Navigating Strange Worlds

[David Johnston2]

Originally Posted by lwcamp

If the orbital plane of the planet-moon system is in (or close to) the plane of the ecliptic, then once every day-month (which will be the same thing on this world) you will go into full eclipse. The planet will be entirely dark, and the moon will be in the planet' shadow with the sun behind the planet. You could still see where the planet is, because you'll have a spot in the sky without stars. The eclipse will last on the order of an hour or so (with considerable variation depending on orbital radius and planet size), after which the moon will go out of the planet's shadow and you will be able to see at least a crescent of light on the limb of the planet.

If the orbital plane of the planet-moon system is not particularly closely aligned with the plane of the ecliptic, eclipses will be much rarer. As long as you are not in eclipse, you will be able to see some sun reflecting off the planet somewhere from the moon, although it might only be a thin sliver.

Luke

As I understand it new moons don't happen because of eclipses. They just happen because it's night on the visible side of the moon. If the primary is optically large, then it wouldn't be a very precise position indicator. If it's small then it would be difficult to pick it out in the sky when it isn't illuminated.