Arc units in astronomy???

[KittyAttack7139]

Does anyone know what "arc units" are in astronomy? I can't find an explanation anywhere....Edit: I think it's supposed to have something to do with grades, minutes and seconds...But I'm not 100% sure.If so, how does that work?

[Al]

Arc unit refers to the mesurement of length of an arc on the celestial sphere.The one you will hear about most is "arcsecond" or 1/3600 of a degree. If an object exhibits a "parallax shift" of 1 arcsecond, that object is said to be 1 "Parsec" distant, or about 3.26 light years... See link if you want to read more.UPDATE: Saw your additional question. See the link for an in-depth explanation... If you want to see this effect for yourself, do this. Hold you thumb out at arms length in front of your face. Close one eye. Rapidly close one eye and then the other, keeping only one eye open at any one time. You will see your thumb appear to "shift" with relation to distant objects. This is a parallax shift. Since your eyes are a few inches apart, you are viewing your thumb from a slightly different angle from each eye with relation to the background. Hope that helps...

[fonsv]

Indeed! It has to do with minutes and seconds.You can express the size of an object in the sky in minutes and seconds as it appeared to be from your point of view.It does not say anything about the real size because you do not know its distance.

[GeoffG5947]

In astronomy we use minutes and seconds of time (to measure right ascension) and minutes and seconds of arc (for most angles including declination). One degree contains 60 arcminutes and 3600 arcseconds; one hour contains 60 minutes and 3600 seconds. Arc measures are abbreviated as ° '", time measures as h m s.

[PeterT]

360 degrees in a full circle60 minutes to a degree60 seconds to a minute.When you give celestial coordinates in right ascension and declination the right ascension number is usually a time in hours, minutes and seconds while the declination is giving in degrees, minutes and seconds.In right ascension 24 hours corresponds to 360 degrees so 1 hour of right ascension equals 15 degrees and 1 minute of right ascension is 15/60 = 0.25 of a degree or 15 minutes. Since we have the unit minute referring to two situations here we refer to the angle measure as "minutes of arc" or "arcminutes". 1 minute of right ascension is thus 15 arcminutes and 1 second of right ascension is 15 arcseconds.

[Raymond8288]

One problem is that, in astronomy, there are two sets of units that use the same names: angle and time.Worst, astronomers sometimes use units of time to indicate angles.Time: hours, minutes, seconds.Angles: degrees, minutes, seconds.To distinguish one system from the other, if there is a risk of confusion, astronomers will use 'arc-minutes' and 'arc-second' for angles.Our time system uses 24 hours per day. The Earth rotates 360 degrees in 24 hours. That is 15 degrees per hour. In setting coordinates for the celestial sphere, it is convenient for astronomers to measure the 'Right Ascension' (the same idea as longitudes on Earth) in hours instead of degrees. You will often see the position of a star given in mixed units(for example, Capella R.A.=5h17.2m dec=+46 00)The Right Ascension is 5 hours and 17.2 minutes (at 15 degrees per hour, this is the same as 79 degrees and 18 minutes, in angle) East of the prime hour angle (Right Ascension is measured from where the ecliptic crosses the celestial equator: the point where the Sun appears to be at March Equinox).Declination is given in degrees (and minutes). Capella's declination is 46 degrees North of the celestial equator (South would have been with a minus sign -).We inherited our celestial calculations from the Babylonians: They would divide things in 60 parts.We use decimals, dividing by 10 at each step, because we have calculators. They did not. 60 is a practical number when you have no calculator because it is easy to divide 60 by 2, 3, 4, 5, 6, 10, 12, 15 and 30. If is also a convenient divisor for 360.The modern names of the divisions came much later, when scientists were writing in Latin. When measurements became too small, they would use a unit 1/60 of the main unit. This was 'the first small part' (pars minuta prima); the adjective 'minute' in English comes from the original Latin word, minuta, meaning 'small'. In that sense, the emphasis is on the second syllable: miNUte.e.g., It's not fair: her piece of cake is huge, while mine is minute.When calculations became even more precise, the first part had to be divided again, so we had the 'second small part' (pars minuta secunda). Popular usage retained the words 'minute' (emphasis on the first syllable: MINute) for the first division and 'second' for the... ahem... second division. This was used for any calculations, not just angles and time.At one time many centuries ago, some exaggeratingly performed calculation to five level of division by 60 (pars minuta quinta, the fifth small part); fortunately, this did not really catch on.That is why we are now stuck with the same names (minutes and seconds) for two sets of units.