March 4, 2004

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Ptolemaeus Peak Heights


Image Credit: NASA Chart LM 77

Ptolemaeus Peak Heights

Lunar observers, long impressed by lengthy shadows of peaks, have often speculated on the their heights. Galileo used shadow lengths to estimate peaks heights, and Schroter, Madler and especially Schmidt determined a few thousand peak heights and crater depths this way. Shadow measurements at the telescope require great care, and are susceptible to systematic error. As part of my job in the early 1970s of directing the daily work of cataloging the Moon with Lunar Orbiter photos I also measured many shadows. Similar techniques (but mechanized) of measuring feature positions on Apollo 16 Metric stereo photographs were used by the US Defense Mapping Agency to produce maps with 300 m contours. This excerpt from map LM 77 - Ptolemaeus (1:1m; 1978) shows the same area as in yesterday's LPOD. A larger, unmarked version can be seen by clicking on this image. Now, with these contours and spot heights we can determine more accurately than ever before the heights of peaks rimming Ptolemaeus. Accepting 6900 m (relative to the average radius of the Moon) as the average level of the relatively flat floor of Ptolemaeus I have determined the following peak heights above that floor:



1810 m
910 m
1327 m
2945 m
1830 m
1635 m
1220 m
2370 m
1220 m
2740 m
3267 m
2743 m
1500 m
2710 m
900 m

Comparing peak heights I determined using the LM 77 chart with values reported by Neison in 1876 and Schmidt in 1878 shows a systematic under-estimation of Neison's values. It is perplexing that his values are as much as 50% less than the modern values; Schmidt's two heights are 500 m too high and 500 m too low. However, in Wilkins & Moore's 1955 The Moon, the only height given is eta at 9000 ft which equals nearly exactly the modern value! Someone needs to write a program that will allow amateurs to routinely measure heights from their own ccd images.

Related Links:
Ptolemy - the Man
Height of Lunar Mountains
Identification of suspect features in Ptolemaeus crater (domes?)

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