rlow wrote:oops...I should have explained in my earlier post...
One parsec is defined as the distance of a star (nearby) which has a PARallax (apparent shift in position against the background stars) of one arc SECond (hence PARSEC) when viewed from the Earth six months apart (ie, from the two extreme opposite positions of the Earth's orbit).
I hope this clarifies the above issue.
rlow
In 6 months, the Earth would have moved 2 AUs.
By definition, a parsec is the distance to a star which has a parallax angle of 1"
The parallax angle is the angle measured divided by 2. Check out the website I mention above to get a better understandering.
Basically, the defination is: The parsec is approximately the distance at which the RADIUS of the orbit of the Earth around the Sun covers an angle of 1 second of arc.
Note it is radius, not diameter. Hope the information is useful.
Have a nice day.
Yang Weixing "The universe is composed mainly of hydrogen and ignorance."
"One parsec is defined as the distance of a star (nearby) which has a PARallax (apparent shift in position against the background stars) of TWO arc SECond (hence PARSEC) when viewed from the Earth six months apart (ie, from the two extreme opposite positions of the Earth's orbit). "
Hi,
Your definition is not correct. May be you didn't "catch" the concept. As you see, it use radius and the angle use is the only half the angle measure.
Anyway, for better understanding, please refer to the website I mention eariler and you will see what it mean... it got drawings to show the concept.
Have a nice day...
Yang Weixing "The universe is composed mainly of hydrogen and ignorance."
weixing wrote:Hi,
Your definition is not correct. May be you didn't "catch" the concept. As you see, it use radius and the angle use is the only half the angle measure.
Anyway, for better understanding, please refer to the website I mention eariler and you will see what it mean... it got drawings to show the concept.
Have a nice day...
Hold on......i'm getting confused here. My impression is that the info on the website agrees with my definition. In 6 months, the Earth would have moved 2AUs (distance between the blue dots). The angular movement of a star at 1 parsec would have shifted 2" (angle between the red lines), right? Therefore, modifying the earlier definition given by rlow, i got:
"One parsec is defined as the distance of a star (nearby) which has a PARallax (apparent shift in position against the background stars) of TWO arc SECond (hence PARSEC) when viewed from the Earth six months apart (ie, from the two extreme opposite positions of the Earth's orbit). "
Hi,
The angle of 1 arc second is the 'p' in the drawing, which is the angle form using earth, the target star and the sun. So it is only 1 arc second and the distance use for calculation is only 1 AU.
Get it..
Yang Weixing "The universe is composed mainly of hydrogen and ignorance."
Oops again!! I didn't realised the 'magnitude' of this problem when I submitted my very first post above in singastro!!
I quickly went to check several sources in my library and I found out that you guys are right; it should be based on 1 AU or the Earth's radius. In fact, several books did not define the parsec clearly and I did somehow remembered the 'incorrect' info many years ago from one of these books. So... I have learned something new today (thank you guys) and sorry for all the subsequent confusion.
So kindly allow me to revise the definition in the following way:
"one parsec is defined as the distance of a star or object (nearby) which has a PARallax (apparent shift in position against the background stars) of one arc SECond (hence PARSEC) when viewed from two points 1 AU apart. (1 Astronomical Unit is traditionally defined as equal to the average distance of the Earth to the sun. Now this value is defined as fixed at 149,597,870 km)."
The other shorter but more 'mathematical' definition is, as pointed out earlier, "One parsec is the distance at which 1 AU subtends an angle of 1 arc second."
rlow wrote:
I quickly went to check several sources in my library and I found out that you guys are right; it should be based on 1 AU or the Earth's radius.
Sharp optics you have, chaosknight; you are spot on. That was indeed some sloppy work I did. Next time I better open my eyes and think twice before I consider to post here.
Yes, it should have read as "...or the radius of the Earth' orbit."
rlow wrote:Sharp optics you have, chaosknight; you are spot on. That was indeed some sloppy work I did. Next time I better open my eyes and think twice before I consider to post here.
Yes, it should have read as "...or the radius of the Earth' orbit."
Hahaaa.....well....to be fair the definition i posted abt 1 AU subtended at 1 arc sec isn't so complete either.
"The universe is composed mainly of hydrogen and ignorance." 
