NavList:

George H, you wrote:
"I don't understand. Except at very low Sun altitudes, the Moon correction
for (parallax and refraction) is almost always far bigger that that for the
Sun, which is mostly just refraction.
So I would change Frank's suggested order to Sun-Moon-lunars-Moon-Sun."

No. That would actually be worse --not a lot, but it's definitely better to put the Moon altitudes at the ends of the sequence. You don't have to measure them or interpolate them as carefully.

Why?? Well, remember that "lunars miracle" that I've discussed with you several times in the past few years?? The altitude of the Moon matters LESS than the altitude of the other body, and when the lunar distance is close to 90 degrees, the altitude of the Moon can be wrong even by several degrees with no significant effect on the lunars clearing process. You're still thinking of the altitude as an index to get into the altitude correction table. That's only part of it. The altitudes also affect the geometric factors determining what portion of the altitude corrections act along the lunar arc. Those factors or "corner cosines" change when the altitudes change. By a lucky accident, the change in the corner cosine cancels out the change in the Moon's altitude correction as the distance approaches 90 degrees. Also notice that if you move the Sun up or down, you can significantly change the geometry at the other side of the standard lunar triangle. So even though the Sun's altitude correction may be very small, it will still impact the lunar clearing process. In other words, even if there were no refraction AT ALL, you would still need to get the Sun's altitude correct to better than 6' (actually 6'*sin(LD)/cos(h)) if you want to clear a lunar correctly. Try it yourself with a few simple cases.

-FER

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