The Phase symbols are modifiers for Mystcraft Ages. They control the height of something in the sky, like vertical versions of the Direction symbols. There are four symbols in the set: Rising Phase, Zenith Phase, Setting Phase, Nadir Phase.
They are normally used to control where along a sun's or moon's path it will appear, by marking their nominal "starting position" at game time 0. ("Zenith" means a sun's noon position, "nadir" its midnight position.) This is mostly important when trying either to synchronize an Age's sun or moon with the Overworld's, or to place an unmoving (Zero Length cycle) sun or moon in the sky. For example: "Half length, Zenith Phase, Normal Sun" will complete its circuit twice in each Overworld day, but will be at the noon position whenever the Overworld is having sunrise or sunset.
Phase modifiers blend similarly to the Directions: "Rising Phase, Zenith Phase" will give a position halfway up the sky. As with directions, if a phrase begins with opposed phases such as "Rising Phase, Setting Phase", it is undocumented whether the result will be zenith or nadir.