by Chris Peterson » Thu Nov 12, 2009 4:34 pm
Axel wrote:Now this is the way I see it. If there is a reference plane parallel to the image plane and located between us (i.e. the image on our screens) and the galactic centre, and if a person floating near the galactic centre is looking towards us with his or her body parallel to the reference N-S line and with head pointing north, then that person's "east" will be on his/her right-hand side, which to us will be the left-hand side.
I think you are asking for trouble in two ways here: by talking about "planes", and by changing the position of the observer. The position of an object on the sky is described by 2D spherical coordinates. It is a necessarily geocentric (or heliocentric) coordinate system. And a planar image is actually a projection, and only approximates the actual object when the FOV is small- just like maps of the Earth's surface. It is pretty meaningless to try and assign what we would call "east" to what an observer would see from some other location in the galaxy.
Although the spherical coordinate system used for the sky is related to the Earth's coordinate system (their poles lie on the same line), you might do better forgetting about Earth completely and just picturing a spherical grid on the sky, with coordinates measured in
declination (equivalent to latitude on the Earth) measured from +90° at the north celestial pole, to 0° at the celestial equator, to -90° at the south celestial pole, and in
right ascension (equivalent to longitude on Earth), with a fixed, arbitrary zero (equivalent to longitude 0° on Earth at a specific time) and increasing through 360° clockwise around the north celestial pole. "East" is the direction that right ascension increases, and isn't necessarily the same everywhere in an image. If you take a picture of the sky around the North Star, for instance, "east" is clockwise: typically to the left below Polaris and to the right above it.
When you look at an annotated image like the one under discussion, don't try too hard (or at all) to relate the directions to your local horizon on Earth. The "north" arrow is just telling you the direction that declination
increases, and the "east" arrow is telling you the direction that right ascension
increases.
[quote="Axel"]Now this is the way I see it. If there is a reference plane parallel to the image plane and located between us (i.e. the image on our screens) and the galactic centre, and if a person floating near the galactic centre is looking towards us with his or her body parallel to the reference N-S line and with head pointing north, then that person's "east" will be on his/her right-hand side, which to us will be the left-hand side.[/quote]
I think you are asking for trouble in two ways here: by talking about "planes", and by changing the position of the observer. The position of an object on the sky is described by 2D spherical coordinates. It is a necessarily geocentric (or heliocentric) coordinate system. And a planar image is actually a projection, and only approximates the actual object when the FOV is small- just like maps of the Earth's surface. It is pretty meaningless to try and assign what we would call "east" to what an observer would see from some other location in the galaxy.
Although the spherical coordinate system used for the sky is related to the Earth's coordinate system (their poles lie on the same line), you might do better forgetting about Earth completely and just picturing a spherical grid on the sky, with coordinates measured in [i]declination[/i] (equivalent to latitude on the Earth) measured from +90° at the north celestial pole, to 0° at the celestial equator, to -90° at the south celestial pole, and in [i]right ascension[/i] (equivalent to longitude on Earth), with a fixed, arbitrary zero (equivalent to longitude 0° on Earth at a specific time) and increasing through 360° clockwise around the north celestial pole. "East" is the direction that right ascension increases, and isn't necessarily the same everywhere in an image. If you take a picture of the sky around the North Star, for instance, "east" is clockwise: typically to the left below Polaris and to the right above it.
When you look at an annotated image like the one under discussion, don't try too hard (or at all) to relate the directions to your local horizon on Earth. The "north" arrow is just telling you the direction that declination [i]increases[/i], and the "east" arrow is telling you the direction that right ascension [i]increases[/i].