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 1]]

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.

[[☃ image 1]]

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:

[[☃ video 1]]

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:

[[☃ image 1]]

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.

[[☃ image 1]]

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.

[[☃ image 1]]

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:

[[☃ image 1]]

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.

[[☃ cs-program 1]]

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.

[[☃ image 1]]

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.

[[☃ image 1]]

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

[[☃ image 1]]

It 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 1]]

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 1]]

Unfortunately, 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 1]]

In 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 1]]

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.

[[☃ image 1]]

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!

##Lunar eclipse

The heliocentric model contains one remnant from the geocentric model. Can you find it?

[[☃ image 1]]The geocentric model correctly assumed that the moon orbits the earth. We now know this is the reason for the phases of the moon. Why is this? First let's take a look at a sequence of photos taken of the moon over 29 days:

[[☃ image 1]]The following is a detailed animation that covers an entire year of the earth/moon cycle. In the center there is a large image of the moon as seen from earth each evening. Take this time to convince yourself why the moon has phases:

[[☃ video 1]]

Notice that the moon passes directly through the shadow of the earth on days when there is a full moon. Only when the the sun, moon and earth are closely aligned like this is it possible to observe a lunar eclipse. Several cultures have myths related to lunar eclipses. The Egyptians saw the eclipse as a pig swallowing the moon other cultures view the eclipse as the moon being swallowed by other animals, such as a jaguar in Mayan tradition, or a three legged toad in China.

[[☃ image 1]]

However, the lunar orbit is not perfectly aligned with the earths orbit. It’s shifted by around 5 degrees. This means the moon does not always pass exactly through the middle of the Earth’s shadow, causing the eclipse time to be much shorter. In some cases the eclipse doesn’t occur at all as the Moon sweeps outside the shadow cone.

[[☃ image 1]]

Variations on an eclipse

Astronomers classify eclipses into three types depending on the position of the Moon around the Earth’s two shadows: umbral and penumbral:

[[☃ image 1]]

Total lunar eclipse

This occurs when the entire moon passes through the earth’s umbral shadow. This video follows the moon during a total eclipse:

[[☃ image 1]]

Partial eclipse

This occurs when the moon passes through the earth’s penumbral shadow but only a portion of the much darker umbral shadow.

[[☃ image 1]]

Penumbral eclipse

In some cases the moon only passes through the penumbral shadow of the earth, these are much harder to observe as there is little change in illumination. Look closely as this is one of the most subtle phenomena in naked eye astronomy:

[[☃ image 1]]

Challenge questions

Could the length of an eclipse tell us something about the size of the Moon relative to the Earth?

We have 12 full moons per year, how come we always have far fewer lunar eclipses?

Next, we have an interactive simulation which will allow you to explore these lunar eclipses in more detail.