A Handy Guide to Measuring the Sky
Did you know that you can measure the distances between objects in the sky with just your hands and fingers?
Our conventional methods of measuring distances between two objects on Earth make very little sense when measuring the vast distances between celestial objects.
Because of this, astronomers measure the distance between celestial objects based on the angle they make with an observational point on Earth. Known as angular distances or angular separation, distances are expressed in terms of degrees (°), arc minutes ('), and arc seconds (").
While angular separation primarily describes the apparent distance between celestial objects, as seen from Earth, it can also be used to suggest their actual distance from one another.
Imagining Angles in the Sky
Like our modern-day timekeeping, the angular method is based on a sexagesimal system – a numeral system with 60 as its base. There are 360° in a circle or sphere, each degree is divided into 60' and each arc minute is further divided into 60".
Imagine the sky as a big hollow sphere with the Earth at its center – let’s call this the celestial sphere. This sphere is 360°. If you look above at the sky, you will see only half of the sky i.e. 180° of the celestial sphere. The other half of the celestial sphere is below the horizon and cannot be seen. The point right above you in the sky is the zenith. The zenith is always 90° from the horizon.
A “Handy” Way to Measure Distances
Hold your hand at arm’s length and close one eye.
- Make a fist, with the back of your hand facing you. The width of your fist will approximately be 10 degrees. This means that any two objects that are on the opposite ends of your fist will be 10 degrees apart. The North Star (Polaris) and Dubhe, one of the northern pointers of the Big Dipper are 3 fists apart. This means that angular distance or angular separation between the two stars is 30°.
- Open up your fist, stretch your little finger and thumb as far as you can and curl down the rest of your fingers. The tip of your little finger and your thumb will span about 25°. The Big Dipper spans around 25°.
- The tip-to-tip span between your index finger and your little finger is 15°.
- Your three middle fingers will span about 5°.
- Your little finger at an arms length is about 1° wide.
It is important to note that such measurements are approximate – not everyone has the same sized hand.
With these simple measuring rules in your hand, you can not only understand basic stargazing jargon but also tell other budding stargazers where to look for a specific celestial object in the sky.
Angular size or angular diameter of a celestial object is the angular separation between opposite edges of the object. The Sun and the Moon are the only objects in the sky whose angular size is visible to the naked eye.
Remember to never look at the Sun directly without any eye protection!
The angular diameter of a full Moon is about 30', while the angular diameter of the Sun is around 32'.
Find Your Latitude
If you are in the Northern Hemisphere, you can use your hands to find your current location’s latitude. To do this, stretch your hands in front of you and measure the angle between the visible horizon and the North Star. This angle is your latitude in degrees.
Unfortunately, there is no bright star equivalent to the North Star in the Southern Hemisphere.