Celestial Observations of the Moon
The moon is easy to identify and is often visible during the day. Yet, the navigator taking the sight must follow the moon across the horizon. It appears to move very fast, so the navigator must be quick in obtaining the sextant reading. The moon's proximity to the earth requires only some additional corrections (Parallax) to (Ha) to obtain (Ho). The rest of the sight reduction is the same process as an observation of the sun.
Parallax (P)
Parallax is the difference in the direction of an object at a finite distance when viewed simultaneously from two different positions. It enters into the sextant altitude corrections because (Hs) is measured from the earth's surface, but Ho is calculated from the earth's center. Since the moon is the celestial body nearest the earth, parallax has its greatest effect on lunar observations. If the moon is directly overhead, such as with an altitude of 90°, there is no parallax, as its direction is the same at the center of the earth as for the observer. As the moon decreases in altitude its direction from the observer begins to differ with its direction from the earth's center, and the difference in direction increases continuously until the moon sets. The same effect occurs in reverse when a body is rising. Parallax ranges from zero for a body with an altitude of 90° to a maximum when the body is on the horizon, with 0° altitude. At altitude 0°, it is called horizontal parallax (HP).
In addition to increasing as altitude decreases, parallax increases as distance to a celestial body decreases. Venus and Mars, when close to the earth, are also affected by parallax. The sun is slightly affected, the parallax correction for the sun being +0.1' from zero altitude to 65°. All other celestial bodies are too far from the earth to require correction for parallax when observed with the sextant. The correction for parallax is always positive. and is applied only to observations of the moon, sun, Venus, and Mars. When it is applied to (Hs), the sextant altitude is corrected to the value it would have if the observer were at the center of the earth. The value of horizontal parallax (HP) is found on the daily pages Nautical Almanac under Moon for every hour of GMT.
As with other observations the sextant corrections for dip and altitude are in the front of the Nautical Almanac. But for the Moon, the altitude and dip tables are in the back of the Nautical Almanac. Which means you cannot correct your (Hs) to (Ho) until you get the HP from the daily pages.This procedure goes against the format that I use, but you have to be flexible as you will see in the next blog titled "How to Compute the Intercept and Azimuth of the Moon".
