Physics-related topics II


As asteroids move in their orbits, they can pass in front of background stars. This process is called occultation. You may not be able to see the asteroid, but the background star will dim or entirely disappear for several seconds before reappearing. 


Accurate timing of the disappearance and reappearance of the star (using a specialised GPS-enabled camera) will provide valuable data on where the asteroid is. If more than one observer records the occultation we can even get an idea of the shape of the asteroid.


How can you get a really accurate measurement on an asteroid? The answer is through occultation. 


Accurate orbital information about asteroids’ orbits is helping in planetary defence. NASA has recently conducted its first test of deflecting asteroids, and this type of data is invaluable in aiming probes as well as assessing the effects the collision had. Most data is collected by accredited amateurs including schools.


Note that shadow paths from occultations can be narrow and it may be rare that one passes directly over the facility. A mobile telescope will be required to travel to likely occultation sites, and the methodology is advanced. 

Our Solar System

Humans have been trying to explain their surroundings for millenia. Different cultures have had different explanations for the movement of visible planets over time.


  • Which planets are visible without a telescope?

  • Discuss a couple of non-European cultural descriptions of planetary movement

  • What are superior and inferior planets? Why are they called that? What is the difference in their apparent motion? Show the difference using an orbital diagram.

  • Using a telescope and camera, take a photograph of an inferior planet. Does the image show a circular planet? Why or why not?

  • Using a telescope and camera, show that a planet moves against the background of the stars over time (because the inner planets are so bright, you might have to show an outer planet like Neptune or Uranus) 

  • What is retrograde motion? Can you capture images of a planet moving into or out of retrograde? (This may take several images of the planet over a few months.)

  • What is the plane of the ecliptic? Why did the planets form here during the formation of the solar system?


The telescope inside the dome can track stars while taking images using the camera. Satellites move quickly through the frame, leaving a line across the image. The line is normally solid and constant in brightness, but if the satellite is spinning, its brightness may vary.


Use the camera and telescope to record the passage of a satellite. You are most likely to do this by accident while doing something else!

  • How does the satellite produce the light picked up by the camera?

  • Why are most satellites seen after sunset and before sunrise?

  • How high are different types of satellites? 

  • How do satellites get where they are? 

  • What is the difference between LEO (low Earth orbit) and other orbits?

  • Can you see the International Space Station? How can you photograph it?

  • What is Newton’s cannon? 

  • What is escape velocity, and what happens above this? 

  • What is the relationship between orbital altitude and velocity?

  • Investigate the mathematics of geostationary satellites.

Parallax and Proper motion

For most purposes, stars can be thought of as fixed, never appearing to move. However, this is not quite the case. There are at least two ways in which stars near to the Earth may appear to move against the background of more distant stars.


These two are challenging projects. Not only will they take some astrophotography skill, they also need a long period of time to complete the projects, upwards of six months, and preferably over a few years.


The images for these projects will have to be taken with a long focal length, and have a relatively short exposure time, plenty of other stars in the field, and have good polar alignment and tracking. Do not overexpose the star, as this will cause “bloat” which will prevent accurate measurement of the star’s position.


The Earth orbits the sun at a distance of 1 astronomical unit (about 150,000,000 km). This means that its position relative to the sun changes by 2AU each six months. From these two extremes, stars which are relatively close can appear in different places against the background.

  • What is the closest star to Earth that is easy to image?

  • If you took two images of Proxima Centauri six months apart, what differences in the two images would you expect to see?

  • Take two images of a nearby star six months apart. Can you use this photo to estimate the distance of that star from Earth?

  • Why was the long focal length important for these photos?

  • What design of telescope did you use to take these photos? Was it the best design for the image? Why?

  • What do you think another image of Proxima Centauri taken in another six months would look like?

Proper motion

The big bang theory suggests that, in general, stars should be moving apart from each other. However, due to local gravitational effects, not all stars are moving in consistent directions. This is called proper motion. Some stars appear to move quickly, and images taken in different years may show the movement against the background.

  • In March, take an image of Barnard’s Star in the constellation Ophiuchus (challenging) – this star will be rising in the east in March mornings

  • In October, take another image. The star will be setting in the northwest in October evenings. Can you see a difference in the position of Barnard’s star against the background?

  • Why was the long focal length important for these photos?

  • What design of telescope did you use to take these photos? Was it the best design for the image? Why?

  • For a more accurate and scientifically interesting experiment (and if you can), carry out this experiment each year for several years.



An important part of the search for extraterrestrial life is the detection of exoplanets – those orbiting distant stars. 


Medium sized telescopes, such as those found in educational facility domes, are sensitive enough to detect exoplanets through the transit photometry method. When a large planet passes in front of a distant star, the amount of light reaching Earth from the star decreases by a consistent amount. Accurate photometric recordings of the brightness of stars over time can detect these exoplanets.


  • Determine when (estimated) future transits will occur.

  • Take continuous images of the star well before and well after the transit, taking care not to saturate the pixels the star illuminates). 

  • Analyse the relevant pixels using photometric analysis software ( and generate a light curve. If you caught the transit, you should be able to identify it and stages in the transit.

  • Fit the light curve to a model to identify transit duration, beginning, middle and endpoints.


Galaxies (and black holes)

Find and photograph a galaxy – this is challenging, and will require advanced photographic techniques. Galaxies are mostly small and dim. Sometimes they have spiral shapes, but some have less structure. See if you can photograph a pretty one.


How far away is the galaxy? How do scientists know how far galaxies are? Have scientists always known the difference between our galaxy (the Milky Way) and other galaxies?


Describe some different types of galaxies. What does a galaxy’s shape tell us about its age, or what may have happened to it over its history?


What is a “companion galaxy”? What might be the difference between the two Magellanic Clouds and the closest full-sized galaxy to the Milky Way – the Andromeda Galaxy.


Where is the Andromeda Galaxy heading? What will happen when it gets there?

Some galaxies are known to have black holes at their centre.

  • Why can’t you photograph a black hole?

  • How might an astronomer see indirect evidence of a black hole?

  • What are the jets coming from black holes?

  • (very challenging) at least two galaxies, Centaurus A galaxy (NGC 5128), and Virgo A (M87) have been photographed showing jets of gas coming from the black hole. Can you get an image of a black hole jet?


Chemistry of nebulas

Astronomical cameras come in a number of varieties. If you have access to a monochrome camera and a range of “narrowband” filters, you can photograph and highlight clouds (nebulas) consisting of elements such as hydrogen, oxygen and sulphur.


Where did these elements come from?


What is the spectrum of visible light? What is beyond this?


When different elements are ionised, electrons around their nucleus are displaced, and may rise to a higher (more energetic) shell around the nucleus. After some time in this state, the electron may return to its original state, and in doing so, they emit a photon. What is special about the photon that is emitted in this process? What is the difference between the photon that excites the electron and the photon that is eventually emitted? How does this allow us to identify the element that the photon comes from ?


What is the relationship between wavelength and frequency? What does this have to do with the velocity of light?