This apparent variation in angular size of the moon must be taken into account by navigators when making observations. When measuring angular distance to the horizon with a sextant, it is impossible to determine precisely where the center of the orb is, so navigators measure either the upper or lower limb's horizon distance and either subtract or add the lunar semidiameter to correct for the moon's apparent size. This varies about 2 minutes of arc, which is two nautical miles when transferred to the earth's surface. Since a sextant can be read to an accuracy of about a half minute of arc under ideal conditions, this is not a trivial correction.

In addition, since the moon is relatively close to the earth (about 60 earth radii), where it is, relative to the distant background stars ( its position on the celestial sphere), must also be corrected for parallax.

Both of these values, semidiameter and parallax, are printed for every hour of the year in the Nautical Amanac so these corrections can be applied to the observation. In addition, corrections must be made for the distance to the horizon (which varies with the altitude of the sextant above sea level) and with atmospheric refraction (which varies inversely with angular elevation above the horizon, and to a lesser extent, with atmospheric pressure and temperature).

Aren't you glad we have GPS now?