I'm having trouble defining the term "parsec". I've seen two definitions:
1. The distance at which one astronomical unit subtends one second of arc.
2. The distance at which one "solar parallax" subtends one second of arc.
A "solar parallax" (as I understand it) is the diameter of the earth's orbit around the sun. This distance would be different than an AU, thus changing the definition of parsec. Can anyone set me straight?
Parsec

 Apathetic Retiree
 Posts: 20396
 Joined: Mon Aug 28, 2006 2:06 pm
 Location: Oklahoma
Re: Parsec
The parsec (parallax of one arcsecond; symbol: pc) is a unit of length, equal to just under 31 trillion km (about 19 trillion miles), or about 3.26 lightyears.
Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from the Earth and Sun, i. e. the angle subtended at a star by the mean radius of the Earth's orbit around the Sun. The parsec (3.26 lightyears) is defined as the distance for which the annual parallax is 1 arcsecond.
An astronomical unit (abbreviated as AU, au, a.u., or sometimes ua) is a unit of length equal to about 149,597,871 km (92,960,000 miles). It is defined by the IAU, and is defined as the mean distance between the Earth and the Sun over one Earth orbit (i.e. the mean Earth orbital radius).
A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond.
Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from the Earth and Sun, i. e. the angle subtended at a star by the mean radius of the Earth's orbit around the Sun. The parsec (3.26 lightyears) is defined as the distance for which the annual parallax is 1 arcsecond.
An astronomical unit (abbreviated as AU, au, a.u., or sometimes ua) is a unit of length equal to about 149,597,871 km (92,960,000 miles). It is defined by the IAU, and is defined as the mean distance between the Earth and the Sun over one Earth orbit (i.e. the mean Earth orbital radius).
A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond.

 Asternaut
 Posts: 3
 Joined: Tue Nov 11, 2008 3:42 pm
Re: Parsec
I certainly appreciate this question, because I have struggled with parsecs and arcseconds for a couple of years. I still am not clear....What is an arcsecond and how do we know where a star is as observed from the sun when we can only observe it from earth. By calculating the difference between the earth and the sun, but still how do we know? Because it seems to me it would depend on the distance of said star. Help! I need laymens terms, what is a arcsecond and a parsec?

 Apathetic Retiree
 Posts: 20396
 Joined: Mon Aug 28, 2006 2:06 pm
 Location: Oklahoma
Re: Parsec
A degree is 1/360 of a circle, an arcminute is 1/60 of a degree, an arcsecond is 1/60 of an arcminute.
The distance between the Earth and Sun is negligible compared to the distance to even the nearest stars.
The distance between the Earth and Sun is negligible compared to the distance to even the nearest stars.

 Asternaut
 Posts: 3
 Joined: Tue Nov 11, 2008 3:42 pm
Re: Parsec
Nice! Thanks Bystander....actually that helped a great deal. Appreciate it!