# A quick (elementary) check on arsecond conversion to sky distance in parsecs

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I just wanted to double check I had something correct. If an object, say a nebula, is measured to roughly have a diameter or extent of, say, 250 arcseconds, would its diameter in parsec simply therefore be 250 parsecs?

The pedagogical answer is this. You're confusing the

# Width

of something, with it's

# Parallax!

Here's the fixed background

Here's something that happens to have width…

Here's something that has parallax…

That's the deal!

Note that width can just be measured, using one printout of the photo and a wooden ruler.

But parallax means two photos taken 180 days apart, and you measure how much the thing has moved.

(The mind-boggling GAIA does not actually use wooden rulers!)

One huge confusion with "parsecs" is this:

Parallax means the distance an object moves in two photos take six months apart. That technique can be used with objects from 1 to about 100 parsecs away from us.

However, astronomers (for convenience) still use the measure "parsecs" even when the distance is far more than 100 parsecs. And, they even use the measure "parsecs" for things like "size", which just have no connection at all to parallax photo measurements.

If you hear that something is "5000 parsecs away", it is inconceivable the distance was determined using parallax photo measurements!! And, if you hear that something is "1000 parsecs wide", that just has no connection at all to parallax photo measurements (it's a width - nothing to do with the distance from Earth!!)

In the question at hand here, the "width" of an object was quoted in parsecs. The OP was originally assuming that, since parsecs were mentioned in relation to the width, you could use something related to the technique of parallax photo measurement, in relation to the width. In fact, parallax photo measurement just has no connection, whatsoever to width. You literally might as well use smell or taste - heh! The confusion is that parsecs happen to be also used (very confusingly) to measure things like width: but remembering that the technique of parsecs (parallax photo measurement) has absolutely no connection at all to width, and no connection to anything more than 100 ps from us.

Only if it happens to be 206 kpc away, i.e. 1/4 of the distance from here to the Andromeda galaxy. A parsec is the distance at which 1 AU makes an angle of 1 arcsecond. If the nebula is 100 parsecs away and has an angular diameter of 250 arcseconds, then its diameter is $$100 mathrm{pc} imes 250 mathrm{as} = 25000 mathrm{AU} = 0.12,mathrm{pc}$$