galisats

If you are looking at Jupiter through binoculars or a telescope and don’t know which moon is which, then use this package.

galisats is used to determine the positions of the four greatest satellites of Jupiter (called Galilean satellites). Positions are shown on the plot for any given time (UTC – Coordinated Universal Time) with respect to the planet, as seen from the Earth.

The galsat() function calculates numerical values of the satellites’ positions:

x – the apparent rectangular coordinate of the satellite with respect to the center of Jupiter’s disk in the equatorial plane in the units of Jupiter’s equatorial radius; X is positive toward the west

y – the apparent rectangular coordinate of the satellite with respect to the center of Jupiter’s disk from the equatorial plane in the units of Jupiter’s equatorial radius; Y is positive toward the north

The function is based on algorithms in the book:

Astronomical Formulae for Calculators (4th edition), Jean Meeus, Willmann-Bell Inc., 1988

The delta_t() function returns the value of delta-T in seconds unit. It’s useful for converting the Coordinated Universal Time (UTC) to the Ephemeris Time (ET). The conversion is handled as: ET = UTC + deltaT

Installation

You can install the development version of galisats from [GitHub] (https://github.com/) with:

# install.packages("devtools")
devtools::install_github("LechJaszowski/galilean_satellites")

Example

There are examples of using galsat() and delta_t() functions:

library(galisats)
galsat(2025, 10, 13, 21, 40)

#>       moon          x          y
#> 1       Io   4.089507  0.1125276
#> 2   Europa  -8.417919  0.1080519
#> 3 Ganymede   6.644450 -0.3518624
#> 4 Callisto -24.735531  0.2536266
delta_t(1999, 10)
#> [1] 63.78768
delta_t(c(-200, 1610, 2030), c(1, 10, 12))
#> [1] 12791.65348   107.80766    78.25045