This module has provided you with an introduction to the way that astronomers view the night sky, and you have begun to develop an understanding of how the observations we make here and now on Earth translate to a greater cosmological picture. At the end of the module, we explored how this picture leads to a simple explanation for the seasons on Earth.
In a few places, important details had to be neglected because they were not critical to this week’s discussion. These were silently left to be covered in upcoming modules. For instance, I told you that the stars’ positions are fixed, and that their only apparent motion is a daily revolution about the Earth. However, the Earth’s axis is not quite as fixed as I said. As you’ll learn in Module 2, the Earth actually has a slow wobble, like a toy top, which causes the positions of the celestial poles and the equinoxes to precess slowly over the course of 26,000 years. The effect was in fact discovered by Hipparchus, who invented the magnitude scale and created the first comprehensive map of the stars. Also, I mentioned a number of times that the Earth’s orbit is roughly circular. What I did not say is that the orbital paths of all planets are really ellipses. This will be a key concept in the learning material of Module 3. The Earth’s atmosphere was discussed in connection with the seasons, noting that it is at least partially responsible for the contrast in temperatures between the northern and southern hemispheres at different times of the year. However, I had no opportunity to discuss why the Earth even has an atmosphere, which we will come to in Module 7. Finally, I mentioned the IAU, the international group of astronomers who come up with definitions and conventions for describing astronomical objects, but said nothing about the 2006 definition of a planet which resulted in Pluto being left off the list for our Solar System. This will be explained in Module 9.
These are just a small sample of the many details that will be discussed as we make our way through the course. And all of them were learned from observations made here and now, by looking up at the night sky and searching for explanations of the things that we observe, applying the method of hypothesis formation, verification and rejection that we call science.
cosmiCave.org 