Angular Distance Between Stars - Printable Version +- HP Forums (https://www.hpmuseum.org/forum) +-- Forum: HP Software Libraries (/forum-10.html) +--- Forum: HP Prime Software Library (/forum-15.html) +--- Thread: Angular Distance Between Stars (/thread-8498.html) Angular Distance Between Stars - Eddie W. Shore - 06-12-2017 12:15 PM Given the right ascension (α) and declination (δ) of two stars of the same epoch (J2000.0 is the most current), the distance between the stars are: d = acos( sin δ1 * sin δ2 + cos δ1 * cos δ2 * cos (α1 – α2) ) The distance is usually given in decimal degrees. Enter α in terms of hours, minutes, seconds (standard notation) and δ in terms of degrees, minutes, seconds (standard notation). HP Prime Program: ANGSTAR Code: ``` EXPORT ANGSTAR(α1,δ1,α2,δ2) BEGIN // 2017-06-08 EWS // Angular Angle // Degrees HAngle:=1; LOCAL d; α1:=15*α1; α2:=15*α2; d:=ACOS(SIN(δ1)*SIN(δ2)+ COS(δ1)*COS(δ2)*COS(α1-α2)); RETURN →HMS(d); END;``` Example Distance between Regulus (A) in Leo and Sadalmelik in Aquarius: (data via Wikipedia) Regulus: α = 10h8m23.11s, δ = +11°58’01.95” Sadamelik: α = 22h5m47.03593s, δ = -0°19’11.4568” Distance: 168°20’05.1793” Source: Meeus, Jean. Astronomical Algorithms William-Bell, Inc. Richmond, VA 1991. ISBN 0-943396-35-2 RE: Angular Distance Between Stars - KeithB - 05-15-2019 04:45 PM Replying to an old thread, but this is a good way to measure a lens's focal length, since the angle of view is known for the focal length and image size (i.e., Full Frame vs APS-C) Here is a good method: http://www.bobatkins.com/photography/technical/measuring_focal_length.html