It can be very frustrating when you encounter a word or phrase repeatedly, but you’re not sure what it refers to. When it comes to astronomy, there are many such phrases. As with most sciences, astronomy is replete with its own language that is foreign to those new to it. In this article, we will cover what is an arcsec.
If you’re keen to use a telescope, for example, you might have read over and over again the word “arcsec”, and wondered what exactly this means.
In short, arcsec is a shortened version of “arcsecond”, and this is a measurement used in astronomy which refers to three main things; a declination distance in a star chart, a unit of something’s astronomical size and the resolving power of a telescope (Also check out How To Use An Astrolabe How To Use An Astrolabe).
We’ll unpack this a little further with our very handy guide below. So, if you want to learn more, read on for more information!
Arcsec Explained
As we said, arcsec is a shortened version of the word arcsecond, and this unit of measurement is used in three main ways. It’s a good idea that we explore this a little further individually.
Arcsec: Star Chart
Let’s start by looking at how arcsec is used in the context of a star chart and the visible night sky. Possibly, the easiest way to explain this unit of measurement in this context is by visualizing the night sky dome as the face of a clock.
A typical clock is divided into hours, minutes and seconds. Much like a clock, the night sky is divided into degrees, which is further divided into arcminutes and arcseconds. There are sixty arcminutes per degree and every arcminute is devised of 60 arcseconds.
If you’re wondering how “big” this actually is, we’ll use the full moon as an indicator. A full moon covers half a degree of the night sky – which is 30 arcminutes, or 1,800 arcseconds (arcsecs).
These measurements are often shortened further, so you might see for example 30’ as a way to denote 30 arcminutes, or 1,800” to denote 1,800 arcseconds.
You may notice that this system is the same way that latitude and longitude is represented. Only the names are changed.
Practical Use
When using a star chart, you will notice degrees of declination, which are measurements from North to South marked along the edge.
When you are observing – you will be given coordinates, which is pretty much like an address for celestial objects. These coordinates will use arcseconds so you might see something like:
RA 12h 22m 13s — Dec +22° 44′ 11″
The important area to look at here is the second set of figures. This shows you that your object is 22 degrees, 44 arcminutes, 11 arcseconds north of the celestial equator.
Singular arcseconds are too small for us to determine visually when we’re looking at the night sky, but they are critical units to use when using celestial catalogs. Think of them as house numbers or door numbers for space addresses!
Incidentally, the same coordinates can be presented digitally as RA 12.37s Dec +22.74.
Astronomical Size
When we are looking to understand the size of an astronomical object, we might use arcseconds to do so. We do not assess this by its “true” size but by its angular size when seen from Earth. However, if we know the distance to the object we can determine its real size.
These diameters tend to be very small as the objects are so far from Earth, and as a result – their measurements will be denoted with arcminutes, but more often arcseconds.
Star charts should provide you with the object’s scale in relation to other objects nearby. One of the best examples to look at is Polaris. Our main bright star, Polaris A, is separated from the smaller faint star, Polaris B, by 18″ (18 arcseconds).
Understanding how this works can best place you for judging sky conditions, which is incredibly useful when you’re planning to conduct observations.
Resolving Power Of A Telescope
A final way we might use arcseconds is within a telescope’s specifications – the resolving power. This refers to your telescope’s ability to observe a size or distance under ideal observation conditions.
Now, how arcseconds are used in this context is extremely complex and involves a lot of different mathematical formulae.
To simplify it though, all you need to know is that if your telescope has a resolving power of X arcseconds, this means it is capable of clearly resolving a distance of up to X arcseconds.
Most manuals will explain this, at least to a simplified degree, when you purchase a telescope. However, a basic understanding of how this works can be useful.
If you wish to calculate the resolving power of your telescope you can use our Telescope Calculator.
We appreciate that this particular use of arcseconds can be extremely confusing. Still, it is beneficial to understand, and the more you use arcseconds in this way – the easier you will find and understand it.
Frequently Asked Questions
We’ll now go through some of your most asked questions.
Why Do Astronomers Use Arcseconds?
Astronomers use arcseconds primarily as a way to navigate the night sky. As we mentioned earlier, it’s an astronomer’s way of placing a “house number” to a celestial object, which makes navigation and observation much easier (Also check out What Is The Earliest Time In The Universe We Can Directly Observe?).
Why Is It Called Arcsecs Sometimes?
Arcsecs is just a more simplified way of saying arcseconds. These are exactly the same unit, but the word has been shortened for time, which is further reduced by using symbols such as “ and ‘.
How Many Arcseconds Can The Human Eye See?
Angular resolution of our own eyes can normally perceive about 40 arcseconds, and at the very most, an arcminute. A lot of this will depend on light and what the light conditions are, along with your own eyes and other third party factors of course.
The Bottom Line
Arcsec is just a shortened version of the word arcsecond, a measurement unit we use in astronomy for a few reasons. There are 60 arcseconds in an arcminute and 60 arcminutes in a degree. Hopefully, we have been able to explain how and why this unit is helpful.
Good luck with your observations!
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
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