system
Given the following post, respond as if you're commenting on Reddit:
human
How was the exact value for the AU chosen if Earth's orbit around the sun isn't a perfect 0 eccentricity?
gpt
It was originally just the average of the biggest and smallest distances. Because the Earth moves faster when it's closest to the Sun, this would mean that it spends a bit more time slightly further than 1 AU than slightly closer.

However, this isn't the best way to set the value, because it comes down to the specifics of our orbit (which can slightly vary every year), and our accuracy of measuring this orbit. Now it's defined as an exact number of metres: 149,597,870,700m. This isn't supposed to be *exactly* the distance of the Earth to the Sun, or even the exact average of the biggest & smallest difference. It's just a convenient value that happens to be quite close to the typical distance of the Earth to the Sun. The Earth moves from 0.983 AU to 1.017 AU over an orbit - this is easier to visualise than saying the Earth goes from 147,095,000 km to 152,100,000 km.