In: Physics
We often use telescopes to capture beautiful pictures of the planets and distant galaxies. If optical telescopes like the Hubble telescope can capture pictures of galaxies millions of light years away, why can’t we get detailed photos of stars and planets in our own galaxy that are only a few hundred light years away?
The apparent size of an object is a result of just two numbers, its true size (diameter) divided by its distance: A 10-foot object located 100 feet away appears the same angular size as a 100–foot object located 1,000 feet away. And the bigger an object is relative to its distance, naturally, the larger it will appear: a 20-foot object at 100 feet will look twice as big as a 10-foot object at that distance.
Consider a prominent nearby galaxy, the Andromeda Galaxy. It is approximately 150,000 light years wide and 2.2 million light years away. The ratio of its size to its distance is 1:15, and the full Andromeda galaxy (if you could see all the way out to its dim edges) stretches nearly 4 degrees in the sky. It’s actually much larger in the sky than the sun or moon. It is so large that the Hubble Space Telescope had to take 7,398 separate images to make this partial mosaic portrait.
Now let’s consider Pluto. It has a diameter of 1,473 miles and is currently located just over 3 billion miles from Earth. The ratio of its size to its distance is about 1:2,000,000. Put another way, the Andromeda Galaxy appears more than 100,000 times as wide in Earth’s sky as Pluto does. It is, roughly speaking, the difference between looking at a poster on your living room wall and trying to look at a bacterium on the wall next to it.
That is why Pluto is so hard to see clearly from Earth.