Major Richard C. Henry's paper (Journal, 16, 449) is timely. These comments are restricted to mid-course navigation and are intended to expand and clarify some parts of this excellent paper.

Major Henry divides space navigation into two parts, orbit and trajectory navigation: the first about one body, and the second, about two bodies. For example, Echo Satellite is in orbit about the Earth, while if its path were to include an orbit about the Moon as well as about the Earth, its path would be called trajectory. If we consider the path of a lunar capsule as determined by lunar perturbations of a body in orbit about the Earth, astronomers should be able to compute its trajectory, or path, or ephemeris, or orbit, and supply the information to the space navigator in the form of a Space Almanac. The guild of space navigators would use this shelf item, along with an accurate timepiece, and a marine type sextant, as the basic equipment for accomplishing mid-course navigation.