Cosmic ladder (Part 1)

It’s a lot more difficult to measure distances in space than you might think. [1 of 2]

Sombrero Galaxy
NASA/ESA and The Hubble Heritage Team (STScI/AURA), Public domain, via Wikimedia Commons

Back in the post on the island universe debate, I noted that early estimates for the size of the galaxy varied widely. And in my post on the dark universe – that period of hundreds of millions of years when nothing was producing light – I noted that measuring distances in outer space is really really difficult. Well, it’s time to talk about the cosmic distance ladder.

If you look at the landscape from the side window of a moving car, you might notice that nearby objects seem to be moving faster than distant ones. Fenceposts by the side of the road zip past, but trees on the horizon fall behind at a much more stately pace. This is parallax.

Parallax animation
NuclearDuckie, CC0, via Wikimedia Commons

With a bit of simple trigonometry, you can figure out how far away those fences and trees are, based only on your own speed and their apparent motion relative to the objects behind them. And it turns out that this technique can scale up. Way up. We can use parallax to measure distances in space.

The Earth orbits around the sun. As we move from one side of the solar system to the other, our view of nearby stars – the position of those stars in our sky – changes too. If we measure those changes relative to our own motion, we can use that parallax to calculate the distance to those stars.

(Parallax, by the way, is so important to celestial cartography that the standard measurement of distance in space – the parsec – is an abbreviation of “parallax of one arcsecond.”)

So far so good, but now we hit our first hurdle. The distances are so great that beyond a certain point, we just cannot measure the change accurately enough to make the calculations. This measurement doesn’t work even for distant stars in our own galaxy; the parallax is just too small to detect.

We know of no way to directly measure the distance of anything further away. The distance to other galaxies is a mystery… but one that we can solve by measuring indirectly. Tomorrow, we’ll explore the standard candle.

[Part 2 comes tomorrow.]

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