Are you wanting to find your way around the night sky based on angles in the sky? The citing of key stars and star groups by degrees in the sky can be confusing if you’re not used to this sky measurement of celestial distances. But, to make sense of it, all you need is your hand. Here I delve into sky measures with your hand.
Do you want to know the angular distance between two stars? Rather than an app or refering to an astronomy book for measurements, you can figure it out by using a simple hand measurement guide. Read on…
What is angular distance? Angular distance is the measure of degrees between two stars in the sky as observed from Earth. You might want to know angular distances when you are using a star map or an app such as Stellarium on your phone.
This is the apparent distance from one celestial body to another, as measured in degrees or part thereof.
Things to know about how to measure with hand against the sky
The following are a foundation to measuring distances in the sky.
- How many degrees in a sphere
- How to find angular distance
- Hand width measurements as degrees
Degrees in the sky according to hand width
Let’s start simple, using the width of a finger…Hold up your hand at arm’s length to the sky. The width of the end of the little finger is 1 degree. This is one hand measurement – see others below, including putting your fist to the sky, the width of your fingers or distance between certain digits when held at arm’s length towards the sky.
The width of the end of your little finger is about one degree.
The width of your three middle fingers, tip-to-tip, is about five degrees when held to the sky at arm’s length.
The hand width as a fist held up to the sky at arm’s length is ten degrees wide.
The width between your index and little finger, tip-to-tip, when extended from your fist is 15 degrees and held at the sky at arms length.
That width between you little finger and thumb, tip-to-tip when extended from your fist is roughly 25 degrees – when held to the sky at arms length.
The trick is ‘at arms length’.
These distances generally work for everyone, because arm length is proportional with hand size. The exception is the hand-span measure extending the thumb and little finger apart. Some people have more or less flexibility in their thumb than others and so this can vary from person to person.
A way to check the span is to use the Big Dipper or Orion’s seven brightest stars (aka the Saucepan in the Southern Hemisphere) as a gauge and see whether your span for this hand measurement is closer to 20º or to 25º.
How to get to know your personal sky measures by using your hand to measure angles…
Using the Big Dipper for this, gauge your hand measure between Dubhe, the point where water pours if a real dipper, and the end star in the handle to see how well it fits the 25º between these two. This will indicate your personal measurement and any adjustment needed.
Especially if you’re in the Southern Hemisphere, the Saucepan is an easy way of doing this…
Sky measuring guide…How Big Is A Degree
A degree is 60 arc minutes in astronomy measurements featuring degrees (º) arc minutes (‘) arc seconds (“).
One minute equals 1/60º and one second equals 1/60′. Put another way, 60 seconds make up a minute, and 60 of those minutes make up a degree. This is a sexagesimal system, not unlike that used to measure time.
It will make sense if you’re familiar with latitudes and longitudes as degrees, minutes, and seconds for positioning on Earth.
The distance between close celestial bodies is usually down to units of arc minutes / arc seconds.
But how big is a degree by hand measurements — the width of your pinky held at the sky at arms length.
Putting sky distances (degrees) into perspective
How many degrees are in a sphere? The number of degrees in a sphere is 360 degrees. Now picture Earth centered inside that sphere.
The sky you see from one horizon to the other side represents half a sphere, 180º, and the angle from the horizon to half way (Zenith) is 90º.
We can relate the apparent movement of the Sun across the sky to this in that it moves 15º each hour (24 hr in a day and 360º in a full circle).
At night, our naked eyes can distinguish two close celestial bodies if they are at least 6 minutes (one tenth of a degree) apart.
For stars or other celestial bodies closer than 6 minutes or a tenth of a degree (e.g. double stars or triple stars), they’ll appear a single entity, unless viewed through a set of binoculars or a telescope.
A 2-inch (60 mm) telescope will discern two stars of equal brightness if they are at least 2 seconds apart. For a 3-inch (90 mm) this gets better, up to 1.5 seconds.1
How to read degrees when referring to altitude/azimuth: The altitude is the angle of the celestial body (in degrees) above the horizon. Zero is the horizon (rising), 90º the zenith (overhead), and 180º the horizon (setting). From the Alt/Az coordinate system method, we can work out the exact position of objects in the sky. I refer to the azimuth, the horizontal angle, in explaining the axis of alt/az telescope mounts.
Use this coordinate system to find planets in the night sky.
Sailors in the past would use the angle of Polaris above the horizon to work out their position for navigational purposes.
How to calculate angular distance
An app for degree measurement might help. But hand measurements can also provide a no fuss angular separation calculator.
You can work out the angles in the sky using the guide given above.
Let these measurements be a reference when you’re looking for angles in the night sky!
- Dickinson, T. 2019. NightWatch: A Practical Guide to Viewing the Universe. Firefly Books.
- Time and Date