gam wrote:All the meteor tracks appear to pass through the same sequence of colors. Why is that?
The green color is from ionized oxygen, and only shows up for the right combination of temperature (which essentially means meteor velocity) and atmospheric concentration (which essentially means height). Since most of the meteors have similar velocity-height profiles, they also have similar colors.
The stars in the background do not appear to have moved over the six-hour exposure. Why not?
The exposures themselves as short enough that the stars don't move much. Each meteor image has been stacked after using the stars as alignment references, so the stars appear stationary. The only tricky bit is dealing with the foreground, which can only come from one exposure. The usual method is to mask it out on all but one image when stacking.
The moderator says that the apparent divergence of parallel meteor tracks is due perspective. Rays of sunlight from clouds also seem to diverge for the same reason, I suppose, which gives one a clue about how great the distances of even earthly phenomena can be.
The reasons are the same, although large distances are not required. The meteors are only about 100 miles away, similar to the distance to horizon clouds. But even train tracks appear to diverge as they approach you, and the distance there is only a mile or less. You can see the effect with wood or tile floors, inside a single room.
[quote="gam"]All the meteor tracks appear to pass through the same sequence of colors. Why is that?[/quote]
The green color is from ionized oxygen, and only shows up for the right combination of temperature (which essentially means meteor velocity) and atmospheric concentration (which essentially means height). Since most of the meteors have similar velocity-height profiles, they also have similar colors.
[quote]The stars in the background do not appear to have moved over the six-hour exposure. Why not?[/quote]
The exposures themselves as short enough that the stars don't move much. Each meteor image has been stacked after using the stars as alignment references, so the stars appear stationary. The only tricky bit is dealing with the foreground, which can only come from one exposure. The usual method is to mask it out on all but one image when stacking.
[quote]The moderator says that the apparent divergence of parallel meteor tracks is due perspective. Rays of sunlight from clouds also seem to diverge for the same reason, I suppose, which gives one a clue about how great the distances of even earthly phenomena can be.[/quote]
The reasons are the same, although large distances are not required. The meteors are only about 100 miles away, similar to the distance to horizon clouds. But even train tracks appear to diverge as they approach you, and the distance there is only a mile or less. You can see the effect with wood or tile floors, inside a single room.