Trying to locate key bright stars and constellations by degrees in the sky can be confusing. Degrees and angles are used to measure celestial distances. Once you know how to estimate sky angles, it’s easy. Here I show how, for sky measures, all you need is your hand. Read on…
What is the angular distance between two stars? Rather than an app or refering to an astronomy book for angular sizes, you can figure it out by using a simple sky measuring guide using your hand, as I show in my chart below.
What is angular distance? Angular distance is the measure of degrees between two stars in the sky as observed from Earth. You might come across angular distances when you are using a star map or an app such as Stellarium on your phone to find celestial objects.
Things to know about finger & angles in sky
The following are fundamental in measuring distances in the sky.
- Knowing degrees in a circle (360º)
- Determining angular distance
- Hand width measurements as degrees in the sky
Knowing sky distances – degrees in circles
A circle is 360º. Imagine the sky at night as a semi-circle, which is 180º from horizon to horizon and with zero being one horizon and 180 the opposite horizon. From zero to zenith is 90º. And, so a celestial body in the sky sits at an altitude of so many degrees above the horizon of the east.
On the other plane, the azimuth, the angle between direct east and south, south and west, west and north, and north and east is 90º — This being the complete circle (360º) divided into four 90º. Direct north is zero on this horizontal or azimuth plane, east is 90º, south 180º, and west 270º.
From the Alt/Az (aka horizontal coordinate system) method, we can work out the exact position of objects and so find planets to observe in the night sky. I wrote more on this in explaining the axis of alt/az telescope mounts.
Measuring a sky object by angles: Let’s take the Sun and the Moon. When looking at the Sun or the Moon, it will be roughly 0.5º in diameter in the sky. In arc-minutes, it’s 32 for the Sun and 30 for the Moon.
Movement across the sky in angles: The Sun moves across the sky 15º each hour (24 hr in a day and 360º in a full circle).
Distance discerning close stars: At night, our naked eyes can distinguish two close celestial bodies if they are at least 6 minutes (one tenth of a degree) apart. Stars or other celestial bodies closer than 6 arc-minutes or a tenth of a degree (e.g. double stars or triple stars) will appear as a single entity, unless viewed through a set of binoculars or a telescope. A 2-inch (60 mm) telescope can discern two equally bright stars if they are at least 2 seconds apart. For a 3-inch (90 mm), this gets better, up to 1.5 seconds.1
Angles for navigation: 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 can help. But hand measurements provide a no fuss angular separation calculator. You can work out the angles in the sky using the chart provided. Let these measurements be a reference when you’re looking for angles in the night sky!
Chart: How to measure degrees in the sky with your hand
Let’s start simple, using the width of your finger…Hold up your hand at arm’s length to the sky. The width of the end of the little finger (pinky) is 1 degree. This is one hand measurement — see others below, including putting your clenched fist to the sky, the width of your fingers, or distance between certain digits when held at arm’s length towards the sky.
Simple sky Measuring Guide
The width of the end of your little finger (pinky) 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 clenched 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 your pinky 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’.
How do you find the degrees above the horizon?
You can use the finger and hand measurements shown in the chart to find the degrees above the horizon. Another way is to divide the area between the zenith (90º) and the horizon (zero) into equal distances to find the corresponding angle.
For an object at 25º above the horizon, you’d be looking for it low in the sky, as in between the first ¼ and ⅓ part of the sky between the horizon and zenith (90÷25 = 3.6 .˙. 1/3.6). Or, you could use your pinky finger to thumb hand measurement.
The finger and hand distances generally work for everyone because one’s arm length is proportional to their 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…
How Big Is one Degree in the sky
One degree in the sky is also the width of your pinky when held up to the sky at arms length. This is the same as 60 arcminutes, which is the width of two Moons in the sky.
Astronomy measurements are in degrees (º) arcminutes (‘) arc seconds (“), and 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 is not unlike the latitudes and longitudes reported 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.
Also, one degree is equal to 52.5 feet at a 1000 yard distance, when talking about the field of view. This means that what you are seeing at a 1000 yards from where you’re viewing will be 52.5 feet from edge to edge of the circular viewing frame.
Some distances to familiarize yourself with
- Betelgeuse to Rigel in Orion is 19º.
- Orion’s belt is a bit less than 3º.
- End star of Big Dipper handle to end star of the bowl is 25º.
- Between two end stars of the bowl: 5º.
- The Great Square of Pegasus sides on average: 15º.
- The W of Cassiopeia, from one end to the other: 13º.
Summing it up
Angular distance is the apparent distance from one celestial body to another, measured in degrees or part thereof. A simple convenient way of measure is to use your hand.
- Dickinson, T. 2019. NightWatch: A Practical Guide to Viewing the Universe. Firefly Books.
- Time and Date
- Mosley, John. 2003. Starry Night Companion. Space Holding Corp.