The shape of the Earth, to a good approximation, is a sphere. To the extent that the density of the Earth depends only on the distance from the Earth’s center, the arrow of gravity will point inevitably toward the Earth’s center. With the help of a spirit level, a camera can be directed toward the zenith. A change in the latitude of the observer will show up as a shift in the displacement of the diurnal paths of the stars in the field of view of the exposure. For a displacement of 2 mm on an exposure taken with a camera lens of 135-mm focal length, a north-south displacement of 100 km is needed.
KeywordsFocal Length Zenith Angle Image Scale Spirit Level Plate Scale
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