by Chris Peterson » Wed Dec 14, 2011 4:53 pm
flash wrote:What factors explain the shape of the moon's path through this photo? I understand that stars follow circular paths around the pole. Is that it? It is affected at all by the focal length of the camera used? I have this notion that the path of the Sun (and Moon?) through the sky lies in a plane which is always tilted at the same angle relative to the plane of the equator, but which seems to shift to the north in the summer, and south in the winter. At the equator, at equinox, the Sun rises due east, goes vertically straight up to the zenith, and then straight down to set due west. At the solstices, the path is parallel, but in the summer it begins north of due east (misses the zenith by passing north of it, and ends north of due west. In the winter the same is true but south instead of north. Since New Delhi is 28 deg N, and since the arc of the Moon's path in this photo is concave to the upper left, can I presume that north is to the left?
I think the easiest way to visualize this is to realize that the path of the Moon is approximately circumpolar. Because of its orbit around the Earth, it does move a little against the background stars, but over one evening this amounts to only a few degrees, so we see it move in pretty much the same way as the stars. Thus, its path looks substantially similar to that of stars captured in the same image. And since this was taken in the northern hemisphere, the north polar axis lies at the center of rotation, so yes, north is to the left. This is also clear by noting that from this location, the Moon rose at the start of the eclipse, so we must be looking in a generally eastward direction.
[quote="flash"]What factors explain the shape of the moon's path through this photo? I understand that stars follow circular paths around the pole. Is that it? It is affected at all by the focal length of the camera used? I have this notion that the path of the Sun (and Moon?) through the sky lies in a plane which is always tilted at the same angle relative to the plane of the equator, but which seems to shift to the north in the summer, and south in the winter. At the equator, at equinox, the Sun rises due east, goes vertically straight up to the zenith, and then straight down to set due west. At the solstices, the path is parallel, but in the summer it begins north of due east (misses the zenith by passing north of it, and ends north of due west. In the winter the same is true but south instead of north. Since New Delhi is 28 deg N, and since the arc of the Moon's path in this photo is concave to the upper left, can I presume that north is to the left?[/quote]
I think the easiest way to visualize this is to realize that the path of the Moon is approximately circumpolar. Because of its orbit around the Earth, it does move a little against the background stars, but over one evening this amounts to only a few degrees, so we see it move in pretty much the same way as the stars. Thus, its path looks substantially similar to that of stars captured in the same image. And since this was taken in the northern hemisphere, the north polar axis lies at the center of rotation, so yes, north is to the left. This is also clear by noting that from this location, the Moon rose at the start of the eclipse, so we must be looking in a generally eastward direction.