What is Angular Size in Astronomy?

Angular size is a geometric concept and a key astronomical metric. We look at how angular size measured and what arcminutes & arcseconds are

You may have heard the terms arcminute and arcsecond mentioned on TV, in magazines or on other websites. These are the units of measurement for angular size used in astronomy. Angular size is used to describe the dimensions of an object as it appears in the sky.

Angular Size in Astronomy is measured in arcminutes and arcseconds, which are used to represent angles on a sphere. An arcsecond is 1/3600th of one degree, and a radian is 180/π degrees, so one radian equals 3,600*180/π arcseconds, which is about 206,265 arcseconds. This is useful to astronomers when working out distances between objects or calculating magnifications required for observations. Angular Size is also used as a measure of optical instruments resolving power - basically how small an object can be seen.

Angular size refers to the object's apparent size as seen by an observer on Earth. For example, the Moon has an angular size of approximately 30 arcminutes.

The angular size of an object is determined by its actual size and its distance from the observer. For an object of fixed size, the larger the distance, the smaller the angular size. For objects at a fixed distance, the larger the actual size of an object, the larger its angular size.

Many deep sky objects such as galaxies and nebulae appear as non-circular and are thus typically given two measures of diameter: Major Diameter and Minor Diameter. For example, the Small Magellanic Cloud has a visual apparent diameter of 5° 20' x 3° 5'.

What Are Degrees, Minutes, and Seconds?

Before we start talking about the idea of angular size we need to quickly review how we measure angles. There are 360 degrees in a circle, but the degree is not the smallest unit we use to measure angles. If we divide each degree up into 60 equal slices, each slice represents an angle that is 1 arcminute across. If we further divide an arcminute up into 60 yet smaller evenly spaced slices, those slices each represent an angle that is 1 arcsecond across.

We can measure angles by specifying the number of degrees, arcminutes, and arcseconds that they span. An arcsecond is an extremely tiny angle -it's 1/3,600th of a degree.

What Is Angular Size in Astronomy?

Now we know how to measure an angle, we can begin talking about the angular sizes of objects. If you draw lines from you to each side of an object in the distance, the angle between those two lines is the object's angular size. This is illustrated in the diagram at the top of the page.

There is a handy trick that you can use to estimate the angular size of something using nothing more than your hand. All you need to know is that if you hold your hand at arm's length, the distance across the end of your pinky finger spans an angle of about 1 degree.

Experiment: Calculate the Angular Size of the Moon

Let's try and calculate the angular size of the Moon. All you need is a tape measure and a ruler.

Hold the ruler at arm's length and measure the diameter of the Moon, you may have to wait for a full moon to be able to accurately measure it. You should get a reading of between 5mm and 8mm depending on the season and the length of your arm. Note down your measured apparent size of the Moon.

Next, you need to measure the distance between your hand and your eye (you may need help with this one) and note this down as well. Both measurements need to be the same units, ideally millimetres.

Now, all we have to do is do some simple maths.

Equation 12 - Angular Size Calculation

This will give you the angular size of the object in radians, where S_ap is the apparent size measured and l is the between your hand and your eye. You can then use the formula to calculate its actual size:

Equation 13 - Diameter given Angular size and Distance

The Moons angular size can be converted from radians to arcseconds by multiplying by 206,264. Arcminutes can be found by dividing by 60.

Your answer should be between 30 and 35 arcminutes in diameter.

Angular Size of the Sun

Do you think that the angular size of the Sun is greater or smaller than the angular size of the Moon?

The Sun and the Moon appear to us the same size - almost exactly the same (hence Solar Eclipses), but we know that the Sun is many, many times bigger than the Moon. The Sun is 400 times bigger than the Moon, however, the Sun is also 400 times farther away from us - so the result is that the Sun and the Moon have the same angular size.