earth and sun

We see sun, moon, planets and stars rise in the East (roughly), climb higher towards the South and descent and set roughly in the West. In the North sky we only see stars, which in their daily movement seem to revolve around the North Star (Polaris).


start of autumn


The Sun

In summer the sun rises more to the North, in winter more to the South. Look at the Sundial and the two gnomon shadow patterns to the right:

  • The time of sunrise is connected to the direction
  • Length of day (arrow circle) and shadow pattern depend on season and location: the closer to the poles the more it varies through the year

The daily and yearly movement of the Sun can be explained as follows:

  1. The Earth revolving daily around the Sun (not seriously considered)
  2. The Sun revolving daily around the Earth, spiraling up and down: Ptolemy and Tycho
  3. The Earth revolving around its axis and orbiting the Sun (with a tilted axis): Copernicus.

Models of the Universe

The stars

Stars show the same daily movement as the Sun but a star day is about 4 minutes shorter than a solar day. This is explained as follows:

  1. Stars revolve around the Earth daily (but without the spiraling up and down): Ptolemy and Tycho
  2. Stars have a fixed position: the Earth revolves around its axis and travels through space to orbit the Sun: Copernicus

To prove Copernicus right parallax of stars had to be shown (Bessel in 1838) - which is very small, the nearest star being 4 light years away. See for an explanation of parallax the box below


Three models

Eventually three models compete from about the 16th till about the18th century:

  1. Ptolemy needed a lot of math toget his calculations right. His model works pretty well mathematically wiskundig and it could predict the positions of the planets correctly. But he neede epicycles, excenters and equants.
  1. Copernicus put the Sun in the middle, partly to save the circel - but he also needed adaptations: the Earth makes four different movements.
  1. Tycho presented a compromise: an immovable Earth, but the planets orbit the Sun. Semi-Tychonian models don't have all planets orbiting the Sun.

The planets

These wandering stars show a retrograde movement relative to the background of the fixed stars. This phenomenon can be explained as follows:

  1. Planets revolve around a imaginary point orbitting the Earth: epicycles in the model of Ptolemy
  1. Planets orbit the Sun: models of Tycho and Copernicus

The almost unchanging brightness of Venus supported Ptolemy until Galilei saw Venus having phases (like the Moon), changing size at the same time (see picture below). which can only be explained assuming Venus orbits the Sun (Tycho and Copernicus: see picture below).


Three models shown: from left to right Ptolemy, Copernicus and Tycho

Above left an animation of the retrograde movement of the planets is shown.

Next to it is shown how Copernicus explains this apparent movement.

Below you find to drawings to show how Ptolemy explains it:

To the left the system with deferent (the big circle), the epicycle (A). The centre of the deferent rust of centre: the blut dot is the Earth, B is the equant. To the right the movement of planets around the Earth is shown.

LINKS and downloads:

  1. Download the SCIENCE star map
  2. Download Worksheet Sun
  3. On how Erathosthenes measured the size of the Earth
  4. A nicer one on Eratosthenes (password: circumference99)
  5. A short article about Inventing the flat Earth
  6. Go to sun and moon scope
  7. Download Stellarium: a freeware planetarium programme
  8. Sites of the NASA: and
  9. The European Space Agency: ESA
  10. An elaborate BBC-site on space science
  11. A theory (with animations) on the Star of Bethlehem
  12. A good English site about our Solar System.
  13. An article in the Guardian about travelling to Mars
  14. Wikipedia on parallax

Arguments in the debate

Quite a few arguments were put forward:

  1. The speed arguments were very important at the time, the highest speeds being obtained and experienced on horseback or in a horse and carriage (you could not help noticing this speed and it was hard to imagine a much higher velocity would not be damaging).
  2. A revolving Earth should have an effect on winds (which it has: the Coriolis effect - but this seems only a small effect), birds flying in the air and stones falling from a tower.
  3. The Earth seemed to be the centre of creation en the Bible seems to say it stands still.
  4. The Earth orbiting the Sun should show a parallax of at least some stars (see Universe 1).
  5. Venus has an almost unchanging brightness - later on phases were discovered (Unverse 1).
  6. The less movements (and the less complicated the model) the better.
  7. Foucaults pendulum (click here for a demo)
  8. Other planets having moons orbiting them (shown by Galileo using the telescope in 1609)


This phenomenon can be seen everyday: see the animation to the right. When you move (like in a train), objects far away seem to move slower than objects nearer to you.

Stellar parallax can only be observed with good telescopes because of the distance of the stars. How it works is explained in the picture below to the right.


He was very good in math. Using the exact observations of Tycho he could not get his calculations right assuming planets were orbiting in circles - as was assumed to be perfect, divine movement (a Greek thought). He concluded the planets orbit the Sun in ellipses and proposed some famous laws (used by Newton for his laws). He is considered to be one of the founding fathers of modern science.



His contribution to the discussion seems to be vital. Two of his laws are essential:

Planets orbit around the Sun because of the law of gravity - the Sun being the largest.

You do not notice speed because of the law of inertia: nothing changes speed unless a new force is applied upon it. We experience this in a plane cruising over the Atlantic at 900 kms/h.