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Milichius π : Observations, measurements and classification. ( published on TLO, March 2005, p.4-5)

by Raffaello Lena, Rodrigo Viegas, Jim Phillips and Maria Teresa Bregante- GLR group

In a previous paper [1], some of us described the highland dome located near the crater T. Mayer-B. Moreover, we reported some images of the very well known Milichius region. A study about two lunar cones located near the crater Milichius [1] will be published in a next issue ofSelenology, the Journal of the American Lunar Society (ALS).

The dome field near crater Milichius has been thoroughly studied. The A.L.P.O. Lunar Dome list reports several low domes in this region. Another comprehensive study of lunar domes was written in 1964 by Brungart [2]. The author compiled a catalogue of 261 domes where he reported their coordinates, diameters, heights, slopes and morphological characteristics [3]. Here, under the entry #190, Brungart reported for Milichius π (Xi = - 0.510 Eta = + 0.175) a height of 742 m with an average slope of 9 º. In clear agreement with Brungart, the height of Milichius π (Xi = -0.510 Eta = +0.175) was computed as 720 meters in [4], where a drawing by Donald Watts was used.

Domes are one of the most difficult types of lunar objects to study quantitatively. Domes are such low and gently sloped features that it is very difficult to measure accurately their shadow lengths to derive topographic information. Recently, some of us described in [5] the dependence of an ideal hemispherical dome shadow length (R) on solar elevation and its diameter-height (D/H) ratio. A recent LPOD item by Wood [6] focused on our study concerning artificial domes.

For Milichius π we found [5] the best fit for D/H = 40, yielding an average dome slope of 2.9° and a height of about 200 m.


Fig.2

On the best image (Fig.1), the dome diameter and the length of its shadow were both measured in pixels. The corresponding scale of the image was obtained (0.296 km per pixel), allowing diameters and shadow lengths to be expressed in kilometres. The image by Phillips, here proposed as fig.1 and 2, was also compared with a high-resolution version of the Lunar Orbiter frame IV-133-H2. The solar altitude (Alt) as seen from Milichius π (located at 31.20° W and 10.08° N) and the colongitude (C) were calculated using the Lunar Observer's Tool Kit software by Harry Jamieson. According to Ashbrook [7], the average slope of the dome flank is equal to the solar altitude when x = 0.25, where x is the fraction of the dome diameter that is covered by black shadow. The height (H) of the dome was then calculated by the formula (1):

H = r (tan s)

where r is the radius of the dome and (tan s) is the tangent of the average slope angle when the dome is ¼ covered by black shadow (x = 0.25). Our results are reported in Table 1.

Fig. 2

Table 1: measurements on the image taken by Jim Phillips on December 22nd 2004 at 02:37 UT using a TMB 8” F/9 Apochromatic refractor. Scale of the image is 0.296 km per pixel. The coordinates ofMilichius π are 31.20° W and 10.08° N (Xi = -0.510 Eta = +0.175).

This dome requires a specific solar altitude for it to be observed clearly. Moreover, we were able to distinguish between the black shadow and the dark grey shading of the dome flank which represents grazing illumination by sunlight. From Table 1 and [7] it follows that the maximum slope angle of the dome is 2.72°. The height of the dome was then estimated using the formula (1). It turns out that the summit of the dome is 211 metres higher than the surrounding plain. This estimated height is comparable with the preceding value determined (200 m) using an artificial model, as described in [5].

These measurements provide some insight into the possible explanation for the difference with the heights reported in [3-4]. Likely, the variation is due to an incorrect evaluation of the black shadow on the dome flank.

The results obtained suggest strongly that previous estimation of Milichius π height were wrong.

Future observing schedules of the GLR group are being planned to investigate different lunar domes on a case by case basis. It is hoped that by eliminating many of the less reliable reports in the A.L.P.O. catalogue, we will be left with a core set of observations upon which more reliable statistical analysis can be performed. The activity of the GLR group is at www.glrgroup.org .

References:

[1] R. Viegas, R. Lena and J. Phillips, TLO, February 2005, pp.4-5

[2] C.Wood,Brungart Catalog of lunar domeshttp://cwm.lpod.org/DataStuff/Brungart-Domes/BrungartDomes.html

[3] D.L. Brungart, Airforce Institute of Technology Wright Patterson Air force Base, Ohio, 15 July 1964.

[4] H. Jamieson,JALPO, vol 37, 1 (1993).

[5] R. Lena, C. Fattinnanzi, F. Lottero, Lunar Domes and Artificial Domes: Two Tools for Lunar Observers, Selenology,vol 23 –2, 2004.

[6] C. Wood, LPOD-http://www.lpod.org/LPOD-2004-10-25.htm

[7] J. Ashbrook, JALPO, vol 15, 1-2 (1961)

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