Poles of Astronomical Bodies - Near, Far, Leading and Trailing Poles

Near, Far, Leading and Trailing Poles

In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io for example; this moon of Jupiter rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith, exactly overhead – this is the near pole, also called the sub- or pro-Jovian point. At the antipode of this point is the far pole, where Jupiter lies at the nadir; it is also called the anti-Jovian point. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) – this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. These poles are mean poles because the points are not, strictly speaking, unmoving: there is continuous libration about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.

These poles also apply to planets that are rotating synchronously with their primary stars, as is likely the case with many hot Jupiters and as was once thought to be the case with Mercury. Other synchronously-rotating objects, such as Pluto and some asteroids with large asteroid moons, can also be described as having "near" and "far" poles – though "leading" and "trailing" may not be as significant in these cases.