20 ms·

The Astronomical Unit is based on the Earth-Sun average distance. It seems a lot of people care about this metric.

hn_throwaway_99 8y ago
Oh, come on. There is a big difference in what "average distance" means when comparing planets (which do not orbit around each other) vs. a planet and the sun (where one body has a nearly circular orbit around the other).
Rebelgecko 8y ago
It used to be that an AU wasn't a constant, and gradually changed over time since the definition depended on things that weren't constants (e.g. mass of the sun). For the last few years, the AU has been hardcoded to a certain number of meters close to its notional value. The AU is a handy unit when measuring how far things are from the sun, but I think most people's intuition breaks down when using it to measure objects with different orbits and varying angular separations. On top of that I think the argument can be made that just because people care about a particular unit of measure doesn't mean that the definition is particularly meaningful. There's lots of people that care about kilograms and acres, but that doesn't mean they care about a hunk of metal in a French vault or the plowing speeds of European oxen.
exDM69 8y ago
These days the Astronomical Unit (AU) is defined exactly (149597870700 m), but historically AU was used to measure relative distances in the solar system because the distance from the Earth to the Sun was unknown. The transits of Venus in 1761 and 1769 were used to determine the distance to the sun and hence the scale of the solar system in a major scientific endeavor. Several expeditions were shipped around the globe to do simultaneous observations of the beginning and end times of the transit, using the positions of moons of Jupiter as a time reference. The AU is still in modern use because it can easily communicate distances around the solar system to humans. It is less useful in computation and it's a somewhat arbitrary number given that the orbit of the Earth has minor perturbations.

Oh, come on. There is a big difference in what "average distance" means when comparing planets (which do not orbit around each other) vs. a planet and the sun (where one body has a nearly circular orbit around the other).

Rebelgecko 8y ago

It used to be that an AU wasn't a constant, and gradually changed over time since the definition depended on things that weren't constants (e.g. mass of the sun). For the last few years, the AU has been hardcoded to a certain number of meters close to its notional value. The AU is a handy unit when measuring how far things are from the sun, but I think most people's intuition breaks down when using it to measure objects with different orbits and varying angular separations. On top of that I think the argument can be made that just because people care about a particular unit of measure doesn't mean that the definition is particularly meaningful. There's lots of people that care about kilograms and acres, but that doesn't mean they care about a hunk of metal in a French vault or the plowing speeds of European oxen.

exDM69 8y ago

These days the Astronomical Unit (AU) is defined exactly (149597870700 m), but historically AU was used to measure relative distances in the solar system because the distance from the Earth to the Sun was unknown. The transits of Venus in 1761 and 1769 were used to determine the distance to the sun and hence the scale of the solar system in a major scientific endeavor. Several expeditions were shipped around the globe to do simultaneous observations of the beginning and end times of the transit, using the positions of moons of Jupiter as a time reference. The AU is still in modern use because it can easily communicate distances around the solar system to humans. It is less useful in computation and it's a somewhat arbitrary number given that the orbit of the Earth has minor perturbations.