A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2016 Jul 29, 00:39 -0400
You asked if I had any comment about the 15' adjustment. I thought Bill B did an admirable job of it, so to avoid the risk of sounding like an echo, I will try to explain it in a different way.
In every celestial navigation observation, we are attempting to determine the angular altitude of a the body observed. What you wish to obtain is the angle from the center of the earth to the center of the body observed, referenced to a plane formed orthogonal to your body. What you physically measure is somewhat different. We make adjustments to the observation to change the physical observation into a calculated observation, as if the observer was at the center of the earth.
One adjustment has already been dealt with. That was the adjustment for dip. You are using a bubble horizon, and thus there is no dip adjustment, as would be required when using a natural sea horizon.
Another adjustment is due to the apparent diameter of the Moon. You may take your observation using the Upper Limb (UL) or top edge of the Moon. Similarly, you may take your observation using the Lower Limb (LL) bottom edge of the Moon. In either case, there is an angular difference between the edge selected and the center of the Moon. The 15' adjustment must be applied in those cases, the sign dependent on the limb selected. Bill B applies a useful trick to avoid this adjustment. He centers the bubble on the moon. In that case, no adjustment for the semi diameter (radius) of the Moon is required.
The 15' figure of merit is just an approximation. The moon's orbit causes it to change distance and thus the apparent diameter varies. But 15' is good enough for now!
There is another adjustment which you haven't asked about, but likely will in short order. When you take your observation of the Moon, you are standing on the surface of the earth, not it's center. The moon is relatively close to us, and the angle observed will change as a function of the relationship between the earths center, your position on the earths surface, and the position of the Moon relative to the earth. This is a parallax adjustment. Take your time puzzling this one through.
In each adjustment there is a physical reason for it. Celestial Navigation isn't some magical numerical incantation that causes your position to squirt out at the end. Think for a moment, dip is caused primarily by the curvature of the earth and the height of your eye. Refraction is caused by the atmospheric bending of light. The moon's semi diameter must be adjusted for, because the moon has an apparent disk, not just a point of light like a star, and thus the parallax caused by the earths radius must be removed. And on and on.
Please try to think of WHY an adjustment is to be performed, in addition to simply how it is performed. You'll be better for it!