How do you calculate the distance to celestial objects?

Ever wondered how do astronomers calculate the distance to celestial objects such as a distant star? Ever since I was a kid, I remember looking at the night sky with my sister using our telescope

Ever wondered how do astronomers calculate the distance to celestial objects such as a distant star? Ever since I was a kid, I remember looking at the night sky with my sister using our telescope (which our father had purchased while on a business trip) and focusing on one star per night (usually the brightest) through the different lenses of the telescope and deciphering whether it was a planet in our solar system or just a bright star shining millions of miles away.

And since we were new to this, whenever we focused our lenses on an object and discovered (a big term in the astronomical world, but for us was true) that it was a planet (Mars, Saturn, Jupiter etc.) and a group of moons orbiting (in case of Jupiter), the joy and smile on our faces could only be truly understood by those astronomers or cosmologists who would experience it while making those great discoveries of new galaxies, stars, planets or black holes.

However, the thought of how could the astronomers come to the conclusion as to how far a particular celestial body was, has captured my imagination ever since. I knew that unless I get an answer to this basic question, I will never truly enjoy and understand the enormity of our universe and just how gargantuan it is.

Today, I wanted to shed some light on this topic, hoping to answer this question in basic terms and help you understand this great, but oh so important concept while talking about our universe. Through a simple home experiment, I will try and explain just how do our scientist use basic mathematics and laws to calculate the distance from our planet to some of the most distant objects in outer space. But first, we need to understand a very important concept called as Parallax.


Parallax is the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g. through the viewfinder and the lens of a camera.

Home experiment anyone can perform

It’s a concept which is very easy to understand even from the comfort of your living room. Hold your index finger straight in front of you at an arm’s length, close your left eye and view the finger with your right. Now do the same again with your left eye open and right eye closed. You will notice that the finger appears to have moved slightly with reference to the background when looked from each eye. This effect is called as Parallax.

And this is the same concept that is applied while measuring distance to objects in space, but just on a very large scale.

In space study, this is the angular amount of parallax in a particular case, especially that of a star viewed from different points in the earth’s orbit. Thus the parallax is the angle at the star in an Earth-Sun-Star triangle.

Basic mathematics and trigonometry

Let’s get to the chase here. The formula used to calculate the distance to any star is as below:         

For example, the black hole discovered in the 1970’s called as the Cygnus X- 1 was officially named a Black hole in 2010 when as per calculations using the above formula, the distance was computed as 6000 light years from Planet earth. And with some other calculations which helped find the mass of the object (we will cover in later chapters), this body was deemed to be a black hole.

However, to calculate the distances to such far away bodies (in hundreds or thousands of light years), astronomers take help of some of the largest telescopes to determine these parameters.

Using one single telescope would not provide enough meat in terms of data points to help calculate the above angle. So they used a group of 10 giant telescopes located across continental USA to find the missing parameters in the above equation to determine its distance and mass, thus coming to a conclusion that it was indeed a black hole that they were looking at.

So, did this help you understand better about the distance calculations?

I hope I have been able to simplify this distance calculation for you and helped you understand the very basics of astronomy. This understanding is vital for anyone who has ever faced the same question I had when I was a kid and struggled to come to terms with how large distances are calculated sitting here on earth.

The conclusion or beauty lies in the fact that most of astronomy is pure mathematics and if we can get our heads around the complex concepts and equations, the sheer joy of astronomy can be as infinite as the space itself.

Leave a Reply

Your email address will not be published. Required fields are marked *