Astrometry of mutual approximations between natural satellites. Application to the Galilean moons

TL;DR

This paper introduces a new astrometric method called mutual approximations for accurately measuring the positions of natural satellites, especially useful when traditional methods are limited, demonstrated on the Galilean moons with high precision results.

Contribution

The paper presents a novel method for satellite astrometry that works during any close approach, not just mutual phenomena, achieving high precision with small telescopes.

Findings

Achieved an average precision of 3.42 mas in central instant measurements.

Method shows less than 10 mas difference compared to mutual phenomena results.

Applicable to small telescopes and non-equinox observations.

Abstract

Typically we can deliver astrometric positions of natural satellites with errors in the 50-150 mas range. Apparent distances from mutual phenomena, have much smaller errors, less than 10 mas. However, this method can only be applied during the equinox of the planets. We developed a method that can provide accurate astrometric data for natural satellites -- the mutual approximations. The method can be applied when any two satellites pass close by each other in the apparent sky plane. The fundamental parameter is the central instant $t_0$ of the passage when the distances reach a minimum. We applied the method for the Galilean moons. All observations were made with a 0.6 m telescope with a narrow-band filter centred at 889 nm with width of 15 nm which attenuated Jupiter's scattered light. We obtained central instants for 14 mutual approximations observed in 2014-2015. We determined $t_0$ with an average precision of 3.42 mas (10.43 km). For comparison, we also applied the method for 5 occultations in the 2009 mutual phenomena campaign and for 22 occultations in the 2014-2015 campaign. The comparisons of $t_0$ determined by our method with the results from mutual phenomena show an agreement by less than 1-sigma error in $t_0$, typically less than 10 mas. This new method is particularly suitable for observations by small telescopes.