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. It also gives you precise control over the corrections SkySafari makes when computing the positions of objects in the sky.

    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. The Earth's axis defines both the Equatorial (or RA-Dec) coordinate system. Because of precession, an object's right ascension and declination change over time - not because the object is moving, but because the coordinate system is moving.

    Use Current Epoch: if turned on, SkySafari will always report right ascensions and declinations for the current year ("epoch"). 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, if "Use Current Epoch" is turned off. Star atlases and ephemeris predictions (e.g. as in the Astronomical Almanac) often use a fixed epoch, such as 2000.0, for reporting RA and Dec.

    Include Nutation: a small wobble in the orientation of the Earth's axis superimposed on its overall precessional motion. Nutation causes a small change in an object's position, typically amounting to about 8-10 arc seconds.

    Other Corrections

    Aberration: a systematic shift in star positions caused by the Earth's velocity through space. It is a result of Einstein's theory of special relativity. Aberration causes objects to appear to shift in the direction that the Earth is moving by about 20 arc seconds, and affects all objects in the same part of the sky equally.

    Proper Motion: a slow change in the positions of the stars due to their physical motion through space. For all except the nearest stars, proper motion is only a small fraction of an arc second per year. When this option is turned on, a star's proper motion in right ascension and declination is displayed adjacent to its coordinates in the Object Info window.

    Light Time: adjusts the positions of objects in the solar system for the finite velocity of light. We see Saturn in the sky not where it is right now, but instead where it was about 90 minutes ago, because light from Saturn requires about 90 minutes to travel to Earth.

    For most objects, the effect of light time amounts to only a few arc seconds. Where light time makes a noticeable difference is in the positions of the outer planets' moons, and especially in planetary rotation.

    Dynamic Time: also called Terrestrial Dynamic Time (TDT), this is the standard for precise time keeping in astronomy. It differs from Universal Time (UTC or GMT) because the Earth's rotation is slowing irregularly, due to the gravitational influence of the Moon. We periodically add or subtract "leap seconds" to/from our civil time scale, to keep it in sync with where the Earth is actually pointing. The accumulated difference between UTC and TDT is called Delta T, and its current value is about 67 seconds. Delta T affects the local time when an astronomical event is observed on Earth.

    If you turn on the Dynamic Time option, SkySafari adds Delta T to the civil time obtained from your Android device before computing the positions of solar system objects. If your leave Dynamic Time off, SkySafari will assume that there is no difference between UTC and TDT. This is technically incorrect, but it may be useful to compare SkySafari's results against another reference (such as the Astronomical Almanac) which tabulates an ephemeris of planetary positions against Dynamic Time instead of Universal Time.

    Refraction: a distortion in an object's apparent altitude caused by the Earth's atmosphere, which bends light as it passes through. Refraction can amount to over 1/2 degree near the horizon, but decreases rapidly as an object's altitude increases. Refraction only affects an object's apparent altitude, not its azimuth, right ascension, or declination.