How Can I Measure Degrees in the Sky With My Hands?

Visualizing and measuring night sky distances in degrees can feel very tricky.

But measuring distances in degrees is not hard at all because, surprisingly, you have the perfect tool to do the job already… and it won’t cost you a penny: it’s your hands!

Yes, seriously!

You can measure degrees in the night sky with nothing more than an outstretched hand and I’ve created this in-depth guide showing you exactly how to do it.

Key Takeaways

Measuring degrees with your hands is as simple as holding your hand out at arm’s length and holding your fingers in various patterns.
For example, the distance between your outstretched thumb and pinky finger is about 25°, the width of your closed fist 10°, and your middle three fingers side by side cover about 5° of the night sky.

You can calibrate your hand measurements using distances between the bright stars of the Big Dipper. The distance between Dubhe and Megrez, for example, is 10°.

Sky-Measuring Terminology

Before I get into the ‘how’ let’s have a quick recap of the units astronomers use to measure distances in the night sky.

Since we think of the sky as the inside of a globe-shaped dome surrounding Earth, we don’t measure distances in straight lines but in “angular separation”, which takes account of the sky dome’s curvature. What we see overhead is half of that entire dome, known as a hemisphere.

The basic unit of measurement is the degree, with the symbol °. There are 360° in a full circle, and 180° in half of one. Imagine the line stretching overhead from due east to due west on a flat horizon. Since this is half a circle, that line measures 180°. The same is also true measuring from due north to due south, which means that the entire hemisphere can be mapped as a 180°x180° grid.

Now, let’s half that east-west line we just measured.

Instead of going all the way from due east to due west, let’s just go from due east to directly overhead (known as the zenith). This time we’ve only traveled half as far, a distance of 90º, which also means that halfway between the horizon and zenith is 45º.

If you use Stellarium or sky maps, you’re probably already familiar with the concept of the distances between stars being given in angles. What we need to do now then is to show you how to estimate degrees with your hands.

How to Easily Measure Distances in the Sky Using Your Hands

The three great things about using your hands to measure degrees are:

  1. Your hands are readily available at all times
  2. It’s always roughly accurate no matter who you are
  3. You can always calibrate your own measurements for greater accuracy

The reason these approximations work for everyone, old or young, tall or short, male or female, is because the size of your hands (and fingers) is proportional to the length of your arm.

The span of everyone’s hand, i.e. the distance between your splayed-out thumb and pinkie finger, held at arm’s length is roughly 25 degrees of sky!

But we’re not limited to just 25° – there are a variety of measurements we can make, like these:

Five hand shapes for measuring angles in the sky
Five hand shapes for measuring angles (click to enlarge)

Easy Distances to Measure With Your Hands

The table below summarizes the most useful measurements.

Distance in DegreesHand Shape
The width of your pinkie finger*.
The width of your three middle fingers held next to each other.
10°The width of your clenched fist.
15°The distance between your outstretched pointer and pinky fingers.
25°The distance between your splayed-out thumb and pinky finger.

These are great ways to approximate distance. For example, if you’re told that “Jupiter is 10° east of the moon“, then your clenched fist held by the moon should give you enough confidence that the bright ‘star’ you can see there is actually Jupiter.

Keep in mind that all of the measurements in the table assume that you’re holding out your hand at arm’s length with the back of it facing you.

You might have guessed already that this system can be used to measure other angles too. For example, two 25° angles placed side by side measures 50°, and so on.

This method is useful but not perfect (for example, you should *take the width of your little finger on its own being equal to 1º with a pinch of salt as finger sizes vary greatly) but you can improve accuracy by calibrating your hands using a few known night sky measurements.

Let’s take a quick look at how to use the Big Dipper to measure degrees in the night sky.

Calibrating Your Hand Angles With The Big Dipper

The Big Dipper (also known as the Plough) is an asterism in the constellation of Ursa Major, or the Great Bear.

What makes it really useful for measuring distances in the night sky is that:

  • It’s bright and easily recognized, even if you’re new to astronomy
  • Because it’s circumpolar for many of us, it’s always visible at night, and
  • The distances between some of its brightest stars are a round number of degrees
Sky distances in angles using the Big Dipper
Standard distances in The Big Dipper (click to zoom)

You can see in the image that the spacing between the stars in this constellation offers simple ways of checking how closely your hand shapes match 5°, 10°, 15° and 25° against the night sky.​

For example, if your own ‘Y’ shape from pointer to pinky doesn’t quite measure 15 degrees, move them away from or nearer to your eyes until it does. Note how you’re holding your arm and always hold it in the same way from now on to get a consistent and accurate(ish) measure.

Use this next time you’re outside with your telescope to get a sense of how to use your own hands effectively.

Using Other Stars as a Sky Measure

The stars themselves can serve as handy yardsticks in the sky. For instance, The belt of Orion measures about 3 degrees wide. In the constellation of Leo, the distance between Regulus (at the bottom of the sickle) and Denebola (tip of the lion’s tail) is 25 degrees.

Measuring Less than 1° in the Night Sky

Most distances between objects are measured in degrees, with the obvious exception of the separation between double stars.

We’re much more likely to measure the width of objects using units smaller than a degree. For example, even the full moon is only about half a degree wide. The width of Jupiter or Mercury is much less, so we need a finer standard to measure them.

To achieve smaller units, we divide each degree into 60 equal parts called arcminutes. So, 1 degree is 60 minutes, and half a degree is 30 minutes. The symbol for arcminutes is ‘.

As I just mentioned, the width of the full moon is about 30′, or half a degree.

How much do you think our sun measures?

Well, it’s the same: 30 arcminutes! This is why the moon covers the sun almost perfectly in a solar eclipse.

Some of the sights we’re seeking with our telescope are so small that we need to divide up the arcminutes into even smaller measurements. 1 arcminute is divided into 60 arcseconds, with the symbol “.

This means that each degree is 3,600 arcseconds.

In the sky, Jupiter has a diameter of about 50” (arcseconds) when it’s at its closest, whereas Mars might be as small as four arcseconds across at its most distant.

A good telescope can resolve objects down to 1 arcsecond but we’re not measuring such small lengths with our hands!

Our eyes can resolve distances down to about 10 arcminutes. In Big Dipper, the star at the bend of the handle is Mizar, which has a fainter companion called Alcor. Together, they are known as the ‘horse and the rider’, and because they are about 12 arcminutes apart, the ability to resolve these two stars can be used to test your own eyesight.

Video Guide to Measuring the Night Sky

I’ve recorded a video to show you how simple it is to use these hand shapes for yourself. Watch it below.

One Last Little ‘Trick’

With what you’ve learned in this article, you can use Polaris (the Pole Star) to work out your approximate latitude.

Calculate the angular distance between Polaris and the horizon. Since the Pole Star is almost directly over the North Pole (see my video about this), the resulting angle is a good estimate of your latitude.

Note that this method is valid only for the northern hemisphere. There is no similarly easy method to use for the southern hemisphere since no star lies exactly over the south pole.

Wrapping Up

Now you’re comfortable using this simple technique to navigate the night sky, I recommend regularly using these hand gestures in your skywatching sessions. This way, you’ll soon be darting across the sky like a pro.

The next time you read a newsletter stating that “Jupiter will be seen 3 degrees northwest of Moon”, you will know just the right place to look for it.

Author