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)
- Will the "Jupiter" be always fully visible during daytime? Will there be phases?
- How bright will it be during nighttime? Will there even be a proper dark night outside of solar eclipses?
- 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?)