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Re: Lunars: altitude accuracy
From: Bruce Stark
Date: 2004 Dec 4, 12:48 EST
From: Bruce Stark
Date: 2004 Dec 4, 12:48 EST
For 28 years I've been underestimating the effect inaccurate altitudes can have on short lunar distances. This shows up in the advice in my Tables for Clearing, where I say:
"Fortunately, altitudes are not critical. They are wanted for their refraction and parallax corrections, and to determine the shape of the triangle formed by the two bodies and the observer's zenith. Still, try to get the altitudes within 1' or 2' of the truth, especially if the distance is short."
This would give the impression that, even with short distances, altitudes aren't critical.
To make matters worse, I used an extremely short distance as one of the examples. It was scarcely over three degrees, cleared.
Now I find that a 2' error in either of the altitudes of that 3 degree distance causes a 0.'5 error in the outcome: more than five times what I'd expected.
I dearly wish I'd actually checked the effects of altitude errors on short lunars a quarter century ago. But, rather than take time for it then, I put it on the "To Do" list and set it aside. There's no honest excuse for this failure.
Now the "Lunars: altitude accuracy" thread has brought the question up again, and the last couple of days I've been trying various distances and altitudes. Results tend to agree with the formulas Frank R. has posted on this thread.
The upshot is: Add a third caution to the two that are discussed under "Extremely short Distances" in the introductory part of my Tables.
Bruce
"Fortunately, altitudes are not critical. They are wanted for their refraction and parallax corrections, and to determine the shape of the triangle formed by the two bodies and the observer's zenith. Still, try to get the altitudes within 1' or 2' of the truth, especially if the distance is short."
This would give the impression that, even with short distances, altitudes aren't critical.
To make matters worse, I used an extremely short distance as one of the examples. It was scarcely over three degrees, cleared.
Now I find that a 2' error in either of the altitudes of that 3 degree distance causes a 0.'5 error in the outcome: more than five times what I'd expected.
I dearly wish I'd actually checked the effects of altitude errors on short lunars a quarter century ago. But, rather than take time for it then, I put it on the "To Do" list and set it aside. There's no honest excuse for this failure.
Now the "Lunars: altitude accuracy" thread has brought the question up again, and the last couple of days I've been trying various distances and altitudes. Results tend to agree with the formulas Frank R. has posted on this thread.
The upshot is: Add a third caution to the two that are discussed under "Extremely short Distances" in the introductory part of my Tables.
Bruce
