# HP Forums

Full Version: Angular Distance Between Stars
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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
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
Reference URL's
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