system
Given the following post, respond as if you're commenting on Reddit:
human
When they measure the distance between objects like planets and suns, at what point do they start the measurement?

Do they start at the center of the planet and how do they handle elliptical orbits?
gpt
It depends on the application. Typically in the solar system, we do it between the centers of mass of objects. This is good for figuring out where planets will be if you want to send a lander to them, for example, but then you really care about where the lander will actually land and so you also need a measurement of surface feature to surface feature.

For objects like stars, we don't know their distance precisely enough to care about the difference between start the measurement at the Earth or the Sun. And any ellipticity, while we can measure that because we can see the relative distances change or the position on the sky change, don't affect the overall measured distance, again because the errors on the absolute distance are so large.