Notes on Saturnian Satellite Offsets and Elongation Times


Three tables of data are presented for the eight closest satellites of Saturn. Also, a table of data for Phoebe is presented. These tables can be used by an observer to compute the position of these nine satellites at any point in their orbits throughout the year.

The first table (found using the Satellite Offsets link) gives the angular separation, position angle, and a/Δ values for the eight closest satellites at 0.0 hours UT for each day of a particular year. The time is expressed as a Julian date. The angular separation is given in arcseconds and is the distance from the center of Saturn to the center of each satellite as viewed from the geocenter. The position angle, given in degrees, is measured to the observer's East from an axis that points towards the Celestial North Pole with origin at the center of the apparent disk of Saturn. The last quantity, a/Δ, expressed in arcseconds, is the ratio of each satellite's semimajor axis length at unit distance (arcseconds/AU) divided by the distance between Earth and Saturn at the time of interest (in AU).

The second table (also found using the Satellite Offsets link) gives the position of the eight closest satellites at equally spaced times over the course of one orbit. That orbit is the one that starts at the first Eastern Elongation after opposition. At each interval within one orbital period, a ratio F and a position angle p' are computed. F is a scaling factor for the specific orbit chosen. It is the apparent size of each satellite's semimajor axis at that time in the chosen orbit, expressed in arcseconds, divided by a/Δ. The position angle, p', is that of a given satellite at that moment in time in the chosen orbit. Due to the observed orientation of the Saturnian system, rapid changes in position angle can occur as the satellites cross the polar axis of Saturn.

The third table (found under the Satellite Elongation Times link) is the table of elongation times; it gives the time of each eastern elongation throughout the year.

To summarize, the first table serves to give positional information for the eight closest Saturnian satellites with respect to the planet at the beginning of each day; the second table gives the positional information of those satellites within one orbital period; the third table gives the time of each new orbital period.

Phoebe's offset data are presented differently. Its orbital period is just under two years, therefore it moves much more slowly relative to Saturn. For Phoebe, positional offsets are given as differences from Saturn's position in right ascension (minutes and seconds) and declination (Dd), in arcminutes, for several dates throughout the year.

The Astronomical Almanac Online! 2018
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