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Seeka
2009-Feb-19, 05:30 PM
Hi all,

I need some clarification in the easiest terms possible with some astronomical measurements if anybody can help i would appreciate it.
I know the parsec is 3.26ly's but what is the explanation of a parsec?
How does the arc second fit in? I had it explained last evening with a type of triangle diagram but it did not sink in:(

Much Appreciated

Steffanie

aurora
2009-Feb-19, 05:39 PM
I'll take a stab, without referring to triangles.

A circle is 360 degrees. A half circle is 180, so think of the sky from horizon to horizon (assume a flat horizon) is 180 degrees.

Each degree is further divided into 60 minutes. (a full moon is about one half degree, or 30 minutes, across as seen from Earth).

Each minute is further divided into 60 seconds.

And arc second is one second of arc in the sky.

A Parsec is the distance at which an object would have one arc second of parallax. Parallax is how much an object would appear to shift when the Earth moves from one side of the Sun to the opposite side (which takes roughly 6 months).

See here for more:
http://en.wikipedia.org/wiki/Parsec

hhEb09'1
2009-Feb-19, 05:52 PM
A Parsec is the distance at which an object would have one arc second of parallax. Parallax is how much an object would appear to shift when the Earth moves from one side of the Sun to the opposite side (which takes roughly 6 months).That would be two parsecs :)

See here for more:
http://en.wikipedia.org/wiki/ParsecAs that link says, it's only the radius of the earth orbit, not the diameter.

Since the time it takes light to go from the sun to the earth is about 8 1/3 min, the angular distance is 8.33/(3.26x365.25x24x60) radians, about 5x10-6 radians, about one arcsecond.

Nick Theodorakis
2009-Feb-19, 05:54 PM
If you like this topic, you might be interested in the book Parallax: The Race to Measure the Cosmos (http://www.amazon.com/Parallax-Measure-Alan-W-Hirshfeld/dp/0805071334/), by Alan Hirshfeld.

Nick

aurora
2009-Feb-19, 10:06 PM
That would be two parsecs :)As that link says, it's only the radius of the earth orbit, not the diameter.

That's what I get for trying to avoid using triangles. I thought I was being so clever. :doh:

Seeka
2009-Feb-19, 10:07 PM
Thanks guys, still a bit confused as to when an arcsecond is actually used?

Can someone break this definition down for me?;
The parsec being the distance at which 1AU perpendicular to the observers line of site subtends an angle of 1 arcsecond...

aurora
2009-Feb-19, 10:13 PM
Did you check the wiki page we linked to? Try that, it has a diagram that may help.

As to when an arcsecond is used, it is used to measure the size of something in the sky. As I said above, a full moon is 30 minutes in apparent size, so that would be 30 x 60 or 1800 seconds. It is also used in the measurement of position in the sky, for example declination is measured in degrees/minutes/seconds.

Seeka
2009-Feb-19, 10:35 PM
Yeah i did have a look Aurora:)

I seem to understand things better if it's explained here than going directly to Wiki esecially if someone relates it to something easily understandable if that makes sense. I will have another look at wiki :)

Jeff Root
2009-Feb-20, 08:31 AM
An arcsecond is just a tiny fraction of a degree of angle. So arcseconds
can be used whenever you measure very small angles.

Astronomical distances are only expressed in parsecs because distances
to the nearest stars are found by measuring the tiny difference in angle
when they are observed six months apart. Since those measurements
are the basis for determining distances to all farther objects, professional
astronomers use parsecs for everything outside the Solar System.

Popular literature more often uses light-years, which has a closer
connection to something most people can relate to, even if the speed
of light is absurdly fast. Eight minutes, twenty seconds from the Sun
to the Earth; 5.5 hours to Pluto; 4.2 years to Alpha Centauri.

-- Jeff, in Minneapolis

Tog
2009-Feb-20, 09:13 AM
Here (http://www.bautforum.com/685711-post33.html) is an old post of mine about Parallax.

Arcseconds are to degrees what seconds are to hours. They are just a smaller version of the same thing.

One hour is made up of 60 minutes. Each minute is made up of 60 seconds. That means that each hours is made up of 60*60, or 3600 seconds.

It works the same way with degrees. The Moon is about 0.5 degrees across (as seen from Earth). That's the same as about 30 arcminutes or 1800 arcseconds.

There is also a term used in shooting called "Minute of Angle" or MOA accuracy. In one of those odd coincidences that pop up from time to time, one minute of arc is equal to 1 inch at 100 yards. A rifle that is capable of placing 5 bullets in a 1 inch circle is said to have MOA accuracy.

The way it's used is the same as in astronomy though. The diameter of an object as seen from a fixed distance is the angular size. That size is measured in degree, arcminutes, or arcseconds, depending on which makes the most sense.

I've got an example that has a pretty good real-world way to see it hands on, but the problem is, it's for right ascension, so it's off by a factor of 4 when dealing with degrees. I don't want to toss that out an make things more confusing.

Peter B
2009-Feb-20, 09:18 AM
Thanks guys, still a bit confused as to when an arcsecond is actually used?

Well, as far as I can tell, it's simply a measurement used by astronomers as an alternative to the light year. I could easily be wrong, though.

Can someone break this definition down for me?;
The parsec being the distance at which 1AU perpendicular to the observers line of site subtends an angle of 1 arcsecond...

Think of a right angle triangle (you know, like a 3-4-5 triangle), but one which is extremely long and not very high. The short (vertical) side is 1 Astronomical Unit high, and the very narrow angle at the opposite end of the triangle is 1 arc second. Knowing those two figures, you can use trigonometry to work out the length of the very long (horizontal) side of the triangle. And that long side is one parsec long.

aurora
2009-Feb-20, 06:28 PM
Well, as far as I can tell, it's simply a measurement used by astronomers as an alternative to the light year. I could easily be wrong, though.

That would be the parsec, not the arcsecond.

Peter B
2009-Feb-20, 06:48 PM
Oops.

Seeka
2009-Feb-20, 10:19 PM
You've been a great help thanks to all especially Tog that link made alot of sense to me:)

jwestern
2009-Nov-27, 09:06 PM
I am new to the site. I started watching Astronomy Cast about a year ago and have been listening to the OSU Astronomy Classes...141, 161 and 162 on line. I have a number of questions but one simple one has me perplexed.

How was the parallax vertical angle physically measured when stars distance measurements were first made? Is it simply a protractor places on the earth-sun line measuring the angle as referenced to the star.

I have been trying to find reference to this question but have not located it on line nor in any of my text. All references assumes one understands how this simple angle measurement is made.

Thank you. :)

Hornblower
2009-Nov-27, 11:06 PM
I am new to the site. I started watching Astronomy Cast about a year ago and have been listening to the OSU Astronomy Classes...141, 161 and 162 on line. I have a number of questions but one simple one has me perplexed.

How was the parallax vertical angle physically measured when stars distance measurements were first made? Is it simply a protractor places on the earth-sun line measuring the angle as referenced to the star.

I have been trying to find reference to this question but have not located it on line nor in any of my text. All references assumes one understands how this simple angle measurement is made.

Thank you. :)

Bessel and others used precision micrometers in the focal planes of powerful telescopes to measure the angular separations of closely spaced stars. If one star was near enough, it would appear to oscillate annually with respect to the background stars.

Nick Theodorakis
2009-Nov-27, 11:14 PM
I am new to the site. I started watching Astronomy Cast about a year ago and have been listening to the OSU Astronomy Classes...141, 161 and 162 on line. I have a number of questions but one simple one has me perplexed.

How was the parallax vertical angle physically measured when stars distance measurements were first made? Is it simply a protractor places on the earth-sun line measuring the angle as referenced to the star.

I have been trying to find reference to this question but have not located it on line nor in any of my text. All references assumes one understands how this simple angle measurement is made.

Thank you. :)

Read the book I mentioned in post 4 upthread.

Nick

StupendousMan
2009-Nov-28, 03:15 PM
I suggest you read these lectures, which include
plenty of diagrams and examples.

http://spiff.rit.edu/classes/phys301/lectures/parallax/parallax.html

http://spiff.rit.edu/classes/phys440/lectures/helio_para/helio_para.html

jwestern
2009-Nov-29, 01:03 AM
Thank you for the references. I have reviewed all of these references earlier. My question still remains. How is the angle physically measured. What is the mechanism for taking the measurement (and what does it look like) and how is it placed (any direction). The mathematics are simple. What I am looking for are specific details on how the angle reading was taken originally. I have ordered ... Parallax: The Race to Measure the Cosmos, by Alan Hirshfeld as suggested. Maybe there is something on the measurement details. Thanks again to all.

StupendousMan
2009-Nov-29, 02:06 AM
Thank you for the references. I have reviewed all of these references earlier. My question still remains. How is the angle physically measured. What is the mechanism for taking the measurement (and what does it look like) and how is it placed (any direction).

If you read the lectures I posted two items above this one, you would have seen examples of images in which one star appears to shift its position relative to other stars in the field.

Perhaps you should read them again, and click on links which state "Look at examples of the observed shift in position."

But perhaps you want to know how astronomers turn a shift in apparent position into an angle -- is that it?

ngc3314
2009-Nov-29, 11:59 PM
Here's an example somewhat closer to home. These images show an asteroid in the main belt, in images taken simultaneously from Kitt Peak, Arizona, and from our campus observatory, at a time chosen so that the baseline was perpendicular to the direction of the asteroid. The baseline for parallax is then about 2330 km. A careful comparison will show that the asteroid position is slightly different on the two images (which have been registered to the much more distant background stars). Knowing the pixel scale (about 0.4 arcsecond per pixel in this case) translates that into an angle, and then into a parallax (the error bars here give us about a 10% error from these single exposures). For stars, one complication is that one needs to correct for the fact that reference stars are not infinitely far away, or use an all-sky position grid with simultaneous solutions for every object such as the Hipparcos mission did.

George
2009-Nov-30, 12:28 AM
For stars, one complication is that one needs to correct for the fact that reference stars are not infinitely far away, or use an all-sky position grid with simultaneous solutions for every object such as the Hipparcos mission did.Your two real-life images are excellent at illustrating the basis of parallax.

I think this background issue is where many, like me, tend to bog down when they first contemplate what parallax is all about. We just don't do measurements like this in the every day world, and we don't think of distant stars as if they are stuck on an extremely distant spherical shell. We already know that stars are sprinkled all around and it is not easy to appreciate that they might as well be slammed up against that proverbial distant sphere.

Maddad
2009-Nov-30, 12:41 AM
When using the distance from the Sun to the Earth as a unit of measure, it is interesting to track how early astronomers determined that distance. They used a pair of transits of Venus across the sun in the 1780’s I believe to get that measurement. Someone else here might know how.

StupendousMan
2009-Nov-30, 01:24 AM
Enjoy.

http://spiff.rit.edu/classes/phys235/venus_t/venus_t.html

StupendousMan
2009-Nov-30, 01:27 AM
http://spiff.rit.edu/classes/phys440/lectures/ny40/ny40.html

http://spiff.rit.edu/richmond/ritobs/aug16_2002/aug16_2002.html

George
2009-Nov-30, 02:40 AM
Enjoy.

http://spiff.rit.edu/classes/phys235/venus_t/venus_t.html

Nice, and the history was interesting, too.

jwestern
2009-Dec-01, 02:34 AM
Thanks to all who supplied information on this topic. Wow… the data collection effort for determining this angle is difficult, time consuming and complex. Just think of the huge efforts of past astronomers in determining these small star angles. I do appreciate their contribution.

jwestern
2009-Dec-06, 12:37 AM
I just received the Parallax book that Nick Theodorakis recommended: "The Race to Measure the Cosmos", by Alan Hirshfeld. It does a great job in providing the history/background of Parallax in measuring the distance to stars. I would recommend, everone read it. Thanks Nick for recommending it to me.

Best Regards................Jeff

Maddad
2009-Dec-28, 12:36 PM
Enjoy.

http://spiff.rit.edu/classes/phys235/venus_t/venus_t.htmlI do not swing by here as often as I should, so I just found the page and read it. Thank you very much. It is amazing that people from so long ago could work out a way to measure the solar system, even though the precision for exact results would be poor until modern times.

OldAstrofan
2009-Dec-30, 08:13 AM
Since many have already explained the meaning of an arcsecond, i see no point in clarifying it again.
(it's just 1/3600 of a degree)

Firstly, the definition of a parsec is like this (to make it easy):
If one were somewhere one parsec from the sun/earth orbit, then the person would measure the average distance (more formally i should use the term "seperation") of sun-earth as one second of arc (arcsecond).

An arcsecond is just a tiny fraction of a degree of angle. So arcseconds can be used whenever you measure very small angles.

Astronomical distances are only expressed in parsecs because distances
to the nearest stars are found by measuring the tiny difference in angle
when they are observed six months apart. Since those measurements
are the basis for determining distances to all farther objects, professional
astronomers use parsecs for everything outside the Solar System.

Popular literature more often uses light-years, which has a closer
connection to something most people can relate to, even if the speed
of light is absurdly fast. Eight minutes, twenty seconds from the Sun
to the Earth; 5.5 hours to Pluto; 4.2 years to Alpha Centauri.

-- Jeff, in Minneapolis

I'd like to add that a parsec is actually a more accurate measuring unit to use because parsec is defined by "arc" (degree) and when we are dealing with astronomical objects, we observe by arc (mostly).