Precession Help

The settings in this view let you set the precession epoch of the Equatorial coordinate system that SkySafari uses to report the right ascensions and declinations of objects.

Precession & Nutation

Precession is a very slow "wobble" in the direction of the Earth's rotational axis, which takes about 25,800 years to complete. Because of precession, an object's right ascension and declination change over time.

• Use Current Epoch: If turned on, SkySafari will always report right ascensions and declinations for the current year. If turned off, SkySafari will report RA and Dec for the precession epoch entered below.
• Precession Epoch: The precession epoch (or year) for which equatorial coordinates should be reported.
• Include Nutation: A small wobble in the orientation of the Earth's axis superimposed on its overall precessional motion.

Other Corrections

• Aberration: A systematic shift in star positions caused by the Earth's velocity through space - about 20 arc seconds.
• Proper Motion: A slow change in the positions of the stars due to their physical motion through space.
• Light Time: Adjusts the positions of objects in the solar system for the finite velocity of light.
• Dynamic Time: Uses Terrestrial Dynamic Time (TDT) instead of Universal Time for precise time keeping.
• Refraction: A distortion in an object's apparent altitude caused by the Earth's atmosphere - can amount to over 1/2 degree near the horizon.