Astronomical Applications Department, U.S. Naval Observatory - novas c 2 doc (Page 25)

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Discussion:

This function takes the topocentric celestial coordinates of an object and computes the

equivalent local horizon coordinates. The function uses a method that properly accounts
for polar motion, which is significant at the sub-arcsecond level. Atmospheric refraction
can be included in the transformation, and if so, refraction is applied to both sets of
coordinates (this can be useful for telescope pointing). Refraction, when requested, is
computed by function refract.

ra

and

dec

, the input topocentric right ascension and declination, can be obtained

from topo_star or topo_planet.

tjd

is the TT time at which the topocentric place was

computed. The difference TT­UT1 (often called

T) is passed to the function via

argument

deltat

. Values of

T are published in the annual Astronomical Almanac or

can be obtained from the National Earth Orientation Service (NEOS) home page on the
World Wide Web. The coordinates of the pole,

x

and

y

, can be obtained from IERS

Bulletins A and B, although

x

and

y

can be set to zero (0.0) if sub-arcsecond accuracy is

not needed. (If refraction is requested, sub-arcsecond accuracy is unlikely.)

The height of the observer and meteorological conditions at the observer, contained in

structure

location

, are used only for refraction, if

ref_option

is not equal to zero.

In this function, the directions

zd

= 0.0 (the zenith) and

az

= 0.0 (north) are considered

fixed in the terrestrial frame. Specifically, the zenith is along the geodetic normal, and
north is toward the IERS reference pole.

If

ref_option

= 0 (no refraction), then

rar

=

ra

and

decr

=

dec

.