{ Practical astronomy | Computing }

# Computing

Perhaps more than any other science, astronomy is a numeric science. It is also very attractive to less numerically inclined citizens, but to go beyond looking at the night sky in awe and knowing it is all so very very far away and Earth is so very very small, you really need to use number and sensible measuring units.

Make no mistake, the ever popular light year (ly) is not the least intuitive. The name of the unit sounds easy enough, the distance that light travels in one year. We all have a good grasp of a year, but who has really seen light move and has a sense of its speed? There is really no way around it, light moves at almost 300000 km/s. Those are two units we can relate to, a few km is how far I travel to work, a second is a little less how long it takes to say "one kangaroo".

A light year then is a bit under 10,000,000,000,000 km or 1013 km. Almost all numbers in astronomy are very much larger than 1, or sometimes very much smaller than 1. It is futile to try and relate them to things we know, to hook astronomy into our existing intuition of the world. Rather, we have to work out the numbers initially, and slowly build up a new intuition about the astronomical realm.

For example, the hard facts are that one au (astronomical unit) is about 150,000,000 km, that one pc (parsec) is about 200,000 au (about 3.26 ly), that beyond that we use prefixes like kilo (1000), Mega (1,000,000) and Giga (1,000,000,000). The intuition develops when we memorise and internalise knowledge like:

• The Moon is about 400,000 km away.
• The Sun is about 1 au, or 150,000,000 km, away.
• Planets are between a third and 50 au away.
• Stars are between 1 pc and many kpc away. The Milky Way is a disc roughly 100 kpc in diameter and 1 kpc thick.
• Galaxies are between 1 Mpc and a few Gpc away, although "nearby" dwarf galaxies may be as close as 50 or 100 kpc.

This just cannot be done without numbers: If the Moon is "far away" and the planets are "very far", what do we call the distance to the stars and the distance to the galaxies? And it still makes a difference whether a galaxy is 100 kpc or 100 Mpc away. We must use numbers, and it helps to get used to them and then get "a feeling" for the numbers.

In this main section, we build up some software based on the Python programming language. It takes a little bit of setting up, then copying ("installing") the software or writing ("developing") some software of your own. Finally running the software to process input data into output data, like "my lens is 50 mm f/4, what is the field of view?"

In a software appendix are two larger, older items of non-Python software that I wrote. One item is about ephemeris calculation (time, coordinates, orbits of Moon, planets and artificial satellites). The other item is about image reduction, including stacking of images.