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Page 94 in Harold Hill's A Portfolio of Lunar Drawings: The Lichtenberg crater - a relatively small isolated ring 12 miles in diameter - is a celebrated object because it was here between the years 1830 and 1840 that Madler observed a strong reddish tint closely east of the crater. No one appears to have seen this subsequently until its recovery in 1940 by Barcroft in the USA when it was described as 'a pronounced reddish-brown'. Haas, at a later date, observed the same effect and in 1951 Baum in the UK reported a rose-pink coloration which persisted for a time and then faded. Between 01:15 and 03:20 on April 1 1988 (no significance in the date!) the author saw, for the first time in his experience, rosy-tinged areas fringing the northern edge of the lava sheet. The moon had hitherto always looked quite neutral in colour to his eyes but the effect on April 1 was unmistakeable and certainly not due to atmospheric dispersion or other false effects, because a most careful check was made with bright objects, such as crater Lavoisier A. The telescope employed was a 10" reflector using various eyepieces as a safeguard, but the colour persisted until weather conditions deteriorated. The area has been described as 'red sensitive' but, if so, it is curious that the coloration is rarely in evidence, even when specially looked for under the most suitable conditions. There the matter must rest....at least for now. Note: Mr. Hill's observation of April 1 1988 was performed during Waxing Gibbous Moon (Full Moon was on April 2, 9:21 U.T.). The hypothesis and query of D. Caes: Is it perhaps an optical phenomenon created by large amounts of microscopic transparent glass spherules in (or on) the moon's regolith, showing the most distinct color of the spectrum (red) at an angle of 22°30' from the anti-solar point (180° from the sun). In other words, Mr. Hill and others observed a very tiny part of the Primary Glass-Spherule Bow (R: 22°30'). If they had a linear polarizer at hand, they could have "turned" the red coloration "on" or "off" by rotating the filter (that is: a handheld filter between the eye and the eyepiece). Note: the rainbow and the glass-spherule bow are very strong polarized phenomena. Could it be that the less known phenomenon Lunar Dust Levitation (LDL), aka Lunar Dust Migration (LDM) is also involved? Thus: every now and then there are concentrations of levitated and migrated regolith particles (and very tiny transparent glass spherules) at certain locations on the moon, for example near Lichtenberg, or at Gassendi (the legendary observation by Patrick Moore and other British moon observers). The Primary Glass-Spherule Bow is noticeable on dark grey roads when white markings are painted. These markings contain retroreflective transparent glass spherules. When these glass spherules are spilled all over the sunlit dark grey road, it is possible to observe the complete circle of the Primary Glass-Spherule Bow, with the shadow of your head at the very centre of the circle. A distinct sort of retro-reflective white projection screen for home movies or slides has a surface full of microscopic transparent glass spherules. If you place this screen in the sun, you will also see the Primary Glass-Spherule Bow. To make it much more visible, be sure to place the screen "offset" as seen from the sun's location, to minimize the strong blinding retro-reflection! Only the spectral colored arc will be visible, not the strong Heiligenschein effect. The Secondary Glass-Spherule Bow shows up at an angle of almost 90° from the sun! (a very weak and broad spectral colored appearance). WARNING: If you want to observe the moon through a handheld rotating linear polarizer (between your eye and the telescope's eyepiece), beware of the fact that you will also see the yellow-lilac colored Haidinger's Brush, which is an entoptic phenomenon related to polarization. For those who want to observe the First Quarter Moon or Last Quarter Moon in broad daylight, try to look through a rotating linear polarizer! The blue sky's most powerful polarization occurs at 90° from the sun, which is the region of the sky where you could find the First Quarter Moon and/or the Last Quarter Moon. |
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| Are there moon connoisseurs in Astrobin who made color-enhanced photographs of the Waxing Gibbous Moon at an angle of 22°30' from the anti-solar point? In case of slightly reddish colorations near, for example, Lichtenberg or Gassendi, these would show up as pure red spots or streaks! Color-enhanced photographs made through rotate-able linear polarizers might be used to detect red colored Transient Lunar Phenomena. |
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For the Primary and Secondary Glass-Spherule Bows, see Carl B. Boyer's The Rainbow, From Myth to Mathematics (Princeton University Press, 1987), page 244. Edme Mariotte (1620-1684) calculated and found 22°48' for the radius of the Primary Glass-Spherule Bow, and more than 80° for the radius of the Secondary Glass-Spherule Bow. |
