Notes on Neptunian Satellite Offsets and Elongation Times


Three tables of data are presented for Triton. Also, a table of data for Nereid is presented. These tables can be used by an observer to compute the position of Triton and Nereid 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 Triton 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 Neptune to the center of Triton 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 Neptune. The last quantity, a/Δ, expressed in arcseconds, is the ratio of Triton's semimajor axis length at unit distance (arcseconds/AU) divided by the distance between Earth and Neptune at the time of interest (in AU).

The second table (also found using the Satellite Offsets link) gives the position of Triton 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 Triton's semimajor axis at that time in the chosen orbit, expressed in arcseconds, divided by a/Δ. The position angle, p', is that of Triton at that moment in time in the chosen orbit.

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 Triton with respect to the planet at the beginning of each day; the second table gives the positional information of Triton within one orbital period; the third table gives the time of each new orbital period.

Nereid's offset data are presented differently. Its orbital period is just under one year, therefore it moves much more slowly relative to Neptune. For Nereid, positional offsets are given as differences from Neptune's position in right ascension (Da cos d) and declination (Dd), both in arcseconds, for several dates throughout the year.

The Astronomical Almanac Online! 2018
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