# Thread: How do we know how far away objects are in space

1. The title is fairly straightforward but let me show an example.

The galaxy, SXDF-NB1006-2, which lies 12.91 billion light-years from Earth was spotted by the Subaru and Keck Telescopes in Hawaii.
However, my question is that how do we know how far that object is away from us? From my understanding, we would have to wait 12.91 billion light years for the light to hit our telescope before we can find even faintest trace of it.

It may be a fairly simple question to answer, however I've been curious for a long time.

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4. This is where I find confusion. If the measurement is based on photons moving within an expanding universe, then how accurate is the best guess? Within how many parsecs is typically deemed acceptable? I understand the expansion varies based on whether the stars/galaxies are gravitational bound, or not.

5. Originally Posted by MrMojo1
This is where I find confusion. If the measurement is based on photons moving within an expanding universe, then how accurate is the best guess? Within how many parsecs is typically deemed acceptable? I understand the expansion varies based on whether the stars/galaxies are gravitational bound, or not.
There are no guesses. However, there are error bounds (as with everything in science) on the measurements and calculations. Someone with more knowledge of the field might be able to tell you what the typical accuracy is.

6. Hubble's law - Wikipedia, the free encyclopedia

might be a good introduction.