Stars
...e historic planets are conspicuous naked-eye objects moving in looped paths against the stellar background. The size of these loops, which are caused by the Earth's own motion round the Sun, are inversely proportional to the planet's distance from the Earth. The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more manageable bits. Around the world, farmers know that for most crops, you plant in the spring and harvest in the fall. But in some regions, there is not much differentiation between the seasons. Since different constellations are visible at different times of the year, you can use them to tell what month it is. For example, Scorpius is only visible in the northern hemisphere's evening sky in the summer. Some historians suspect that many of the myths associated with the constellations were invented to help the farmers remember them. When they saw certain constellations, they would know it was time to begin the planting or the reaping. The nearest star, Proxima Centurai (part of the Alpha Centurai group) is just over four light years away. The diameter of the orbit of this comet cloud would be about 200,000 Astron...