# How is an Astronomical Unit (AU), which is equal to the distance between the Earth and Sun, determined if the distance between the two isnt constant?

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How is an Astronomical Unit (AU), which is equal to the distance between the Earth and Sun, determined if the distance between the two isnt constant?

In: Mathematics It is essentially the average between the furthest and closest distance between earth and the sun. An AU is defined as the *average* distance between the Earth and the Sun, not the *absolute* distance, which charges throughout the orbit.

The distance itself was determined over many years by sending out probes to help measure the true distance between the Earth and Sun, in order to model the orbit mathematically and find the average. Wikipedia has a perfect explanation:

> Originally conceived as the average of Earth’s aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres, or about 150 million kilometres (93 million miles).

I’m just surprised it took them so long to define it as a fixed distance/number since Earth’s orbit (and our measurements of it) can change ever so slightly.

For everyone asking (or being annoyed) why it’s not 150Gm: I guess they didn’t want to make past equations invalid, so the definition had to fit the last measurement. Like the speed of light, the AU is something we used to measure, but is now a defined value.

Originally, though, the AU was the mean distance between the center-of-mass points of the Earth and sun. More specifically, it was the *geometric* mean of this distance, not the time-averaged distance. If you average over time instead of spatial position, you get a different (larger) value for the AU. Not the original post because I was very wrong. We define the Astronomical Unit as the mean distance from our planet to the center of our sun. To get a rough estimate, we take the furthest point in our orbit (aphelion) and the closest point in our orbit (perihelion), add them together and divide by 2. This gives us a number close to 150 million kilometers. Hope this was a bit more accurate this time, thanks for the corrections everyone. Hooray for peer review!

Phil Plait does a great job explaining it in this Crash Course Astronomy video if you are more of a visual learner! https://www.youtube.com/watch?v=CWMh61yutjU