Yes. I’m talking about Gliese 581g again. So I may have a minor obsession. Whatever. Who can blame me? I find the prospect of another planet harbouring life simply astounding. While thinking about life on Gliese 581g, a question came up. How would Gliese 581, the planet’s home star, look in the sky? I imagined a sentient being looking up at the star, and wanted to know how it would actually look in the sky, both its size and colour. After a quick search on the internet, I was able to find out its probable colour: white. That came as no surprise. Finding its size, however, was much more difficult. Finding nowhere on the internet that had the star’s size in the skies of Gliese 581g, I decided to set out on a quest to calculate it. I tried a number of different ways of calculating, but finally came across the small angle formula, which suited my task perfectly. The forumla used the principles of trigonometry to approximate the size of arcs in angles, which is exactly what I needed to calculate, considering the size of objects in the sky, at least in astronomical terms, is measured in degrees of angular diameters. For instance, our star, Sol, better known as simply the sun, is about .52º. The formula, after I made some adjustments, was as follows:

θ=360º/2π*d/D

Where:

  • θ (the Greek letter theta) is the angular diameter
  • d is the diameter of Gliese 581 (the star) in kilometres
  • D is the distance between Gliese 581g and its home star in kilometres

Now that I had the formula, I simply had to get the data for both the diameter of Gliese 581 and the distance between it and Gliese 581g. After a quick internet search, I was able to get both the diameter of Gliese 581, which is 3.07520*10^5 km, and the distance, which is 2.243968061*10^7 km. I plugged them into the equation, and after accounting for all the values, came out with the answer of:

θ=1.040530677º

This meant that the star, from the surface of Gliese 581g, would be about twice the size of our own sun in our own sky. Try and imagine that; that the sun was twice as large as it is now. It’d be interesting, to say the least. Luckily for you, I went through the trouble of bringing up a photo and resizing the sun as it would appear in the skies of Gliese 581g, so you don’t have to imagine it. Here’s the image as it appeared on the Earth:

 

And here’s the modified image to make the star seem like it was Gliese 581 being viewed from the surface of Gliese 581g:

And there you have it. Gliese 581, viewed from the surface of Gliese 581g. Have a good new year!