A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 May 12, 18:33 -0700
What does the HP of the Moon mean? You can draw a diagram and define it, but usually navigators treat it as "just one of those things" --a number you look up that's related to the Moon's changing distance from the Earth, without any simple interpretation. Some of you (not all) may enjoy this "simple" interpretation: the Moon's HP is the Earth's SD as seen from the Moon. Thinking about the view from the Moon would have been considered far too hypothetical and literally outlandish even fifty years ago, which is probably why this simple interpretation never made it into the standard books, but now we're quite comfortable imagining how someone standing on the Moon would see the Earth. If the Moon's HP in the almanac is 58.2' then that means an observer at the distance of the Moon's center would see the Earth with a semi-diameter of 58.2' (or a full diameter just under two degrees). And as the Moon moves closer to the Earth, that number increases... farther away, it decreases.
Note: there is a very small geometric difference between the Earth's SD and the Moon's HP, but it's trivial (about half a second of arc if I remember correctly) and it doesn't change the fact that you can easily visualize the meaning of the HP by thinking about the view from the Moon. In the attached diagram, the numbers are reasonable, but I have doubled the range of size change in the Earth images since otherwise it's not at all obvious that anything is changing.