Talcott Method

a method for measuring the astronomical latitude ϕ of a point. The method involves measuring with the eyepiece micrometer of an astronomical instrument the small (16’–20’) difference Δz between the zenrth distances of two stars, one south and one north of the zenith, at the moment the stars pass across the meridian plane. The method was proposed by the Danish astronomer P. Horrebow in 1740. It is named after the American geodesist A. Talcott, who developed its practical implementation in 1857.

If both stars are at upper transit, then

If the northern star is at lower transit, then

Here, δ_s is the declination of the southern star, δ_N is the declination of the northern star, z_s is the zenith distance of the southern star, and z_N is the zenith distance of the northern star. In addition, corrections may be made, for example, for the Talcott level (for the change in inclination of the telescope when it is rotated by 180°), for the difference in refractions ρ_s and ρ_N in the directions of the southern and northern stars, and for the curvature of the diurnal circle.

The Talcott method is widely used in astronomical determinations of the first class and in the work of the Latitude Service in the USSR and abroad at latitudes of up to 65°.