I’ve always wondered how astronomers can actually observe galaxies that are millions or even billions of light-years away. What exactly are we seeing when we look at them?
And when scientists say there are hundreds of billions, or possibly trillions, of galaxies in the observable universe, how do they know that if we obviously haven’t seen and counted every single one?
I chose a simple standard candle for a quick explanation, but yes, these are all independently verified by other methods, which in turn verify each other etc. Parallax is a good one: at one side of the earth’s orbit and the other, you measure the relative position of the thing you want to know the distance to, which is just like closing one eye then the other. Distant objects move less in your vision/against the background stars than close objects do.