why are arc minutes called this name?

Question

ive just got my head around what 1' and 1" actually mean, after much confusion because of their name. but now i understand what ' and '' actually denote, as in, they are simply the angles subtended in their respective periods of time, i still dont understand where the name arc minutes comes from.

so could somebody please explain how a unit goes from being a name like degrees, then breaks down into arc minutes, arc seconds. why pick these names instead (just examples for illustrating my point here) of millidegrees, microdegrees...???

aah i get it, so for arc minutes, or minutes of arc, i should read minutes of angle?

Answer 1

Arc minutes and seconds are named that way to separate them from time minutes and seconds.
all of the 360 degrees, 60 minutes, 60 seconds are based on ancient Babylon and their desire to do math. Remember there was no zero place holder which came later. So fractions were based on the "divide by concept"
60 divides by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. Further much of the division can be done graphically with a compass (divider)
and 360 and 60 are divisible by 3, a powerful number, unlike 10 based numbering.
Actually, the degrees, minutes, seconds came first, the time later.

Answer 2

Your definition is a little off. An arc-minute is one sixtieth of a degree, and an arc-second is one sixtieth of an arc-minute.

These are old measures that far predate the popularity of base ten systems of measurement, and even base ten in general. Minutes and seconds are Babylonian in origin. The Babylonian system of counting was base sixty (see http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Babylonian_numerals.html for a full explanation), so it was natural for them to use a system with minutes and seconds.

To further complicate things, one can also talk about an arc-minute being one sixtieth of an arc-hour, with an arc-hour being fifteen degrees. It can be difficult to tell sometimes which is meant. You have to use context mostly.

You might be interested to hear that the next breakdown would be an arc-third, which is one sixtieth of an arc-second. In astronomy this is not typically used, because the need to break arc-seconds into smaller parts didn't arise until metrication was popular. Astronomers tend to use milliarcseconds.

Answer 3

A minute means 1/60. A minute of time is 1/60 of a hour. A minute of angle is 1/60 of a degree. Same logic with a second. The reason is tradition. Centidegrees would be much more convenient.

Answer 4

Canopus is closest.

The names, though, came much later, when mathematicians (who were also astronomers at the time) needed to divide units into smaller units.

Pars minuta prima (Latin): the first small parts, from which we kept 'minute' with emphasis on the MIN syllable. We also use the word miNUte in its original meaning= small (her piece of cake is huge, while mine is minute -- emphasis on NU).

Pars minuta secunda; the second small parts (of which we kept seconds).

At some point (15th and 16th century) some astronomers were pushing their luck and calculating things down to the fifth level (dividing by 60 at every level); the accuracy of their observations did not warrant such precision but they tried nonetheless. They just gave the units their full Latin names (e.g., 'pars minuta quarta' or simply 'quarta pars') I've seen some 'modern' applications (19th century) where triple and quadruple notches were used, after ' " but no names given to these sub-units

These names were applied to anything that was divided into smaller parts. We have only kept the ones related to angles and to time.

In astronomy, where both systems are used to measure angles (as if things were not complicated enough...), you need to distinguish the kind of 'minute' being used.

Right Ascension (the celestial equivalent to longitude) is often given in units called hours, minutes and seconds, corresponding to the sidereal time at which the object crosses the observer's meridian.

Declination (equivalent to latitudes) are given in degrees, minutes and seconds.

There are 360 degrees in a full circle, but only 24 hours. Obviously, minute parts of degrees are 'more minute' than minute parts of hours.

To distinguish them, if the context is not sufficient, one uses the prefix 'arc' when dealing with the divisions of degrees. For example, the apparent diameter of the Sun's disk is 30 arc-minutes.

However, if I give you the declination of a star, then it is obvious that I'm talking degrees, so I may skip the 'arc' part in front of minutes and seconds.

Answer 5

Arc-second, arc-minute, arc-hour, etc. are not expressions of time. They are expressions of position. Right-ascension and declination. The term "arc' comes from the division of the sky circle (celestial equator) into tiny pieces. This is just like our latitude and longitude.

Right-ascension like longitude is expressed in hours, minutes, seconds and so on except ther is the arc added to it.

Declination like latitude is expressed in degrees, minutes, seconds and so on with the arc added to it.

Answer 6

I dont know what arc means but minute sounds like miniature of a minute