Imagine a tidally locked gas giant moon, which is otherwise earth-like. One side of the moon will have a huge planet constantly hovering in the sky (24 hour day/night cycle with a Jupiter-sized primary works out to about 30° angular diameter)


  1. Will the "Jupiter" be always fully visible during daytime? Will there be phases?
  2. How bright will it be during nighttime? Will there even be a proper dark night outside of solar eclipses?
  3. Does it depend on the exact location of the observer on the planet-facing side? (I.e. is it significantly brighter when the gas giant is directly overhead compared to low above horizon?)
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    $\begingroup$ Note: I'm not sure if you intend it to be inhabited by people, but keep in mind, it would likely have intense radiation from the gas giant, just like Jupiter's moons experience. $\endgroup$
    – Mikey
    Mar 8, 2015 at 0:17

3 Answers 3


I decide to simulate the situation in question. After trying Celestia and Kerbal Space Program with mixed results I stumbled upon Space Engine which turned to be an awesome tool for this. Below are pictures from a roughly earth-like moon in orbit around a gas giant (which I'll call Jupp)

Morning. Jupp is faintly visible

A crescent appears as the visible side of Jupp becomes lit by the Sun.

During the day, Jupp waxes

Late evening. Jupp is almost full which signifies the forthcoming night.

Early night. The full Jupp provides significant amount of illumination. I'd guess reading by jupplight is possible.

As night progresses, Jupp wanes and it becomes significantly darker.

The darkest hour is just before the dawn - literally!

The Sun rises...

...only to hide behind Jupp in a spectacular eclipse! This occurs every day and provides a couple hours of true darkness.

The Sun reappears and the cycle begins anew.

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    $\begingroup$ That looks awesome :D $\endgroup$
    – Erik
    Mar 11, 2015 at 8:04
  • $\begingroup$ Wow! would you share how you used Space Engine to simulate this? A blog post maybe? $\endgroup$
    – Rmano
    Mar 11, 2015 at 9:27
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    $\begingroup$ @Rmano it has an infinite procedurally generated universe, so I simply found a suitable moon with earth-like properties (atmospheric density, temperature etc.) There are all kinds of stars, planets and moons which is awesome for world building - you can fly around, take pictures from surface and the like $\endgroup$
    – Korvas
    Mar 11, 2015 at 10:37

What's daylight/nighttime? Some large fraction of the time, the moon will be on the dark side of the planet. Most of the rest of the time, one side will face towards the planet but away from the sun. There will be a relatively brief period where the planet and the sun are both in the sky. Absent cloud cover, the planet will almost certainly be visible even in sunlight.

There will be phases, as the time when both the planet and sun are visible is when the moon is to the side of the planet. Note that the phases will change based on where on the moon someone is. Looking from the point of the moon that is closest to the planet, the planet will never be full while the sun is in the sky. It will be about half full. From a point where the moon is farthest from the planet, the planet will never be visible and the sun will produce a relatively normal day/night cycle. From a point mid-way between, the planet will be visible low on the horizon and have full phases. There will be a normal day/night cycle from the sun (albeit with short days, as part of the day will be blocked by the planet).

At that distance, the sun will be less bright than it is on the Earth. Note that when the moon is on the far side of the planet, that side of the planet doesn't get light from the sun. So there wouldn't be light coming from the planet to the moon. So the moon will have a real nighttime then. Neither the sun nor the planet will be visible.

This is screaming out for a good image. Hopefully someone will produce one or more.


You've asked a few questions here.

As for the brightness of Jupiter, Jupiter has an albedo of around 0.5, compared to 0.12 for the moon. This means that Jupiter will reflect roughly 4x the light that the moon does. However, Jupiter is significantly larger than the moon. You've indicated a 30 degree angular diameter for your Jupiter, this being 60x the diameter of the moon, will result in 3600x the angular area of the moon. Combined this means the Jupiter will shine roughly 14,400 times as brightly as the moon. Given that the moon shines at 0.05 - 0.3 lux, a "full" jupiter will shine with 720-4320 lux. When half of Jupiter is illuminated, it will shine with half this lux. When a quarter is illuminated, it will shine with a quarter this lux etc... This is very bright.

You've asked how bright Jupiter will be during the night. The brightness of the Jupiter will be the same everywhere on the planet at once, but the time of day everywhere on the planet will be different. Because the Earth is now tidally locked to Jupiter, there will be a certain region where Jupiter is constantly straight up at all times (in reality, the Jupiter will wobble a little bit depending on how circular and angled the Earth is orbiting, but it won't be much). This point will have a fully illuminated Jupiter during its midnight, and a halfway illuminated jupiter at dawn/dusk. The darkest it will ever get in this region will be when the Sun is being eclipsed, which will be often.

Depending on where Jupiter appears to be in the sky, Jupiter's phase will be offset from the day/night cycle by some constant amount depending on the longitude. The lower Jupiter appears in the sky, the less sunlight will be present during Jupiter's "dark" phase, giving a true "night". Also worth noting, will be that half of the Earth will never see this Jupiter, and thus never be illuminated. This part of the Earth will be akin to the Dark Side of the moon.


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