Astronomical Concepts – Week 7

This week – special relativity.


We started with the two main axioms of special relativity:

  1. The laws of physics are the same in all inertial frames of reference

  2. The speed of light is constant (299,792,458 m/s)

Tonight we had to imagine we were floating through space in a spaceship. Are we stationary or are we moving? Are the stars stationary or are they moving? Relative to us of course. With the window blinds down on our spacecraft we have no way of knowing if we are moving or not, there is no experiment we can do to find out.

However, an observer on another spacecraft has a different point of view. On our spacecraft if we bounce a ball we see it go down and then up in a straight line. If an observer on another spacecraft could see us bounce a ball as we drift past them in our spacecraft, and they are stationary they get a different point of view of the ball.


The definition of ‘now’

We accept that certain events that happen can be simultaneous. But how can we determine that two events that happen in different locations are simultaneous?

On Earth we carry clocks and they are synchronised, we accept that two events that happen happened at the same time. What we have to accept is that clocks, time, is influenced by motion. Einstein realised that space contracts and time dilates through motion. A moving clock runs more slowly as its velocity increases, until, at the speed of it stops running all together. So having clocks that run at different rates leads to strange effects – simultaneity is relative. Whether or not two events are simultaneous depend on your frame of reference.


So, one person’s definition of time is not the same as another’s. Also, the faster you travel the slower you age. At speeds close to c the effects are huge, at smaller speeds less so.

Further, the faster you move the more you contract. Close to c the amount of contraction is great, at slower speeds less so, tiny amounts. An observer at rest relative to the moving object would observe the moving object to be shorter in length of motion. As the object increases in speed and gets close to c the object would appear much shorter.


Special relativity is a work of pure genius by Albert Einstein. Our session tonight was an introduction, a mere taster of the theory and we were only able to scratch the surface. I loved the session, the concepts are so interesting, if a little hard to get your head around. Much more information about special relativity can be found here.

So, the only true constant is the speed of light. The faster you travel the more time slows down for you and the more you contract.

With the invention of atomic clocks we can now measure time to billionths of a second and can be accurate to within one second over 3.7 billion years. Einstein said that realising gravity and acceleration were the same thing was “the happiest thought of my life”.

Astronomical Concepts – Week 4

This week’s main topic was light.

James Maxwell

James Maxwell was a Scottish scientist who lived from 1831 to 1879. He is well known for the research he did on electromagnetism and light, building on the work of Michael Faraday. He produced a set of equations that explain the properties of magnetic and electric fields which helped to show that light was an electromagnetic wave. He was able to bring together well established laws of electricity and magnetism and with Faraday’s law they could imply that any disturbance in the electric and magnetic fields will travel out together in space at the speed of light.

He also described Saturn’s rings as numerous small particles, and this theory was proved later on in the 20th century by a space probe.

Einstein’s Photoelectric experiment

Einstein’s experiment showed that packets of light, called photons, contained a fixed amount of energy that depends on the light’s frequency. When a metal plate is exposed to light electrons are expelled. This is the photoelectric effect. The effect was discovered by German scientist Heinrich Hertz in 1887. His observations showed there was an interaction between light and matter. But Albert Einstein was needed to explain the theory further in 1905, his theory of light. Einstein said that light is a particle, called a photon. Einstein was awarded the Nobel Prize in physics in 1921 for his experiment.


Spectroscopy is a technique used to measure the light that is emitted or absorbed or scattered by materials. It breaks light into its component parts and this information can be used to identify and quantify those materials.


When light is absorbed or reflected by materials not all light behaves the same way. Only certain wavelengths of light are absorbed other get reflected. When you seperate the light that is passing through a sample you end up with an emission spectrum or absorption line.

An emission spectrum in the visible light range may look like this.


A spectrum like this would be created when material is given extra energy and that extra energy is later emitted as light energy.

An absorption spectrum would look like this.


A spectrum like this is created when light is passed through a gas or liquid or strikes a solid. Certain wavelengths of light will be absorbed by the material and later emitted in random directions. Most wavelengths will pass through the material without being absorbed. sun_spectrum

The image above shows a spectrum of our sun. From this spectra astronomers can tell what elements the sun is made from, for example hydrogen and helium. It is like a rainbow with holes, the holes are coming from the absorption of energy at a particular wavelength, at a particular colour, by the atoms in the cloud. This goes back to the energy levels of the atom, of only taking energy at very particular energies, as electrons move from one excited state to another excited state. So what you’re seeing is the absorption of photons by atoms. When energy is absorbed you are seeing the energy raising the energy level of an electron. So, we see the rainbow because the inside of the sun is hot and it emits a continuous thermal spectrum. The atoms in the outer layer of the sun absorb some of the energy and use it to promote electrons from a low energy level to a high energy level.


Astronomers also take pictures of light, usually through a filter. Astronomers want to see what the light looks like in red light or green light.

Astronomers also do timing with light, which means to measure the brightness or phase changes as things happen in time.

The combination of spectroscopy, imaging and timing can tell us all kinds of information from the thing we are looking at. We can tell hoe fast something is rotating, if it is moving towards or away from us, temperature, density and composition. Light is our spaceship, we can’t travel to stars and planets light years away but light does travel to us, it is the only way to get the information we need.

Doppler effect

Light waves from a moving source experience either a red shift or a blue shift in the lights frequency. A light source moving away from a stationary observer causes a shift towards the red end of the light spectrum, called a red shift. When the light source moves towards an observer the frequency shifts towards the blue end of the spectrum.


Why is the sky blue?

The sky is blue because atoms in our atmosphere scatter blue light more than they scatter red light. When we look towards the sun at sunset we see red and orange because the blue light has been scattered away from the line of sight. Blue is scattered more because it travels as small and short waves.

That’s enough about light, next week is all about the sun.

Astronomical Concepts – Week 2

The main topics this week were the solar system, gravity and the tidal effect. I have previously written on my blog about the solar system so for this entry I will just write about gravity and the tidal effect.

The two main theories of gravity come from Isaac Newton and Albert Einstein, both are used today, both are brilliant and vastly different. Gravity is one of the 4 main forces of nature, it works on grand scales, the great sculpture of the universe. Our Milky Way galaxy is locked in a gravitational embrace with Andromeda and in a few billion years the two galaxies will collide, just one example of the power of gravity. It holds galaxies together over billions of kilometres.

Gravity is the weakest of the four forces, yet it is so influential. The four fundamental forces of nature are gravity, weak, strong and electromagnetic. Well gravity is by far the weakest, certainly it is very weak here on Earth, but out there in the universe it is quite different. Stand on a planet more massive than ours and you would quickly notice the immense power of gravity. Stand on a neutron star and you would be ripped apart very quickly.


Newton realised that when objects fall to the Earth their must be a force acting on the object, reaching up and pulling it down. He stated that the force of gravity is always attractive, and affects everything with mass. Newton was also able to show that objects with different masses fall at the same rate because an object’s acceleration due to the force of gravity depends only on the mass of the object pulling it, such as a planet.

Newton’s cannon was a thought experiment that demonstrated his theory further. He imagined firing a cannon ball from the top of a mountain. Without the force of gravity acting on the cannon ball it would simply travel in a straight line. If gravity is present then the cannon ball’s path will depend on its speed. If it is slow moving it will fall down to the surface, if it is travelling fast enough it will go into orbit around the planet and if it reaches the escape velocity it will leave the orbit all togehter.


Einstein has a different approach. Einstein says that gravity is not a force but rather a property of space-time geometry. Objects in space, such as planets around a star are all attempting to travel in a straight line through space but that the curvature of the fabric of space means objects are constantly falling towards the mass exerting gravity. Einstein says when you are falling around an object you have cancelled out gravity. Astronauts on the International Space Station are weightless because they are continuously falling to Earth. There is gravity where they are, they are travelling at a speed to stay in orbit around the Earth. The astronauts are continually falling to the Earth but they never reach it, that is why they’re weightless. Being weightless means you are in free fall. When you are in free fall you cancel out gravity. Einstein’s elevator thought experiment explains his theory in more detail, read about it here.


Tidal forces are significant across our solar system. Here on Earth we experience tidal effects thanks to the moon. The Earth experiences two high tides, one on the side of the Earth closest to the moon as the moon pulls the water towards it and on the opposite side as the moon pulls the Earth away from it.

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An extreme case of tidal forces in the solar system is the heating of the moon Io around Jupiter. Jupiter is very massive so the effects on Io are huge,  Jupiter pulls Io inwards and the other moons away from Io pull it the other way, causing Io to distort in shape. This constant change results in lots of friction which in turn drives strong volcanic activity on the surface of Io. Io is the most volcanically active body in the solar system and its surface is constantly changing with large dark spots on the surface caused by collapsed volcanoes.

Our moon is also tidally locked, meaning we see the same side of the moon all the time. It spins once on its axis as long as it takes it to orbit the Earth once, so we always see the same face. The constant tugging from the Earth on the moon has caused this locking to happen.

So what is gravity?


Can’t wait for week 3 – the outer planets.

Exploring the Heavens – Final Week

For the final class the topic was Telescopes. We covered the history of telescopes and the different types. We were meant to go outside and use some telescopes, but due to the heavy Sydney rain this was not possible tonight. The advantage being we got to stay inside out of the cold and were able to ask Dr Payne more questions and learn more about astronomy, including about Edwin Hubble and the Hubble telescope, below:


Telescopes come in many different forms including optical, radio, microwave, infrared, ultraviolet and x-ray. Telescopes are all about extending the capability of the first astronomical detector: the naked eye. The light we capture from telescopes is vital for us to explore and understand the universe.

The telescope was developed in the 17th century and was a Flemish invention. One of the first people to turn the telescope towards the stars was Galileo and in his life he built many telescopes. He made many discoveries, including:

  • The features of the moon such as valleys and craters
  • Jupiter’s moons which meant not everything circled the Earth
  • Many stars
  • Phases of Venus
  • Sunspots

The Galilean moons of Jupiter are: Io, Europa, Ganymede and Callisto.

His most powerful telescope magnified by 33x, meaning everything he saw through it appeared 33x larger. The telescope also captured lots of light so objects through the telescope appeared brighter. He was able to see 10x as many stars than were visible to the naked eye. Brightness is the most important feature of a telescope, not the magnifying power but the brightness.

Galileo built a refractor style of telescope which used lenses, but the problem with this style is that objects appear fuzzy. This problem is called chromatic aberration. This problem was solved by Isaac Newton. Instead of a lens he used a curved mirror and this corrected the fuzziness caused by the lens. This style of telescope is called a Newtonian. All large contemporary telescopes have mirrors as the main optical component. Newton’s original mirror was 2.5 cm in diameter, today they reach 10 m in diameter.

Refractor telescope


This type of telescope is better if it is longer, objects appear closer and with more detail. The function of the objective lens is to form an image close to the opposite end of the tube. The distance from the objective lens to the image is called the focal length. The longer the focal length the larger the image produced. The function of the eyepiece is to enable the eye of the observer to have a closer look at the image. It is like a small magnifying glass. The more powerful, the more the telescope will magnify. The magnification of a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece. For amateurs 40 to 150 is standard.

Reflector telescope


A curved mirror replaces the objective lens. The primary mirror produces an image at its focal length and an eyepiece is used to examine that image. The observer looks into the side of the tube, thanks to the small secondary mirror. Most amateurs use reflecting telescopes. The larger the primary mirror the more light the telescope captured and the brighter the image through the eyepiece. Double the size of a telescope and the light collecting power increases by 4.

The diameter of a telescope matters more than magnification. About 15 cm is a good size to start with. A Newtonian is preferred as you can buy a larger telescope for the same price as a small refractor style telescope. A Newtonian will reveal nebulae that will not appear in the refractor for the same price.

Another type of reflecting telescope is the cassegrain, and they give a superior image to the Newtonian. They have a longer focal length and a completely enclosed tube.


A good mount for your telescope is vital. A good mount is needed to help focus the lens and keep the telescope still. The example below is a Newtonian telescope on an equatorial mount.


The mount has 2 axes of rotation and can be fitted with an automatic tracking device.

What can you see?

We are limited by the type of telescope we have and the atmosphere of the Earth. Light pollution is also a problem for city and town dwellers. It is best to take your telescope into the country to view many stars, Jupiter and Saturn. Depending on the size of the telescope, 20cm will ensure you can see the great red spot of Jupiter and the separation of the rings of Saturn. A 20cm reflector will also let you see Uranus, Neptune, but to see Pluto a 40cm instrument at least is required. With a 10cm reflector bright nebulae will be visible, but as wispy blue clouds.

The biggest radio telescope is currently being built in China at 500m in diameter. The next largest is the Aricebo in Puerto Rico and is 305m across.

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A radio telescope is like a giant ear listening for radio waves from space. Radio waves are a type of electromagnetic radiation similar to light. These signals are very weak, so the larger the telescope the more chance you have of picking up signals.

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This telescope will search for ancient signals of hydrogen, one of the building blocks of the early universe. It will also hunt for new stars, and in particular pulsars, rapidly rotating stars.

The dish is made from 4,500 triangular panels that have been carefully lowered into place. Each panel can be adjusted so the telescope can be moved to view different parts of the sky.

Read more about this amazing telescope here:

The universe began with the big bang and astronomers are building more advanced machines to look further and further back into the past to see what the conditions were like when this happened. One of the main questions to answer is “What is the mean density of the universe?” This will determine how much gravity there is and the eventual fate of the universe. Will the universe keep expanding or will it eventually collapse?

Astronomers are slaves to light and we know that we can only observe about 10% of light from the universe. Or, actually that the universe only gives us 10% of its light. We can’t see dare energy and arm matter. It is thought that 68% of the universe is made from dark energy and 27% is dark matter. We know something is there because of the gravitational influence it has over nearby objects. Everything we know about the universe makes up about 5% of it.

More about dark energy and dark matter in another blog.

This was the final class and the whole course was amazing! Huge thanks go to Dr Paul Payne for his amazing lessons, 3D presentations and humour. Huge thanks also to everyone at Sydney Observatory for putting the course on. I will be back for more astronomy later in the year.

Here is a link to sign up for the next instalment of this course:

Here is a link to Paul’s website: