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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 CruiseObservations 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:

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 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. BrunierAll 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 KnappEudoxus, 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 (Bibilotèque 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.