What are arcseconds and arcminutes?

Imagine standing across the road from your house. How wide is your house? There are many different ways you could talk about the width of your house, and even more units that you could use to express this information - metres, feet, armspans, footsteps... I want to describe a way of expressing this size that may not have occurred to you yet:

In your mind, draw a straight line from your feet to one end of the wall that's facing you. Draw another straight line joining your feet and the other end of the wall. On the floor in front of you there should now be two lines that meet at the same point at your feet. In situations like this you could measure the angle between the two lines, and this would be one way of saying how far apart the two ends of the wall are from your particular viewpoint.

By Sriram.aeropsn (Own work) [Public domain], via Wikimedia Commons
Obviously the answer you would get would change depending on where you were standing and wouldn't make a lot of practical sense so we don't tend to use it on such small scales. But to astronomers this is an incredibly useful tool for describing both the sizes of objects and their related positions in the sky: the massive distances between Earth and most astronomical objects compared with the relatively tiny movements we make over time means that this is actually a pretty convenient and accurate way of measuring things.

Measuring sizes in this way doesn't give us their actual size; it is actually an apparent size, and we talk about angular sizes. For example, the angular diameter of the Moon as seen from Earth is around half a degree. Now, the Moon is one of the largest objects (from our viewpoint) in the sky, so it's easy to see that the numbers are going to get very, very small if we keep on talking in degrees. Which is where...


Arcminutes

... come in.

You'll remember protractors from school- those plastic, transparent circles or semi-circles that you used to measure angles. A protractor splits a circle into 360 equal slices called degrees. If you take just one of those degrees and slice it up into 60 even smaller slices, each slice is called an arcminute (or 'minute of arc').

So 30 arcminutes is the same size as half a degree: the Moon, from Earth, appears about 30 arcminutes in diameter.

Venus, occasionally the largest (in terms of apparent size) planet only manages to get as large as 1.1 arcminutes in diameter as seen from Earth. That's about 0.018 degrees. Many things out there appear even smaller to us, so we need an even smaller unit of measurement.


Arcseconds

After chopping a circle up into 360 equal slices (degrees), we took one of those slices and chopped it up into 60 tiny slices (arcminutes).

Now we need to take one of those slices, and chop it up into 60 even smaller slices. Each of these tiny new slices is one arcsecond (or 'second of arc') in size. So Venus has an angular diameter of 66 arcseconds.


Some angular sizes compared

Here are a few astronomical objects listed comparing their angular sizes in arcseconds, arcminutes and degrees*. Notice how the numbers are much nicer if we use arcseconds than if we use degrees!

  • Jupiter: Reaches about 50 arcseconds, 0.8 arcminutes, or 0.01 degrees in angular diameter.
  • Betelgeuse: 0.055 arcseconds, 0.0009 arcminutes, or 0.00002 degrees
  • Proxima Centauri: 0.001 arcseconds, 0.00002 arcminutes, or 0.0000003 degrees.






* For the pedants out there, I've rounded most of these numbers to one significant figure!

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