Distance
Two Minute Astronomy, 1
When you look up at the night sky, what do you see? Tiny pinpricks of light, and the Moon too sometimes. Some of these pinpricks are planets (characterised by the fact that they don’t twinkle like actual stars). Have you ever wondered how astronomers find their way around this seemingly arbitrary-looking expanse of the sky?
The answer is simple. Similar to how the world has been mapped and represented as globes and maps, the sky and positions of stars have been recorded in the form of a celestial sphere (remember this word, we’ll come back to this in the next article). How is that possible?
Before we delve deeper, let’s recall the concept of light-time that many of us would have encountered in school. We know that celestial bodies are at such long distances from us (and each other), that light from them takes considerable time to reach our eyes, even when travelling at 2.998 x 10⁸ m/s. For example, sunlight takes around 8 minutes and 19 seconds to travel from the Sun to the Earth, while the closest star to the solar system, Proxima Centauri (I’ll cover star etymology in a different chapter) takes 4.24 years to beam its light to us.
The consequences of this phenomenon is apparent: if Proxima was to disappear in an instant (highly unlikely though), we wouldn’t know until more than four years after it had actually happened. Or sample this: if someone is observing us right now from the Andromeda Galaxy (which is one of the most distant celestial objects in our night sky visible with the naked eye) they would see the Earth as it was 2.5 million years ago, long before modern humans even existed!
So we’ve established that celestial bodies outside of the Solar System are far, far away from us. In fact they are so far away that we can actually pretend they don’t move around at all over the course of a human lifetime, even though they are all hurtling through space at various velocities (the whole Solar System is moving at a whopping 230 kilometres per second right this instant, as it revolves around the center of our galaxy, and as the Milky Way itself is moving across space). So for all practical observational astronomy purposes, we can consider that celestial bodies (apart from Solar System objects) are fixed and stationary. Of course, over thousands of years, the positions of stars with respect to each other will change, and the constellations we know today will no longer be around.
This brings us to the concept of the celestial sphere, which we will explore in the subsequent episode. Until next time then!
PS: If you want to try to grasp the sheer scale of the distances we are dealing with here, check out this video!
