Imagine a tennis ball. Now imagine that you stuck a spike through the center of the ball. It’s spinning like a globe on its poles.
What if you stopped the spinning and sliced the ball into two even halves using the holes left by the spike as your guides? You’d have approximated something called an hour circle. If you were careful about slicing precisely through the middle, you’d also have created a geometric figure known as a great circle.
This concept of drawing great circles on round objects is an old one, but it still influences our scientific measurements and perceptions about time. The idea underpins Earth’s longitude lines and the North-South borders between our time zones. In other words, if your tennis ball was a planet home to a civilization of beings, then your great circle might have been their prime meridian. Of course, if that was the case, then you probably shouldn’t have stuck a spike through their world and sliced it in half in the first place.
These simple geometric concepts play special roles in celestial astronomy. For instance, hour circles are used in coordinate systems that help observers calculate the positions of objects in space. By measuring apparent motion against hour circles like the celestial meridian, accounting for observational factors and using math, people can determine how things are moving and positioned in the heavens.