NavList:

Paul H, you quoted Chauvenet:
"for all zenith distances less than 85° the contraction of the horizontal semidiameter is very nearly constant and equal to one-fourth of a second."

And for zenith distances less than 45 degrees, the contraction in the vertical and horizontal semi-diameters is almost exactly equal (and by my calculation, closer to a third of a second of arc than a quarter, but that's probably just the limits of the approximation). For altitudes above 45 degrees, refraction shrinks all angles by one part in 3000 regardless of orientation. Refraction "compresses" the constellations.

-FER

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