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Apparent vs. True Azimuth and Elevation, calculations and causative factors
ncc1701d
post Nov 2 2016, 04:46 PM
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what is the difference between "apparent" azimuth and elevation? and non apparent azimuth and elevation? Is there even such a difference and how do I convert between them?

Since your location on a planet can either be in planetcentric lat long coordinates or planetographic lat long coordinates i would think that would effect your azimuth and elevation locations coordinates for the sun but not sure. Its confusing to me.

the internet is very inconsistent when using the word 'apparent" when defining azimuth and elevation.
Not mentioning the word apparent might mean they are assuming "apparent" but Iam confused.
thank you.
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Greenish
post Nov 2 2016, 10:54 PM
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If you're discussing remote observations of astronomical objects, "apparent" includes various aberrations that affect how it would be seen vs. the purely geometric solution.

More detail here at JPL Horizons documentation for how they calculate apparent positions.
http://ssd.jpl.nasa.gov/?horizons_doc#specific_quantities

The key bit for Az & El is as follows:
QUOTE
4. Apparent AZ & EL:
Apparent azimuth and elevation of target. Compensated for light-time,
the gravitational deflection of light, stellar aberration, precession and
nutation. There is an optional (approximate) correction for atmospheric
refraction (Earth only).
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JRehling
post Nov 15 2016, 03:17 PM
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Just one evocative example of this that I've read about recently… the southerly declination of Canopus plus the northerly latitude of Lick Observatory add to almost exactly 90°, but observers there have reported seeing Canopus at about 1° altitude. That is a case where the difference between apparent and ordinary azimuth makes the difference between seeing something and not seeing it.
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scalbers
post Nov 15 2016, 11:44 PM
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Indeed refraction is nominally about 1/2 degree for something on the horizon if you're at sea level. It can be more if you are on top of a mountain looking at a horizon at sea level. On top of that the horizon itself is depressed downward when on a mountain top.


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nprev
post Nov 16 2016, 01:51 AM
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NOTE: Edited the topic title for brevity.


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