Exploring the Heavens – Week 3

The topic for this week was ‘The Characteristics of the Solar System’. The main items covered were the formation of the solar system, the planets and other celestial bodies such as comets and asteroids.

Due to being ill this week I attended the Thursday class instead of my usual Tuesday class, but the format was exactly the same. Paul’s 3D presentation was fantastic as it put us right in the middle of the solar system and we could experience it from many different angles and points of view, which really helped in developing our understanding of how it all works.

One of the main themes throughout this course has been the ‘ecliptic’ – a plane on which all the planets sit and orbit the Sun. Our solar system consists of our star – the Sun – an object which dominates our neighbourhood, consisting of 99% of the mass and also gravitationally. The 8 planets and countless comets and asteroids all belong to the Sun. The unit by which we measure the distance of the planets from the Sun is called an astronomical unit, and the Earth is 1 AU from the Sun (approx. 150m km). It is possible that the distances of all the planets from the Sun can be explained with a mathematical formula that proves they are not just random distances.

A sidereal period is the time it takes a planet to orbit the sun. The further the planet from the Sun the greater the period. Kepler’s third law – the law of periods – found a simple relationship between the distance and the period. The ratio of the average distance from the Sun cubed to the period squared is the same constant value for all planets.


His equation:


Our best guess at how our solar system was formed is called the Solar Nebula theory. Any theory has to explain the characteristics of our system:

  1. The order of the orbits of planets
  2. The categories of planets – terrestrial and jovian
  3. The amount of comets and asteroids
  4. Anomalies


  1. Our solar system formed from the gravitational collapse of a large cloud of gas and dust
  2. As the cloud collapses, conservation of energy, momentum and angular momentum flatten it out into a disk
  3. The diffuse clouds end up as a spinning disk of gas and dust with the young protosun at the middle
  4. The spinning disk is hotter at the centre and colder on the outside, so closer in are more material of rock and iron, further out more hydrogen compounds such as methane, explaining the makeup of our planets
  5. Finally the Sun ignites and releases a strong solar wind to clear away the remaining dust other material. We are left with the planets we have now

We talked about all of the planets and their characteristics. When we were outside looking up we could clearly see Jupiter and Mars on the ecliptic. The signs of the Zodiac pass through the ecliptic. This is a good way to find a planet!

The synodic period is the time it takes for a planet to return to the same angle with respect to the Sun. The synodic period for Mars is 780 days for it to move from opposition to opposition. Prior to opposition is when the planets move in retrograde motion.

We had an amazing view of the Moon tonight through a telescope on the balcony at the observatory. This was my first time viewing the Moon through a telescope and it looked amazing, we could see so much detail. I can only imagine the view from the Moon looking back at Earth.


The Moon has a sidereal period (orbit) of 27.3 days and appears to go through its phases every 29.5 days (synodic period). The basis for our month. The phases of the Moon are due to the changing appearance relative to the Sun, however, we only ever see one side of the Moon as it has been locked in its orbit, due to the tidal effect from Earth.

The Moon is the major force behind tides on Earth. The gravity of the moon pulls the water  up towards it, creating an uneven distribution. The Earth and moon orbit a common centre of mass, located close to the surface of the Earth. As the Earth rotates on its axis each point on the surface is subjected to a sequence of high and low tides every 6 hours. These tides are also causing the moon to slowly drift further away from Earth at 3.7 cm per century.

Finally, we talked about eclipses, both solar and lunar. Paul said that we should all try to make it to a total solar eclipse, when the moon obscures the Sun. This happens when the 3 bodies are in exact alignment, which happens every 6 months or so. This phenomenon is possible because the Sun and Moon look the same size in the sky. The Sun is approx. 400x larger than the moon but is is 400x times further away, hence a total eclipse of the Sun is possible. Very few places on Earth will see a total eclipse due to the precision needed to make this happen.

Next week… The Stars!

Astronomy at Questacon in Canberra

Questacon in Canberra has a pretty cool section on space and astronomy. Check out the pictures I took in the gallery below.

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A few of the highlights from this exhibition.


  • WMAP is the Wilkinson Microwave Anisotropy Probe
  • It launched on June 30, 2001
  • Has mapped fluctuations in the cosmic microwave background (CMB) radiation (the oldest light in the universe) and has produced a full map view of the microwave sky
  • Has determined the universe to be 13.77 billion years old
  • Determined the curvature of space to within 0.4% of flat Euclidean
  • Found that the universe is 24% dark matter
  • Found that dark energy makes 71.4% of the universe, causing the expansion rate of the universe to speed up

Morgan Keenan System (MKK)

  • Classification of stars based on temperature
  • Categories divided into Roman numerals, with sub-categories and classes
  • To completely describe a star the MK luminosity class is appended to the original Harvard classification for the star
  • The Harvard Spectral Classification assigns each star a spectral type with is further divided into 10 sub-classes depending on the absorption features present in the spectrum. E,g, our Sun has a temperature of 5,700 Kelvin and is classified as a G2 star
  • Our Sun is a main sequence star G2 and the full classification is G2V

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Types of orbit

  • The Moon does not orbit the Earth, the Moon and Earth orbit each other around their common centre of mass. The common centre of mass is inside the Earth
  • Kepler showed the orbits of the planets were ellipses with the Sun at one foci
  • Newton showed there were other types of orbit
  • Elliptical and circular orbits are both bound orbits
  • Parabolic and hyperbolic are unbound orbits
  • Comets and asteroids typically have hyperbolic or parabolic orbits, they zip around the Sun once and they go off never to be seen again because they do not have gravitationally bound orbits
  • Look at the circular cone above, how you slice the cone determines what kind of geometric cross-section you get – horizontal makes a circle, an angle creates an ellipse, increase the angle and get to the bottom of the cone you get a parabolic and cut the cone vertically you get a hyperbola
  • So 4 different types of orbits


  • Newton’s laws of gravity showed that Kepler’s first 2 laws, ellipses and equal area in equal time are a consequence of the conservation of angular momentum (Mass x speed x distance = constant)
  • At perihelion the distance to the Sun is smaller so to keep the mass times the speed times the distance the same the conservation of angular momentum the plane must move faster
  • At aphelion, further away from the Sun, conversely the speed must decrease
  • P squared is a cubed (P^2 = a^3)
  • P is the period the time it takes to go around and the semi-major axis of a of the ellipse, a is the average orbital distance

Thank you F.X. Times at Arizona State University for some of the information included.