# What does subtended mean? Parsec definition confusion.

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Thread starter 6 years ago
#1
How would you use it in the sentence? It obviously is something to do with angles.

I am trying to define a parsec.

Which one is correct?
* One parsec is the distance to an object which subtends an angle of 1 arc second to the line from the centre of the Earth to the centre of the Sun.

* One parsec is the distance to an object which the line from the centre of the Earth to the centre of the Sun subtends an angle of 1 arc second.

The definitions are too vague.

And, is the mark scheme for one of the physics papers wrong?
The mark scheme answer confused me.

In one of the old AQA past papers, it stated that one parsec was:
parsec : distance to an object subtending 1 sec of arc to Earth’s orbit.

I believe it is the Earth's mean orbital radius to be precise and the definition in the mark scheme is too vague or incorrect - but I might be wrong.

Any idea?

BTW, I know what a parsec is, I can draw a triangle. But, to put it in words is harder since I don't know how to use subtend in a sentence.

Here's a shorter definition I found from a website:
Parsec - the distance at which one AU would subtend a second of arc.
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6 years ago
#2
Subtend (here when talking about angles) just means "makes" or "creates" an angle.

Definition.
"A parsec is an astronomical unit of length equal to the distance at which a baseline of one astronomical unit subtends an angle of 1 second of arc."

One astronomical unit is defined as 149 597 870 km
which is the mean distance between the centre of the Earth and the centre of the Sun.
(This was the original definition.)
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