Angular Distance Between Stars
06-12-2017, 12:15 PM
Post: #1 Eddie W. Shore Senior Member Posts: 1,319 Joined: Dec 2013
Angular Distance Between Stars
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
05-15-2019, 04:45 PM
Post: #2 KeithB Senior Member Posts: 320 Joined: Jan 2017
RE: Angular Distance Between Stars
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/tec...ength.html
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