I find that when I talk to people about astronomy and space, one of the hardest concepts to understand is the actual scale of things in our Solar System, galaxy, and universe. But this is not wholly unexpected because just every astronomical picture found in the media is depicted with a scale that is either incorrect or not understandable by the viewer. So let's go ahead and talk about the scale of things. First, let's start with the Earth.
The Earth is a sphere with a diameter of about 8,000 miles and a circumference of about 25,000 miles. This isn't too hard to figure out if you know that from coast to coast across the United States is about 3,000 miles and you're looking at a globe that has the land forms shown at a proper scale; you can use the United States as your guide to estimate the circumference of the Earth - give it a try! What is less obvious is exactly how big the Earth is compared to us. The atmosphere of the Earth is 100 miles thick, and if we scale the Earth down to the size of a standard globe, the atmosphere would be thickness of a piece of paper - and we live at the bottom of that piece of paper! No wonder we can't feel the rotation of the Earth (about 800 miles per hour at our latitude) or its motion through space as it orbits the Sun (about 20 miles per second) - we're just too small!
Next week we'll talk about the Earth-Moon system and go out from there!
Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.