Man has looked up at the skies for countless ages. But many are not simply content to admire its majesty but yearn to understand it. To help us understand the heavens, we create models – representations, which while simplified but retain many key elements. It is through models that astronomers “understand” most phenomena from stars to galaxies to the universe as a whole. We've been doing it a long time but the basic process of modeling remains the same: observe and describe those observations as part of the model.
What are some of the astronomical observations that we see?
- All objects in the heavens rise in the east and set in the west.
- The sun travels around the sky in 24 hours.
- The stars travel around the sky slightly faster than the sun (23 hours and 56 minutes).
- There are several thousand stars that we can see at night with good visibility.
There are many ways to describe these events. For example, we could say that a bright luminous god pulling a chariot with a vast drapery of stars flies through the heavens in a great circle, creating day and night. But that wouldn't be very satisfactory to us. Nor was it satisfactory to the ancient Greek philosophers.
To model the heavens, we need a model of the earth. Many in the ancient world believed that it was round. Some of the earliest records come from Ancient Greece (though other cultures were aware of the earth's sphericity). The Pythagoreans and Plato for example taught that the earth was round. Aristotle recorded several observations arguing for a round earth including:
- The lower part of a ship disappears below the horizon first.
- Different stars are visible to different observers and the path they take is different. This implies that “up” is in a different direction as would be the case for those on a spherical surface.
- The shadows of the earth on the moon during a lunar eclipse are consistent with the earth being a sphere.
Several ancient Greeks even measured the radius of the earth. Eratosthenes is perhaps the most famous. Due to a serendipitous cancellation of errors, he came up with a value that was extremely close to the accepted value of the earth’s circumference of 24,900 miles. Other Greeks used more sophisticated and correct methods and got significantly smaller values (which incidentally led Christopher Columbus to believe that he could successfully traverse West across the Atlantic to Asia).
The sun and stars all moved across the sky in a regular, predictable way. The stars moved slightly faster than the sun (by about 4 minutes/day or equivalently 1°/day). The planets, Mercury, Venus, Mars, Jupiter, and Saturn, however behaved differently. The following observations applied to them:
- Mercury and Venus were only seen near sunrise or sunset.
- Mars, Jupiter, and Saturn usually moved slower than the stars (prograde motion)
- Sometimes Mars, Jupiter, and Saturn moved faster than the stars (retrograde motion
- Retrograde motion varied slightly in when and how long it would last.
It is precisely this reason that the planets have their name, which in Greek means “wanderers”. The challenge was how to observe them. Plato is reported to have put forth the challenge to do so with simple, uniform circular motion.