# A Newbie's Guide to Distances in Space

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.

When we talk about distance in astronomy, we are usually talking very, very large numbers. Far too many to describe them in terms of miles or kilometres. When we realised just how big space was, we needed some new units. In modern astronomy, we often use the Astronomical Unit or the Lightyear.

The Astronomical Unit is the average distance from Earth to the Sun. We say average because the Earths orbit is elliptical, varying from a maximum (aphelion) to a minimum (perihelion) and back again once a year.

Due to this variation, the Astronomical Unit is now defined as exactly 149,597,870,700 metres (about 150 million kilometres, or 93 million miles). You can see why we don't express this as kilometres! For objects in the solar system, their orbits are typically given in terms of the Astronomical Unit (AU). Earth being 1AU, Venus at 0.72AU, Jupiter at 5.2AU. These values are much easier to work with. If you want to convert AU to KM or Miles, simply multiply the Earths orbital radius by the AU value.

For distances outside of the solar system, the light year distance is often used. A light year is defined as the distance light travels in a year. Since the speed of light is constant, the distance is also constant. Light travels at around 300,000 kilometres per second, so these numbers can get very big, very fast. In one year, light travels about 10 trillion km. One light-year is equal to 9,500,000,000,000 kilometers, or 63,241 AU.

For large numbers like this, we often use scientific notion. We write a light year as 9.5x10^{12} km. This is called scientific notation. We simply move the decimal place to the left until we get to the smallest significant figure and count the number of times we moved the decimal place. Even using light years as a measure of distance we still deal with very large numbers. The Andromeda galaxy is the nearest galaxy to the Milky Way, and at a distance of 2.5 million light years, it's quite a bit further than walking down the road to the chemist. The furthest observed galaxy is EGS8p7 which is more than 13.2 billion light years away. Because we know how far away it is, and that the speed of light is constant, we know that the light from that galaxy has travelled for 13.2 billion years to arrive here. We are effectively looking back in time, to a point only a few hundred million years after the big bang. How cool is that?

Last updated: 2017-08-23

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