Discuss with respect to celestial navigation Diurnal parallax of moon

Of course. Let’s have a detailed discussion about the diurnal parallax of the Moon, specifically in the context of celestial navigation. This is a critical topic because understanding and correctly applying the parallax correction is the difference between a successful lunar sight and a completely useless one.

1. What is Diurnal Parallax?

In simple terms, parallax is the apparent shift in the position of an object when viewed from two different lines of sight.

Diurnal Parallax (from diurnus, Latin for “daily”) is the specific parallax caused by an observer being on the surface of the Earth, rather than at its geometric center. As the Earth rotates on its axis throughout the day, the observer’s position changes relative to the Earth’s center, causing a celestial body to appear in a slightly different place against the background of distant stars.

All celestial bodies in our solar system (Sun, Moon, planets) exhibit diurnal parallax, but for most, the effect is tiny. For stars, it is completely negligible.

The Key Idea: The Nautical Almanac and other ephemerides provide the positions of celestial bodies (their Geographical Position, or GP) as if they were observed from the center of the Earth. However, a navigator uses a sextant on the surface of the Earth. Diurnal parallax is the correction that accounts for this difference in observation points.