1. Member
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Nov 2002
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I was hoping one of you more experience astronomer-types could clarify something seems to me should be pretty simple, but I must be missing something (not enough coffee today perhaps).

What exactly is the deal with interstellar distances being measured in arc? Like the parsec for example. An arc is an angular measurement, is it not?

How are degrees of arc translated into distance? Do you need to know the distance between the observer and the object to calculate a lateral measurement in arc? That doesn't seem right to me since a parsec is a constant, standard unit of measure.

Can somebody tell me what I'm missing here?

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Oct 2001
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disclaimer: I am, by no means, an experienced astronomer type.

A parsec is indeed a unit of length and is equal to about 3.26 light years or 3E16 metres. A parsec is the distance (from the sun) that an object must be in order to give a parallax of one second of an arc (1/3600 of a degree).

Astronomers prefer using parsec (moreso for nearer objects) since measuring a heliocentric parallax directly measures its distance from our sun.

Useless fact: 1 attoparsec (1E-18pc) is about an inch.

3. Member
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Nov 2002
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disclaimer: I am, by no means, an experienced astronomer type.

A parsec is indeed a unit of length and is equal to about 3.26 light years or 3E16 metres. A parsec is the distance (from the sun) that an object must be in order to give a parallax of one second of an arc (1/3600 of a degree).

Astronomers prefer using parsec (moreso for nearer objects) since measuring a heliocentric parallax directly measures its distance from our sun.

Useless fact: 1 attoparsec (1E-18pc) is about an inch.
Makes sense, thanks for clearing that up for me. I think I'm going to start using "attoparsec" here and there just to confuse people now [img]/phpBB/images/smiles/icon_smile.gif[/img]

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Jan 2002
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On 2002-11-16 23:33, segfault wrote:

What exactly is the deal with interstellar distances being measured in arc? Like the parsec for example. ....

Can somebody tell me what I'm missing here?

Stellar parallax corresponds to distance.

1 arcsecond of parallax = 1 parsec distance

= 3.26 L.Y.

Smaller parallax corresponds to greater distances.
Use this formula:
Parallax(in arcsec) = 1/distance(in parsecs)

<font size=-1>[ This Message was edited by: Gsquare on 2002-11-17 00:12 ]</font>

5. And parallax is defined by the radius of the orbit of Earth, not the diameter. In other words, a parsec is the distance at which Earth would look to be separated from the Sun by one arcsecond.

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