Computed for the Galilean Satellites · Epoch J2000.0

Phases of the
Medicean Stars

Io, Europa, Ganymede & Callisto, as they wax and wane in the sky of Jupiter — with the eclipses, shadows and transits that follow.

—
—
Rate
The Platesseen from the cloud tops, north uppermost
Apparent Sizedrawn to one scale
Every one of the four outreaches the Sun’s disk at Jupiter’s distance, so each in its turn throws a total eclipse across the cloud tops.
Geometrywhy the phase is what it is
Plate V — the system from above Jupiter’s north pole. Sunward is new; the shadow is full, then eclipse. —
Plate VI — the same instant through a terrestrial telescope, as Galileo drew it in January 1610. —
Ephemerisphenomena, next seven days
Universal time Satellite Code Phenomenon Interval
Notes

The phase cycle

A moon is full when it stands on Jupiter’s night side, opposite the Sun, and new when it crosses between Jupiter and the Sun. The Sun scarcely shifts against Jupiter’s sky in a few days, so each phase cycle is simply the orbit, reckoned against the Sun: 1.7699d for Io, 3.5541d Europa, 7.1664d Ganymede, 16.7536d Callisto.

Every full moon is eclipsed

Jupiter’s axis leans only 3.1°, so the shadow never strays far from the orbits. Io, Europa and Ganymede fall into it at every full phase without exception. Callisto alone, at 26 Jupiter radii, can ride clear — it passes years untouched, then keeps a season of eclipses near Jupiter’s equinox, once in about six years.

Jupiter-shine

The unlit face is not black. Jupiter fills 19.5° of Io’s sky, some 38 times the width of our own full Moon, and floods the night side with reflected light. That glow is brightest at new phase — when Jupiter, seen from the moon, is itself full — and it is drawn here.

Authorities & accuracy

Satellite positions follow Meeus, Astronomical Algorithms, ch. 44 (low precision), reproducing his worked example to within 0.04 Jupiter radii and good to about 0.1 throughout. Contacts are refined by bisection against the true umbral cone and the oblate disk. Codes follow the Astronomical Almanac: Ec eclipse, Sh shadow, D disappearance, R reappearance, I ingress, E egress. Good for planning, not for occultation timing.