Circle of equal altitude

The circle of equal altitude also called circle of position, CoP, is the real line of position in celestial navigation. It is defined as the locus of the Earth on which an observer sees a star, at a given time, with the same observed altitude.

The center of the CoP, is the substellar point or geographical position of the observed body, and its radius is the great circle distance equal to the zenith distance of the body.

The equation links the following variables

${\displaystyle \sin(Ho)=\sin(B)\cdot \sin(Dec)+\cos(B)\cdot \cos(Dec)\cdot \cos(LHA)\,}$

Being B the latitude (+N/S), L the longitude (+E/-W). LHA = GHA + L is the local hour angle, Dec and GHA are the declination and Greenwich hour angle of the star observed. And Ho is the true or observed altitude, that is, the altitude measured with sextant corrected for dip, refraction, parallax.

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