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Comment by petercooper | Hacker News Reader

Which means, for the first time since its discovery, Neptune is about to return to the same position in space that it occupied the day it was discovered. And what date will that be?

It will return to the same position within its orbit relative to the sun, but surely not the "same position in space"? (The solar system has moved a little further around in its orbit within the Milky Way too.)

I'm aware I'm questioning a qualified astrophysicist here - something I am not - so I'll blindly assume it's something to do with default frames of reference until corrected otherwise ;-)

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Good source for a layman's explanation of how we measure where things are in the sky is Astronomy Cast: http://www.astronomycast.com/astronomy/ep-171-solar-system-m... http://www.astronomycast.com/astronomy/ep-170-coordinate-sys...

Astronomy Cast is great. I wish I had a similar "not just what we know, but how we know what we know", friendly conversation type podcast for more subjects.

It is! I listen to an episode almost every night. I will listen to the coordinate systems one again, though.

It's a good question - as we are talking about Neptune having completed one orbit it is implied that we're talking about Neptune's position relative to the sun, or more exactly, relative to the solar system's centre of gravity (which is always inside the sun's corona.)

The Milky Way would have also moved a bit further around the centre of gravity of it's local galaxy cluster I'd assume. This is what I was talking to my brother about recently but we didn't look up an answer. To an observer outside our galaxy what speed and possibly direction(probably unanswerable?) are we moving in on Earth?

"Same position in space" isn't well-defined anyway. There's no fixed set of coordinates permeating the entire universe. But it's clear enough what is meant in this case.

I wonder, though, it should be in the same position in the sky, right (after accounting for the Earth not necessarily being in sync with regard to its rotation)?

I'm not sure what you mean "after accounting for the Earth not necessarily being in sync"; since it isn't in sync it won't be in the same position. But if you were viewing it from the centre of the Sun, then yes, it would be in the same position in the sky.

I'm not sure what you mean "after accounting for the Earth not necessarily being in sync"; since it isn't in sync it won't be in the same position.

That from some point on Earth (on a similar latitude - accounting for tilt) Neptune would be in the same position in the sky (because other than the tilt, the time of day would likely be different so the longitude of observation should be too).

In writing that, though, I just realized the flaw in my thinking. Not only would the rotation of the Earth not be in sync, nor would its orbit! So the chance of being able to repeat the precise observation at the specified time would be close to nil.

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