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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...
Wandering stars
Early astronomers observed that a few of the brighter stars seemed to wander in the sky over the
course of many months. Attributing this behaviour to a form of extreme power, these wandering
bodies were assumed to be deities by the Babylonians. The Greeks named them “wandering
stars” or “astēr planētēs” – the root of the word we use today: planets.
On 1 May 2011 five of our Solar System’s eight planets and the Moon could be seen. Image:
G.Hüdepohl The geocentric model couldn’t yet explain why planets would appear brighter and
darker at different times. The most striking issue was how to explain why the planets would
occasionally stop and reverse directions. This is known as retrograde motion and would be
impossible if the planets were the same distance to us at all times. Below is an image of the
geocentric model which assumed that all planets travel around the earth in perfect circles.
Andreas Cellarius, "Ptolemaic orbits", Harmonia Macrocosmica , 1661.The red planet Mars was
particularly difficult to explain. If we assume a geocentric universe and map out the wandering
motion of the Red Planet we’d see this pattern
Retrograde motion of mars when viewed from EarthIt was assumed that something must be
wrong with the model. For early astronomers, there was a strong need to keep the Earth at the
center of the universe. This led to some very creative ways of justifying the geocentric model of
the universe. How could they explain why these pretzel shaped orbits occur if everything travels
in circular orbits? Below is a representation of the apparent motion of the sun and planets from
the earth. It shows the orbit of Venus over 8 years and Mars over 7 years. Notice the
preztel shaped orbits which emerge:
Image: Encyclopædia Britannica (1777)
Epicycles
The Greek geometer and astronomer Apollonius of Perga (262 - 190 BCE) came up with a model
to explain these changes in position and brightness. He believed that this cyclical variation could
be represented visually by mini orbits, or epicycles, revolving around larger circular orbits, or
deferents. The following animation shows Mars traveling around a red orbit, while the red
orbit is also orbiting around the Earth on a secondary orbit.
Image: Peter CollingridgeUnfortunately, the epicycles made the new geocentric model incredibly
complicated, and some planets even needed epi-epicycles for the model to match astronomical
observations. There was, however, a competing theory that greatly simplified things. For this
theory to gain support, astronomers as well as political and religious leaders needed to give up
the dominant view that the Earth was at the center of the universe. This was not going to be an
easy task.
Giving up control
When observing a sunset it is natural to assume the sun is moving around us. Aristarchus of
Samos (c. 310-230 BC), a Greek astronomer, was first to maintain that the earth rotates on its
axis and revolves around the sun. This was a dangerous assertion at the time and could not win
over supporters of the geocentric model, a model that persisted for over a thousand years.
Image Credit: Brit CruiseIn 1543, Nicolaus Copernicus (1473 – 1543) published “On the
Revolutions of the Celestial Spheres”, in which he explained what many had suspected: that the
sun is at the centre of the universe and we move around it along with all the other planets. This is
called the Heliocentric Model.
Image of heliocentric model from Nicolaus Copernicus' "De revolutionibus orbium
coelestium".This theory resolved the issue of retrograde motion by arguing that these pretzel
orbits were an illusion due to our vantage point. It is a perfect example of Occam’s razor: if you
have two competing hypotheses, the one with the fewest assumptions should be selected. The
simplest explanation tends to be correct. If you look closely you'll see his model correctly
assumed the moon is orbiting around us. Notice how this explains the strange wandering motion
of planets when observed from telescopes based on Earth.
It's the motion of the Earth relative to Mars which causes this wandering effect. The next
simulation will allow you to explore both the geocentric vs. heliocentric models of our solar
system. Have fun!
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