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We can, however, measure the extent of M82 in the plane of the sky. That is we can measure its extent across the sky from East to West (or from left to right), and from North to South (top to bottom). The two spatial coordinates used in astronomy to represent these two dimensions are right ascension in the East-West direction, and declination in the North- South direction.
So how do astronomers ever determine all three dimensions of an object?
Recovering the Third Dimension
Astronomers gain back some aspects of the lost third dimension through velocity measurements. Applying Doppler shifts to spectral line measurements, they can calculate the speed at which an object is moving only along the line of sight direction. Fortunately, they can also determine how fast each part, say the left end verses the middle and right end, of an object is moving toward or away from us. It is by comparing these various velocities that astronomers put together a three dimensional picture of an object.
Say, for instance, that the left end of an object is moving away from us faster than the right end. Then we can assume the object is spinning in a clockwise direction. These velocities then indicate how fast that object spins. Finally, its extent in the third dimension can be surmised from its rate of rotation.
Thus velocity provides a means by which astronomers can roughly determine the extent of astronomical objects in the line of sight direction, and acts as a third dimension.
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