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The geocentric universe

A cyclical nature

Astronomy has been practiced for as long as humans have been looking at the sky and wondering what it all means...
Image of the Venus and the Moon which appear close together during a conjunction. - Image: Brit Cruise
The most noticable feature of our sky is the sun. Its appearance and disappearance each day dictates the design of calendars, rhythms of society and even our biological clocks.
During a sunset it appears that sun is moving around the earth -  Image: Brit Cruise
Observations of the sun have led to many key developments in astronomy.  To this day, the sun reveals new and fascinating secrets. Below is a condensed history of our observations of the sun:
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At the center of everything

It is understandable that early observers assumed that the sun was traveling around them. The same is true for the evening sky which seems to contain a countless number of tiny suns. Interestingly, the stars seem to shift position gradually over time, some fast, others slow. There is a cyclical nature to many of these changes. If you take a long exposure photograph of a clear night sky over a full night, you will see something like this:
The long bright stripes are star trails and each one marks the path of a single star across the dark night sky - Image: S. Brunier
All stars seem to rotate around a common point in the sky. It seems that there is a circular nature to the path of all objects in the sky around us. Based on these observations, Plato developed an entire model of the Universe in which everything moved on circular orbits at a constant speed. This was in line with his theory of pure forms, and seemed like a perfect model of the Universe.
In this image the stars appear to streak across the sky about a common center. The effect is created as the earth spins along its axis of rotation - Image: Robert Knapp
Eudoxus, one of Plato's pupils, proposed a universe where all objects in the sky sit on moving spheres, with the Earth at the centre. This model is known as a geocentric model – often named Ptolemaic model after its most famous supporter, the Greco-Roman astronomer Ptolemy.
Bartolomeu Velho, "Figure of the Heavenly Bodies", Cosmographia, 1568 (Bibliothèque nationale de France, Paris)
It puts us in the center of everything, an obviously satisfying conclusion at first glance. However, there was a problem with this model. It is apparent to anyone who observes the sky each evening over a long period of time and tracks the position of the brightest stars. These often appear earlier in the evening. Take a closer look at the first image from this article, slightly later in the evening:
Venus near the Moon slightly later in the evening. Notice how Venus clearly stands out as compared to the background stars - Image: Brit Cruise
Imagine you photographed the sky once each night and animate it over the course of a year, how would the sun, moon and stars move? To save you some time we’ve built an interactive simulation for you below. It represents an image taken each evening animated to show an entire year. While doing so think about why this would present a problem with the geocentric model...
You can scan the sky in both directions using your mouse or finger.

Want to join the conversation?

  • leaf green style avatar for user Larry Ho
    for the planets moving around the sun with the view from earth, what is the time elapse?
    (7 votes)
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    • piceratops ultimate style avatar for user Terry Tolleson
      If you're asking how quickly the time has been sped up, I estimated the answer to this. Short answer, every second represents just over 15 days. For the long answer, see the following explanation as to how I arrived at this estimate:

      1) First, I timed the orbit of Venus around the sun from the perspective of the Earth (essentially, I started the timer when Venus "eclipsed" the view of the sun, and stopped the timer when it reached the next "eclipse"). This was roughly 38 seconds in the time-lapse simulation above.
      2) Next, I used an online orbit simulator (http://lasp.colorado.edu/education/outerplanets/orbit_simulator/) as a cross-referencing tool. The reason I did this is that you can see the orbit of the planets from a "bird's eye" perspective. This is important, because as Venus orbits the sun, so does the Earth.
      3) Given that it takes Venus 224.7 days to orbit the sun, it would appear that it takes about 2.6 orbits (a rough estimate) around the sun between the "eclipses" (as seen from Earth). This would be about 584 days.
      4) Finally, I divided this number by the 38 seconds estimated from the time-lapse simulation above, and calculated that every second of the simulation above represents just over 15 days.
      (25 votes)
  • aqualine seed style avatar for user Cole Topham
    How many galaxies do we know of?
    (7 votes)
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  • blobby green style avatar for user 2013jamesparker
    Is there any point in time where Mars, Venus, or Mercury block any amount of sunlight from planet Earth; due to their (Mars, Venus, or Mercury) orbiting?
    (3 votes)
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  • purple pi pink style avatar for user AkhilFlare10
    why is the sun moving in the picture
    (4 votes)
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  • starky ultimate style avatar for user ericson3026
    I don't understand last simulation.. could you explain more simply?? ^^;
    (4 votes)
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  • winston baby style avatar for user h.hamednalla
    what are the limitations/cons of the geocentric model
    (2 votes)
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    • male robot hal style avatar for user Charles Breiling
      The good news about the geocentric model is that it shows the Earth as a globe, with everything (sun, moon, stars, planets) revolving around us. This matches perfectly with what we see with our vantage point on the ground. But that wasn't your question. :-)

      The problem is when we study the motions of other bodies, especially the planets, and find that their motions are very, very strange if in fact they're orbiting us. We also get some very extreme speeds of these orbiting bodies if every single one of them orbits us once a day. The further they are from us, the faster they must move, to be able to orbit us daily.

      So then it turns into a problem of our frame of reference. Since we're standing on a slowly rotating planet, it appears as though all celestial bodies rotate around us, but if we change our frame of reference we discover we're orbiting the sun. Then all the motions of the other planets make sense.

      This Heliocentric (sun at the center) model of our solar system also makes sense in terms of Kepler's laws of motion, as well as gravity. None of that makes any sense in the geocentric model, the motions of the planets, sun, and moon just happens by magic.
      (5 votes)
  • aqualine ultimate style avatar for user Diego Ponce
    Is it possible to get confused by a star and a planet?
    (3 votes)
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  • winston default style avatar for user ROMAN C00LB0Yn0t7
    how cold is mars
    (4 votes)
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  • duskpin ultimate style avatar for user Tithi Mayani
    How exactly does a long exposure photograph work?
    (2 votes)
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    • piceratops sapling style avatar for user Sydney
      Hi! As you probably know, a long exposure photograph is a photograph that is being taken for a much longer period of time than the normal, less-than-a-second photographs. In a long exposure photograph the shutter (the part of the camera that swings open to let light in and also has a mirror on it) is held open for a long period of time in order to let more light in. This may be done for things like deep sky objects in space or stars to show their movement over a period of time. This is because with the naked eye, the deep sky objects do not seem very bright. However, while taking a long exposure photograph, more light is taken in over a period of time, so in the photograph, the object can appear much brighter rather than dim and barely recognizable. You can think of this as giving the light enough time to come in and print the picture into the camera; it's almost like putting an ink stamp on a piece of paper and then stamping it multiple times to make it brighter and easier to see. A long exposure photograph however, is not multiple photographs. It is only one. The camera may also need to track the object meaning follow it over the period of time the image is taken so the image doesn't appear blurry.
      Hope this helps!
      (4 votes)
  • blobby green style avatar for user Aaliyah
    If the sun is moving what is it rotating around?
    (2 votes)
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