Over on the SF & Fantasy Stack, I've asked a question about a curious watch that shows 15 intervals on its watch face instead of the usual 12 hours. That watch belongs to someone living on Ganymede, close to the end of the 21st century.

Why does Jet's old watch show 15 hours?

Does Ganymede have any day/night cycle, or any other cycle, that would explain this curious watch?

In analogy to earth, we should expect a 30-hour Ganymede-day. But Ganymede is in tidal lock, always showing its same side to Jupiter. Also, calculations show that the light reflected by Jupiter pales compared to that of the sun.

So I would assume that Ganymede's day/night cycle is sun based, possibly complicated by passing through Jupiter's shadow.

What do day and night on Ganymede look like?

  • $\begingroup$ Have you tried Stellarium, just out of curiosity? My gut feeling (which is often wrong) is that Ganymede's day/night cycle would be consistent with Jupiter's 10 (Earth) hour rotation period (plus a little bit more since Jupiter orbits the sun) $\endgroup$
    – user21
    Commented Jul 15, 2017 at 15:11
  • $\begingroup$ @barrycarter no, I haven't, since I'm an amateur; I have no idea what Stellarium even is. $\endgroup$
    – SQB
    Commented Jul 15, 2017 at 15:51
  • $\begingroup$ Free virtual planetarium: stellarium.org $\endgroup$
    – user21
    Commented Jul 15, 2017 at 16:26

2 Answers 2


Here's some pertinent facts:

Apparent magnitudes: Sun from Ganymede = -23.11 Jupiter from Ganymede = -16.02 (full) Difference = - 7.09 Brightness difference = 2.512 E7.09 = 686 times brighter

Earth comparison: Sun from Earth = -26.74 Moon from Earth = -12.74 (mean full) Full moon/ Jupiter from Ganymede difference = 20.5 times

Ganymede facts: Orbital period = 1 Ganymede day: 7.16 days Orbital inclinations: .2 deg to Jupiter equator 2.2 deg to ecliptic Orbital eccentricity: .0011 (circular) Orbital speed (ave.): 10.88 km/s

Jupiter facts: Mean radius: about 70000 km

Summary: If you are on the "far side" of Ganymede you won't see Jupiter (maybe a glow on the horizon from Ganymede's thin atmosphere). The sun will be 2.512 E3.63 = about 28 times dimmer than seen from Earth (which is still pretty darn bright). A day will last 7.16 Earth days.

The near side will be far more interesting. The sun will go through the same cycle at the same brightness as on the far side. However, Jupiter will "hover" in the sky (maybe a little wobble/ libration) and when full will be about 686 times dimmer than the sun in the sky. Jupiter will go through phases over the course of the 7.16 day cycle and be less bright of course as it approaches eclipse. The eclipse (by rough calculations) will last about three and a half hours. The planet will have an angular diameter of about 7.5 degrees (compare with 0.5 for sun/moon from Earth) and be about 20.5 times brighter than a full moon when Ganymede is between the sun and Jupiter.

A fun side note is that the sky won't be totally dark during eclipse. Light from the sun will reflect off aerosols and hazes in Jupiter's upper atmosphere making Jupiter "glow" in Ganymede's sky.

  • $\begingroup$ Another interesting note is that when Ganymede is being eclipsed by Jupiter, the faint light we see reflecting off Ganymede will be that from the light that goes through the hazes and aerosols in Jupiter's upper atmosphere. In other words, this will be a good way to hone our skills at studying light that has been transmitted thru atmospheres of exoplanets. $\endgroup$ Commented Jul 25, 2017 at 4:32
  • $\begingroup$ Been looking for this info and it’s super helpful to a sci if project I’m working on. Where did you get these number from? Hoping to check out the source. Thanks! $\endgroup$
    – Elhammo
    Commented Sep 15, 2022 at 16:38

This answer to a question about the amount of light reflected by Jupiter on Europa, has screenshots from Stellarium, one of which shows the info I needed.

A screenshot from Stellarium, showing Ganymede. Ganymede appears as a brownish moon. Top left shows some info on Ganymede, the relevant part of which is transcribed below.


Type: moon
Sidereal period: 7.16 days
Sidereal day: 171h42m33.4s
Mean solar day: 171h59m36.0s
(Emphasis mine)

Also, this proposal for a calendar system for Jupiter's Galilean moons shows that there is no likely subdivision in 15 intervals (which was the cause of my question here).


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