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HorizontalToEquatorial would be like EquatorialToHorizontal in reverse, something like this: (x, y, z)_hor = rectangular form of (alt, az) (x, y, z)_sid = (x, y, z)_hor rotated by geographic latitude (ha, dec) = spherical form of (x, y, z)_sid ra = local sidereal time - ha


No, this is not possible. Equatorial coordinates only specify the direction of a certain point in space (as seen from Earth), not the distance. During a solar eclipse, the Moon and the Sun have (approximately) the same equatorial coordinates, but the distance to the Sun is 0 or almost 1 AU. It is possible to define equatorial or ecliptic coordinates with the ...


$\xi$ and $\eta$ are just arbitrary names of coordinates, used because the actual position on the sky doesn't matter in this context, i.e. only the relative position in the plot is important. An analogue would be to label the coordinates of your living room $x$ and $y$, instead of using Earth's longitude and latitude. If you really want to transform, you can ...


Or is 0 degrees the direction towards the ascending node 'equinox'? Which is NOT towards the Sun on January first... The answer is yes, more or less. From JPL's HORIZONS, the location of the Sun at Noon Terrestrial Time on 1 Jan 2000 is, ignoring atmospheric effects, a right ascension of 18 hours, 45 minutes, and 9.36 seconds. This is nowhere close to a ...

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