Arc second

[Episkey]

I've seen this term a few times and am struggling to understand what it is. What is it and how do you find/calculate it?

Thanks!

[Bizibilder]

Consider a circle - it can be divided into 360°. Then each degree can be sub-divided into 60' (sixty minutes of a degree) and each minute into 60" (sixty seconds of a minute). Each of these is an "arc second" and is 360x60x60 of a circle = 1/1296000 of a circle.

The Moon (or Sun) is about half a degree across the diameter, as we see it from Earth. This is 0.5° or 30' of arc or 1800" of arc. Hope this helps.

[Moonshane]

hi there

it's 3600th of a degree of sky. one degree = 60 arc minutes one of which equals 60 arc seconds. I think this means that one arc second is approx how far a star would move in the sky (at 1x magnification) in 15 seconds as stars move (obviously really it's the Earth doing the moving) at the rate of 15 degree per hour.

[jimmyjamjoejoe]

hi there

it's 3600th of a degree of sky. one degree = 60 arc minutes one of which equals 60 arc seconds. I think this means that one arc second is approx how far a star would move in the sky (at 1x magnification) in 15 seconds as stars move (obviously really it's the Earth doing the moving) at the rate of 15 degree per hour.

I was just about to ask if the term held any meaning towards time, or whether it was simply a measurement of distance that was named as such due to being a 60x60 scale.

Great to know, now i can finally appreciate just how small mars is...

[Telrad]

jimmyjamjoejoe, Shane's explained it well, it means that one degree of angle is divided into 60 minutes, each minute is divided into 60 seconds. So the number given is a very small angle. But this number is actually the resolving power.

[Episkey]

Thanks! So it's basically the size of something in the sky?

[themos]

Yes, it's a measure of apparent size.

These are small.... these are far away.

[FrankieValley]

Yes. I see angular distances used when talking about field of view i.e. this eyepiece with this focal length gives a field of view of 0.5 degrees, so you'll just about see the whole moon through it or whatever else fits in to that field. You will also see the term arc seconds per pixel. This is a calculation to effectively show the field of view of an individual CCD pixel for a given focal length and given pixel size. For example, one CCD pixel (for a given FL) may have a FoV of one arc second, so if you have a CCD that is 1800 pixels by 1800 pixels, then you can just about fit the moon in. Not sure if that helps or confuses ;-)

[Episkey]

Thanks guys!

[Moonshane]

it can also be used in the distances between double stars.

[Ags]

Or as the current angular size of a planet. Mars is 7 arcseconds in diameter at the moment. Jupiter gets up to 50 arcseconds at its closest.

[Episkey]

Is there a specific website to view this information?

[Telrad]

Is there a specific website to view this information?

Minute of arc - Wikipedia, the free encyclopedia ..under Astronomy

[Episkey]

Minute of arc - Wikipedia, the free encyclopedia ..under Astronomy

Thanks, but I meant the arc seconds of the planets.

[John]

The freeware planetarium software Stellarium tells you the current apparent diameter of the planets. Just click on the planet and a load of data is displayed including the apparent diameter.

[Episkey]

Ah, okay thanks!

[Adrian]

hi there

it's 3600th of a degree of sky. one degree = 60 arc minutes one of which equals 60 arc seconds. I think this means that one arc second is approx how far a star would move in the sky (at 1x magnification) in 15 seconds as stars move (obviously really it's the Earth doing the moving) at the rate of 15 degree per hour.

Close - it moves 15 arcseconds per second (15 degrees divided by 3600)

Otherwise it would take 12 minutes to move across Jupiter...

[Moonshane]

I thought that felt wrong when I said it . what I should have done is 60s x 60m x 15deg = 54,000 arc seconds per hour movement across the sky per hour so as you say on second of movement = 54,000/3600 = 15 arc seconds per second.