exploding in a massive supernovae explosion,.

So, anything that changing that reflects or emits visible light can be detected and by using long-time exposures you can detect very faint very distant objects.

The result of all these historical and continuing contemporary observations with telescopes of all kinds is to give us a pretty good picture of the observable universe, even if we don't fully understand how it all works!

Problems with observations and ways to improve matters

The most obvious problems with the use of optical telescopes is absorption of light by the Earth's atmosphere and light pollution.

The Earth's atmosphere both absorbs and scatters light from an astronomical source which reduces the quality of the image.

Light pollution comes from any light source on the Earth's surface that emits light into the sky at night eg from road traffic, office blocks, street lights etc., all of which makes it more difficult to observe dim-faint objects in the sky

Air pollution eg particulates/dust can also absorb or scatter light diminishing the quality of ANY image.

All of these problems can be considerably reducing by getting a telescope to operate high in the atmosphere OR above the Earth's atmosphere completely.

Observatories using optical telescopes can be sited high up on mountains in dark places where the atmosphere is less dense (thinner), especially in remote places where there is little pollution of any nature - dust/particulates or artificial light sources.

The Hubble Space Telescope has been put into orbit around the Earth acting just like satellite with an adjustable optical telescope. Since it is above the atmosphere, many of the problems of image quality are greatly reduced.

 

How a reflecting telescope works

A reflecting telescope uses a concave mirror

A relatively large concave mirror collects as much light as possible from distant astronomical object e.g. a star.

The collect light is reflected by a small plane mirror at ~45o into an eyepiece or camera to record the image.

By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star.

Advantages over convex collecting lens.

 


More on our Solar System

The Sun is our star at the centre of our solar system.

The solar system is everything that orbits our Sun (and everything that itself orbits anything orbiting the Sun e.g. moons).

A planet is a large body that orbits a star.

There are eight planets in our solar system - listed in the table below in order of distance from the Sun.

A true planet may be defined has having the following properties-criteria:

It must orbit a star (e.g. our Sun).

It must be big enough to have enough gravity to force it into a spherical shape.

It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

Dwarf planets (minor planets)

These objects do not meet the current criteria for planets - usually because they are not big enough.

This is not the case with Pluto, which, due to gravity has a moon called Charon that is half it's size orbiting it - so, Pluto is now classed as a dwarf planet - it just isn't big enough!

Another reason is that some asteroids are bigger than Pluto and could also be classed as planets!

8 PLANETS Distance from Sun in Mkm Mass relative to Earth Size relative to Earth Time to orbit Sun (days or years) Axis rotation time Average surface temperature oC
Mercury 58 0.05 0.4 88 d 58.6 d +350
Venus 108 0.8 0.9 225 d 242 d +480
Earth 150 1 1 365 d 24 h +22
Mars 228 0.1 0.5 687 d 24.7 h -23
Jupiter 778 318 11 12 y 9.8 h -153
Saturn 1430 95 9.4 29 y 10.8 h -185
Uranus 2870 15 4 84 y 17.3 h -214
Neptune 4500 17 3.8 165 y 16 h -225
Pluto (dwarf planet) 5915 0.003 0.2 248 y 153 h -236

Natural satellites

A moon is a relatively large body of material that orbits a planet - they are natural satellites and they have almost circular orbits - actually very slightly elliptical.

We have one moon, Jupiter has dozens and Galileo spotted four of them!

A satellite can be defined as any smaller object, due to its speed and resulting force of gravity, that orbits a more massive object

Artificial satellites

Artificial satellites are 'human-made' satellites that launched into space to orbit a planet or one of its moons and they also have almost circular orbit.

The orbit is designed so an artificial satellite can perform a particular function.

More on artificial satellites in the next section and why one object can orbit another in a steady cycle.

Asteroids

Asteroids are irregularly shaped lumps of rock or metal that orbit the Sun and are found in the asteroid belt between the orbits of Mars and Jupiter.

Comets

Comets are lumps of an ice and dust mixture that orbit the Sun.

Their orbits are often highly elliptical and some can travel from near the Sun to the outer regions of our Solar System.

Unlike the planet orbits, which are all in a narrow plane, comet orbits are at all sorts of angles with respect to this plane of our solar system.

They hurtle around in all directions.

 


The physics of circular motion and the forces involved for solar and planetary systems

Velocity is a vector quantity, it has both size (the speed) and direction.

If either the speed or direction changes, you have a change in velocity - you have an acceleration!

What velocity are we dealing with? What force are we dealing with?

 

So, how do we explain the circular motion of the objects illustrated above!

 

To keep a body moving in a circle there must be a force directing it towards the centre.

This may be a moon or satellite around a planet or a planet around a star - all apply to planet Earth.

This force is called the centripetal force and produces the continuous change in direction of circular motion - which means constantly changing velocity of the moon or planet etc.

Even though the speed may be constant, the object is constantly accelerating because the direction is constantly changing via the circular path - i.e. the velocity is constantly changing (purple arrows, on the diagram).

For an object to be accelerated, it must be subjected to a force that can act on it.

Here the resultant centripetal force of gravity is acting towards the centre, so always directing the object to 'fall' towards the centre of motion (blue arrows on the diagram).

But the object is already moving, so the force of gravity causes it to change direction - speed is constant, but change in direction means acceleration is taking place.

In other words the object keeps on accelerating towards the object it is orbiting and the instantaneous velocity, at right angles to the acceleration, keeps the object moving in a circle.

SO, the actual circular path of motion is determined by the resultant centripetal force (black arrows and circle) ...

... and the circling object keeps accelerating towards what it is orbiting!

The centripetal force stops the object from going off at a tangent in a straight line.

The centripetal force will vary with the mass of the objects in question and the radius of the path the object takes around the other.

You can argue that the path of a given object in a stable constant orbit depends on its distance from the object it orbits and the strength of the gravitational field it experiences.

Summary of the 'rules'

You need to check the text below with the diagram above!

To keep an given object in a stable orbit (moving at constant speed), the faster it moves or the smaller the orbital radius, the greater the gravitational centripetal force needed. See the pattern for the planets of our solar system.

If the speed of an orbiting object changes the orbital radius must also change.

If an orbiting object initially slows down it is pulled by the gravitational centripetal force into an orbit of smaller radius and increases in speed.

Satellites at lower heights above the Earth may experience tiny friction effects from the upper atmosphere. Eventually this causes the satellite to slow down and move to a lower orbit. Ultimately the satellite may be drawn into a higher speed orbit lower in the atmosphere and burn up from the heat of friction. This is sometimes done deliberately to 'safely' remove a defunct satellite.

Conversely, If an object initially speeds up, it partially overcomes the centripetal gravitational force moves to a higher orbit of greater radius, but then it slows down in a larger radius stable orbit.

In positioning a satellite via a 'transporter' rocket, it must be given the correct velocity to go to the right height and with the right speed into the desired stable orbit or specified radius.

We can now apply these ideas to the three situations described below.

1. Circular motion - velocity & centripetal force for a moon around a planet

 

2. Circular motion - velocity & centripetal force for a moon around a planet

3. Artificial satellites - see the separate section.

 

The planets move around a star in almost circular orbits e.g. planet Earth travelling round the Sun once a year.

The same force of gravity keeps a moon orbiting a planet e.g. our Moon orbiting the Earth.

The same arguments on circular motion apply to the movements of planets around a sun, a moon around a planet and a satellite orbiting a planet.

The orbits are usually elliptical, rarely a perfect circle, but the physics is the same.

In these cases, it is the force of gravitational attraction that provides the centripetal force and it acts at right angles to the direction of motion.

You should also realise that they are moving through empty space (vacuum), so there are no forces of friction to slow the object down.

This is why the planets keep going around the Sun and the moon keeps going around the Earth.

 

The pattern in the size of a planet's orbit around a star

8 PLANETS Distance from Sun in Mkm Time to orbit Sun (days or years)
Mercury 58 88 d
Venus 108 225 d
Earth 150 365 d
Mars 228 687 d
Jupiter 778 12 y
Saturn 1430 29 y
Uranus 2870 84 y
Neptune 4500 165 y
Pluto (dwarf planet) 5915 248 y

The closer an object is to the object it is orbiting, the stronger the gravitational force of attraction.

The stronger the gravitational force of attraction, the faster the object must move in order to avoid crashing into the object it is orbiting.

For any object in a stable orbit, the radius of the orbit must match the speed the object is travelling.

Faster moving objects must move in a smaller radius to have a stable orbit.

If the speed changes for some reason, the radius must change too.

You can see clearly that the further the planet is for the Sun (our star) the longer it takes for that planet to orbit the Sun once (relevant data highlighted in bold).

This means, the further out the planet is from the Sun, the slower it is moving AND in a larger radius circle with respect to the Sun as its centre - to move in a stable orbit.

 

Orbiting objects (planets/moons) - summary of the connection between gravity, mass and radial distance (size) of the orbit.

The gravitational field strength depends on the mass of the object creating the field.

The larger the mass of the object, the stronger the gravitational field  e.g. Jupiter > Earth > our Moon

The gravitational field strength experienced by an orbiting object, also varies with its distance from the object it is orbiting.

The stronger the force an orbiting object experiences, the greater the instantaneous velocity needed to balance it e.g. to keep it in stable orbit.

Therefore the closer a planet is to a star or a moon to a planet, the faster the orbiting object must travel to stay in orbit.

To have a stable orbit, the object must have a speed that matches the gravitational 'pull' at a particular radial distance from the object it orbits.

The smaller the radius, the faster the object must travel to have a stable orbit.

8 PLANETS Distance from Sun in Mkm Relative speed km/s Time to orbit Sun
Mercury 58 47.9 88 d
Venus 108 35.0 225 d
Earth 150 29.8 365 d
Mars 228 24.1 687 d
Jupiter 778 13.1 12 y
Saturn 1430 9.7 29 y
Uranus 2870 6.8 84 y
Neptune 4500 5.4 165 y
Pluto (dwarf planet) 5915 ? <5.4 248 y

I've already mentioned the pattern in the distance of planets from our Sun and the speed they are travelling at - as measured by the length of time for one orbit of the Sun.

From the table you can see, the further than planet is from the Sun, the slower the speed it travels at.

You might think that there would be greater variation e.g. between Mercury and Neptune, but remember, the outer planets have a long way to travel in their orbit!

Because the planets are moving at different speeds, ancient astronomers had noted the varying positions of the planets against the relatively constant positions of real stars. They did not realise until later, that these 'wandering stars', as they called them, were actually what we now know as planets orbiting a star (our Sun).


More on natural and artificial satellites

You can define a satellite as a smaller object orbiting a larger object and is kept in orbit by the force of gravity.

Natural satellites are those that occur naturally in a 'solar system'.

e.g. our own moon orbiting the Earth, the many moons of Jupiter.

Artificial satellites are those that we, with our technology, put into orbit around our moon, the Earth or our other fellow planets e.g. Mercury, Venus and Jupiter.

When satellites are put into orbit they are given just the right amount of horizontal velocity so that the resultant centripetal force of gravity keeps the satellite in its a circular orbit.

You can vary this horizontal velocity to position satellites at different distances above the Earth's surface.

Most satellites we launch into space are those that orbit our own planet Earth - diagram on right of geocentric orbits - an orbit of anything orbiting the Earth (over 2000 artificial satellites currently orbiting the Earth).

 

Earth satellites polar orbit geocentric orbitSatellites in polar orbit  (orbit 1 on diagram, actually should be lower than orbit 2)

Satellite orbit 1 passes over (or near) both poles, one after the other in its cycle, and is said to have a polar orbit. It circles around the Earth at about 90o to the equator.

'Polar satellites' have relatively low orbits and the Earth rotates under them.

The lower orbit means they have to have a faster speed (physics above) than geostationary satellites (below) to stay in orbit which may take as little as a few hours.

Because the orbit time is short and the Earth rotates within the orbit, they can monitor the whole surface of the Earth many times in every 24 hours.

Therefore satellites in polar orbit are used for mapping the landscape, military surveillance and producing weather charts for weather forecasting.

 

Earth satellites polar orbit geocentric orbitSatellites in a geostationary orbit (geosynchronous satellites)  (orbit 2 on diagram)

Satellite 2 travels around in the same direction as the Earth's spin and at the same angular speed are moving in a geostationary (geosynchronous) orbit.

Geostationary satellites have a high orbit that takes ~24 hours to go round the Earth (e.g. at a height of 35786 km above the Earth's surface).

This means they keep a constant position above the Earth's surface because they take exactly one day (23 hours 56 minutes) to complete one orbit.

These geosynchronous satellites are very useful for continuous communications e.g. radio, telephone and TV because you can point transmitters and receivers in their direction and they are stationary relative to each other.

Also, the signals can transmitted all around the globe in a fraction of a second ~speed of light.


Atomic emission line spectroscopy - used by astronomers to identify elements in stars

If the atoms of an element are heated to a very high temperature in a flame they emit light of a specific set of frequencies (or wavelengths) called the line spectrum. These are all due to electronic changes in the atoms, the electrons are excited and then lose energy by emitting energy as photons of light. These emitted frequencies can be recorded on a photographic plate, or these days a digital camera.

Every element atom/ion has its own unique and particular set of electron energies so each emission line spectra is unique for each element (atom/ion) because of a unique set of electron level changes. This produces a different pattern of lines i.e. a 'spectral fingerprint' by which to identify any element in the periodic table .e.g. the diagram on the left shows some of the visible emission line spectra for the elements hydrogen, helium, neon, sodium and mercury.

The surface of stars is so hot that all the atoms can potentially emit their characteristic frequencies and from the frequency pattern an element can be identified.

This is an example of an instrumental chemical analysis called spectroscopy and is performed using an instrument called an optical spectrometer and is used to identify elements in stars when attached to a telescope.


See also Cosmology - the Big Bang Theory of the Universe and the Life Cycle of Stars for more detailed notes


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Typical learning objectives and knowledge for the contents of this page

  • Be able to describe how ideas about the structure of the Solar System have changed over time, including the change from the geocentric to the heliocentric models and the discovery of new planets.
    • The geocentric model: The sun, moon, planets, and stars ie everything, all orbited the Earth in a series of concentric circles.
      • This model originates from ancient Greek civilisation 2000-2500 years ago and lasted for 1500-2000 years.
    • The heliocentric model: The Earth and other planets orbited our sun -then considered as the centre of our universe.
      • Astronomers such as Copernicus working in the mid-16th century, were making observations and calculations to explain the movement of the planets without the geocentric model and that a heliocentric model fitted the data better.
        • Copernicus published his heliocentric theory and calculations in 1543, just in time, two months before his death!
      • The Catholic Church was not too impressed by the scientific model challenging the religious view of how our 'universe' works.
    • Our contemporary model: The 8 major planets, minor planets and asteroids orbit the Sun in slightly elliptical orbits (our 'Solar System'), but our Sun is just one of millions-billions of stars in our galaxy (we see part of it as the 'Milky Way') and in turn the observable universe itself contains billions of other galaxies.
  • Be able to show an understanding of how scientists use waves to find out information about our Universe, including:
    • a) the Solar System - the Sun and orbiting planets and asteroids
      • The movement of the planets and asteroids has been observed from visible light (reflected sunlight) for thousands of years, initially with the naked eye and from the early 16th century onwards, with telescopes.
      • With modern techniques, the Sun can be observed by detecting emissions in various regions of the electromagnetic spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray emissions.
    • b) the Milky Way - the view looking through our own galaxy
      • Until relatively recently, the Milky Way galaxy, has been observed with the naked eye and then telescopes on Earth, but now it can be viewed through powerful telescopes on satellites eg the Hubble Space Telescope. Our galaxy, and for that matter distant galaxies, can be continually observed using everything from giant radio telescopes, huge optical\visible light telescopes to gamma ray burst detectors.
  • Be able to discuss how Galileo’s observations of Jupiter, using the telescope, provided evidence for the heliocentric model of the Solar System.
    • Galileo, in the early 17th century, working with the newly invented telescope, found his view of the 'universe' in conflict with that of the Catholic Church, especially after discovering moons orbiting around the planet Jupiter, which meant not everything orbited the Earth and the geocentric model was flawed.
  • Be able to compare methods of observing the Universe using visible light, including the naked eye, photography and telescopes.
    • In observing the night sky, the naked eye, apart from aesthetic appreciation, has been largely replaced by photography, usually coupled to a telescope.
    • However, historically, stars, planes, comets, our Moon have all been successfully discovered, observed, mapped and plotted via naked eye observations and astronomical tables of data assembled.
    • Distant stars can be seen because they are so hot and powerful emitters of electromagnetic radiation eg visible light.
    • Telescopes off much better light gathering power than the naked eye and the lens and lens-reflecting mirror systems can produced greatly magnified images and can peer into deep space totally inaccessible to the naked eye.
    • Photographing the same patch of sky and comparing images from one night to another can show up whether an object is moving eg asteroid or comet or some new star appearing or an old star exploding in a massive supernovae explosion,.
    • So, anything that changing that reflects or emits visible light can be detected and by using long-time exposures you can detect very faint very distant objects.
    • The result of all these historical and continuing contemporary observations with telescopes of all kinds is to give us a pretty good picture of the observable universe, even if we don't fully understand how it all works!
  • Be able to explain how the eyepiece of a simple telescope magnifies the image of a distant object produced by the objective lens (ray diagrams are not necessary).
  • Be able to describe how a reflecting telescope works.

 


WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index

General introduction to the types and properties of waves, ripple tank expts, how to do wave calculations

Illuminated & self-luminous objects, reflection visible light, ray box experiments, ray diagrams, mirror uses

Refraction and diffraction, the visible light spectrum, prism investigations, ray diagrams explained gcse physics

Electromagnetic spectrum, sources, types, properties, uses (including medical) and dangers gcse physics

The absorption and emission of radiation by materials - temperature & surface factors including global warming

See also Global warming, climate change, reducing our carbon footprint from fossil fuel burning gcse chemistry

Optics - types of lenses (convex, concave, uses), experiments and ray diagrams, correction of eye defects

The visible spectrum of colour, light filters and explaining the colour of objects  gcse physics revision notes

Sound waves, properties explained, speed measure, uses of sound, ultrasound, infrasound, earthquake waves

The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave analysis) gcse notes

Astronomy - solar system, stars, galaxies and use of telescopes and satellites gcse physics revision notes

The life cycle of stars - mainly worked out from emitted electromagnetic radiation gcse physics revision notes

Cosmology - the Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse physics


IGCSE physics revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology IGCSE revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology KS4 physics Science notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology GCSE physics guide notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology for schools colleges academies science course tutors images pictures diagrams for astronomy our Solar System, Satellites, Stars Galaxies cosmology science revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology for revising physics modules physics topics notes to help on understanding of astronomy our Solar System, Satellites, Stars Galaxies cosmology university courses in physics careers in science physics jobs in the engineering industry technical laboratory assistant apprenticeships engineer internships in physics USA US grade 8 grade 9 grade10 AQA GCSE 9-1 physics science revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology GCSE notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology Edexcel GCSE 9-1 physics science revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology for OCR GCSE 9-1 21st century physics science notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology OCR GCSE 9-1 Gateway  physics science revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology WJEC gcse science CCEA/CEA gcse science GCSE physics revision notes on astronomy our Solar System, Satellites, Stars Galaxies cosmology

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