Astronomical Concepts – Week 3

The main theme of this week’s session was the outer part of our solar system. This includes:

  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto
  • Asteroids
  • Comets
  • Oort Cloud
  • Kuiper Belt

Oort Cloud

This has never been directly observed but it is believed to exist and it is an area of space on the edge of our solar system between 5,000 to 100,000 AU in distance, so over a vast area. The Oort Cloud consists of millions, perhaps billions of small icy bodies. Every now and then something might disturb one of these bodies and it will become a comet falling towards the Sun. It is named after Dutch astronomer Jan Oort who predicted its existence in 1950.


Kuiper Belt

The Kuiper Belt is another far away region of space that consists of rocky and icy bodies. It extends far beyond Neptune about 30 to 55 AU. It is also predicted to contain over a trillion comets. It takes comets about 200 years to orbit the sun and they travel in a similar plane to the planets. One of the largest and well known objects of the Kuiper Belt is the dwarf planet Pluto. In 2015 the New Horizon’s spacecraft flew past Pluto making it the first mission to a KBO. Another dwarf planet, named Eric, was found in 2005. It is slightly bigger than Pluto and has its own moon. At the time astronomers were considering making Elis the tenth planet, however, in 2006 the International Astronomical Union created a new class of planet called dwarf planets and Pluto and Elis were classified in this new category. The Belt was named after Gerard Kuiper in 1951.

Orbital resonance

A concept described by Paul was orbital resonance. An example to help describe this concept is playing on a push swing. A child can swing by itself at a natural frequency, but the frequency can change by use of an external force, someone else pushing the child on the swing. If the pushes are timed correctly the pushes will build up and the swing gets amplified. With planets and moons when two bodies orbit they exert a regular gravitational influence on each other. This is due to their orbital periods being related by a ratio of two small integers. An example is the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io.


These moons are all in resonance with Jupiter. Io completes exactly 4 orbits and Europa 2 in the same time it takes Ganymede to complete one orbit around Jupiter. During their orbits they sometimes lineup exactly and a gravitational tug is exerted with stretches their orbits into ellipses.

Another example of resonance is the 2:3 resonance of Pluto with Neptune. Pluto completes 2 orbits for every 3 orbits of Neptune around the sun.


Next week … light!

Exploring the Heavens – Week 1

On Tuesday 3 May I attended the first week of my new course on astronomy at Sydney Observatory. The course is titled ‘Exploring the Heavens’ and is led by Dr Paul Payne. The structure of the course is as follows:

  1. History of Astronomy
  2. Celestial rhythms
  3. The solar system
  4. The stars
  5. Telescopes

So the course started with Dr Payne’s two hour version of the history of astronomy in the Sydney Observatory 3D theatre. Some of the main figures covered included:

  • Aristotle (384 – 322 BC)
  • Claudius Ptolemy (~140 BC)
  • Nicolas Copernicus (1473 – 1543)
  • Tycho Brahe (1546 – 1601)
  • Johannes Kepler (1571 – 1630)
  • Giordano Bruno (1547 – 1600)
  • Galileo Galilee (1564 – 1642)
  • Isaac Newton (1642 – 1727)
  • Edmund Halley (1656 – 1742)
  • Charles Messier (1730 – 1817)
  • William Herschel (1738 – 1822)
  • James Bradley (1693 – 1762)
  • Friedrich Bessel (1784 – 1846)
  • John Adams (1819 – 1892)
  • Jean Le Verrier (1811 – 1877)
  • Albert Einstein (1879 – 1955)

Dr Payne kept the pace moving pretty quickly to cover all of these historical and important figures and more about the ancient Greeks, Romans, Babylonians, Renaissance as well as advancements in mathematics, engineering and technology.

Dr Payne also used his homemade 3D graphics to help demonstrate certain themes and concepts, including retrograde motion of planets, constellations and the movement of the moon and planets from Earth’s point of view. The graphics were great and they certainly enhanced the presentation putting us firmly in the cosmic realms.

I was surprised at how important astrology was in ancient times and how seriously it was taken to predict future events. People were also very superstitous and heavenly objects played an important role in how people lived their lives, including Roman emperors. It is also amazing at how much people knew about the solar system hundreds and thousands of years ago without even the aid of a telescope.


As well as learning about the history of astronomy Dr Payne gave us a tour of the night sky on what was a beautiful and clear evening in Sydney. Some of the notable objects we spotted were: Mars, Jupiter, Alpha Centauri, Sirius, Betelgeuse, as well as some notable constellations.


The final part of the evening involved us moving to the south dome of the observatory to use the telescope to view the night sky. We were lucky enough that the sky was clear from clouds and we were treated to an amazing view of Jupiter and the Galilean moons Ganymede, Io, Europa and Callisto.


Image of the telescope in the south dome of Sydney Observatory.

This telescope is the oldest working one in Australia and was built in 1874 by Hugo Schroder in Hamburg, Germany. An interesting article about the telescope can be found here.

This was actually my first time looking through a telescope and the view did not disappoint. I was amazed by how clear Jupiter appeared, being able to clearly identify its white zonesĀ and brown belts, both of which are cloud systems with winds that blow in opposite directions. The Galilean moons, although tiny, were very bright and also so clear to see. This is exactly what I had been hoping to see and has just added more fuel to my growing love for astronomy.

Looking forward to week 2!

5 cool facts about our planets…

5. Venus rotates in the opposite direction to Earth so the Sun rises in the west and sets in the east. It also rotates very slowly so its days and nights are very long.


4. The ‘no atmosphere’ temperature of Earth is 3 degrees Fahrenheit (-16 Celsius). Without the Greenhouse Effect our oceans would freeze over.


3. Saturn is light and fluffy enough that its density is less than 1 gram per cubic centimeter – the density of water. So Saturn would float if placed in water.

2. Jupiter’s coloured bands are different cloud systems. The white stripes are called zones and the brown stripes are called belts. The winds in zones go one way, winds in belts go the other way. When you see different colours on the Jovian planets you are actually seeing clouds at different heights.


1. Earth’s surface gets repaved every 100 million years. This is due to volcanism, tectonics and erosion. Large craters on Earth have been paved over.

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