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Applet: Sky Precession

Precession and Proper Motion Applet

Input field: green
Use TAB key to enter the next input field.

Algorithm from J. Meeus ("Astronomical Algorithms", Chapter 20: Precession).

The current standard epoque is "J2000" (2000, Jan 1 at 12 h TD), corresponding to JD 2 451 545.0. For purposes of calculating positions of stars, the beginning of a "year" differs from the standard epoque J2000.0 by an integer multiple of the Julian year (365.25 days).

Noon on January 1, -4712, i.e., January 1, 4713 BC (JD 0) is J-4711.96441

mean tropical year 365.24219 SI days


(Meeus: Astronomical Algorithms, p. 127)

example precession proper motion Meeus

In case the proper motion in RA is given in seconds (s), the value has to be multiplied by 15 to get arcseconds (as)
(e.g. 0.03425 s = 513.75 mas)

The results of my applet agree with those of Meeus:
RA = 2h 46m 11.331s = 41.547213
dec = 49 20' 54.54'' = 49.348482

The coordinates (Ra, Dec) are also converted to ecliptic values (long. L, lat. B)

2003-2013 J. Giesen

Updated: 2013 Jun 15