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    Re: An exotic lunar distance puzzle
    From: Frank Reed
    Date: 2011 May 5, 17:24 -0700

    Dave, you wrote:
    "So we might expect a 3.4m refraction correction to the 18.3d geometric dip."

    I think that's about right --I came up with 3.7' from a rather different line of reasoning (close enough). We should probably also consider a couple of cases. What's the geometric dip for a cloud deck that's 7 miles high? What the same at 9 miles high? And what's the refraction correction for both cases? That will give you some reasonable error bars on the altitudes you take from the image (in addition to the limitations in the measurement accuracy due to the blurriness of the photo and the difficulty of picking off an exact pixel for a measurement).

    By the way, I thought your initial lat/lon position estimate was quite plausible, though it will probably move a bit after some refinement. Don't jump too quickly to use the ground track from the ISS orbital data.

    Regarding the Moon's parallax correction, yes, it's about a degree. We can be more exact. Find the Moon's HP for that date. Scale it up by a factor of (1+h/Re) where h is 220 miles and Re is the radius of the Earth. Then multiply by cos(-15 degrees) to correct for the Moon's actual altitude. If HP is 57', I get a correction of 58.1'. Note that you don't necessarily need this parallax correction depending on how you choose to analyze the problem. Most (but not all!) astronomy/planetarium software automatically corrects for parallax based on the observer's position because the software directly calculates the objects' positions as seen from a particular point in 3d space. You can test this by locating the Moon relative to nearby stars at some specific instant of UT and then moving the observer's location from one side of the Earth to the other (relative to the direction towards the Moon). It should shift about two degrees. Stellarium does this correctly. Note that the finder charts at the satellite tracking site heavens-above.com do NOT correct properly for this (or at least they didn't recently).

    -FER


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