Development of the Heliocentric (Sun-‐Centered) Model

Development of the Heliocentric (Sun-Centered) Model

The sequence of events that brought most humans to a heliocentric model is a great example of the scientific process. Humans make a model and then change the model after collecting more data. The first big problem with the geocentric (earth-centered) model (in which the sun and all stars move around a stationary earth) was the retrograde motion of planets like Mars. If you looked at the location of Mars each night, it might sometimes do this.

Image: NASA/JPL-Caltech

How does the earth-centric model deal with this new evidence? Here are some highlights. • Ptolemy develops an earth-centric model that has the sun and planets moving around the Earth. However, in order to account for retrograde motion, he put the planets on circles that move in circles. • Copernicus suggests a heliocentric (sun-centric) model. His model has the planets moving around the Sun in circular orbits. This can explain retrograde motion, but his model doesn’t fit all the planetary position data that well. Really, it’s no better than Ptolemy’s earth-centric model. • Kepler proposes that the planets do not orbit in circles. Instead, they have elliptical orbits. This agrees with the observational data very well. • Galileo gets a telescope and looks at the sky. He see stuff that suggests the Earth orbits the Sun. I’ll discuss these in a bit. • Newton develops a model for gravity that also says planets would have elliptical orbits. • This change from earth-centric to heliocentric took a long time. It’s silly to think that people just woke up one day and said “Ah ha! Let’s put the sun in the center!”

How Can You Tell the Earth Orbits the Sun? No one likes to just trust the textbook. You don’t have to. Here are some things you can do to determine for yourself what orbits what.

1. Phases of Venus. The next time Venus is visible in the sky, take a look at it a small telescope. It will probably look something like this.

and on a different night: http://linksthroughspace.blogspot.com/2012_05_01_archive.html

This is something that Galileo saw in his telescope. Although it looks like the moon, this object is the planet Venus. What does this mean? It means three things. First, we can see Venus because it reflects light from the Sun. Second, as the phases change, we are seeing Venus at different angles from the sun. Third, Venus is sometimes closer to us than the Sun and sometimes farther away. You would see a “full phase” Venus when it is on the other side of the Sun. If both Venus and the sun orbit the earth, why is Venus sometimes farther away from the sun, and sometimes closer to earth? And why would its phases change over time?

2. Moons of Jupiter.

This is something else that Galileo did that you can repeat: see the moons of Jupiter. You just need good binoculars. Look at Jupiter and it will look something like this:

Image: NASA/JPL/Malin Space Science Systems Well, it won’t look quite like that. You will probably just see Jupiter as a dot without any details. But you WILL be able to see the 4 big moons of Jupiter. So what? The idea that all the planets (and the Sun) orbit the Earth isn’t as strong once you show that there are objects that orbit another planet. These moons of Jupiter clearly orbit Jupiter and not the Earth.

3. Parallax of Stars.

To collect this evidence, you would need a very high powered telescope and at least 6 months of patience. What is parallax? Parallax is the apparent change in position of objects due to a change in observation location. You can also easily see an example of parallax by holding your thumb out in front of your face. Using just your left eye look at where the thumb lines up with some object in the distant background. Now look with just your right eye – the thumb moved, right? That’s parallax since your two eyes are at different locations.

The ancient Greeks hypothesized that if the Earth is moving around the Sun then the stars should shift their positions due to this orbital motion (called stellar parallax).

http://physics.unm.edu/101lab/lab4/images/Parallax_Stars2_03.gif

It sounds pretty easy, but in practice, it is very difficult. The ancient Greeks couldn’t do it. This is because the shifts in apparent star position are very small (earth’s change in position around the sun is miniscule compared to the distances to stars). The first successful measurement of stellar parallax came more than two hundred years after the invention of the telescope (by Friedrich Bessel in 1838). About 60 stellar parallaxes had been measured by the end of the 19th century. Our current best large set of parallax measurements are from the Hipparcos satellite which retired in 1993. These parallax measurements are only possible because the earth moves around the sun. If the sun orbited earth, star parallax would not occur.

Primary Source: How Do We Know the Earth Orbits the Sun? By Rhett Allain 04.14.14 http://www.wired.com/2014/04/how-do-we-know-the-earth-orbits-the-sun/

Additional Sources & References: http://en.wikipedia.org/wiki/Stellar_parallax http://spiff.rit.edu/classes/phys301/lectures/parallax/parallax.html http://sci.esa.int/gaia/54414-gaia-go-for-science/