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Moon Occultation

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Moon Occultation

Individual observations of the moon follow the same principles as described for the sun. The lunar disk is acquired by the ASM and ESM and tracked as the moon rises through the limb TCFoV. The moon acts both as a target for scientific and for calibration & monitoring measurements, as in case for the sun. The corresponding time interval is named Moon Occultation & Calibration (MO&C) window. Predicting lunar occultation measurements requires analyses of the viewing conditions as a function of the monthly lunar motion. For a full orbit the moon moves, inclined by 5.1° to the ecliptic, in about 29.5 days around the Earth. Whenever the lunar orbit crosses the limb TCFoV of SCIAMACHY, moonrise can be observed. With a total azimuth size of 2 ´ 44° for the limb TCFoV, the mean monthly time interval when lunar measurements can be executed amounts to 7.2 days, displaying a seasonal variation of between 5.5-8 days. Due to the lunar orbital motion, the first moonrise in a monthly period occurs on the left side of the limb TCFoV (azimuth = -44°). Each orbit moonrise progresses with an azimuth rate of about 1°/orbit to the right side of the limb TCFoV. At the end of the monthly visibility, the moon has reached the right edge of the TCFoV (azimuth = +44°). The lunar phase changes continuously within each monthly period. At the beginning of the visibility, the phase amounts to about 0.6-0.7.

Full moon can be observed close to the end of the monthly cycle because the 10 a.m. descending node crossing time criterion of ENVISAT’s sun-synchronous orbit only allows full illumination of the lunar disk when the moon lies at an azimuth of about 30° (fig. 2-13). Moonrise at lunar phases > 0.5 occurs over a large range of geographic latitudes in the southern hemisphere. Different to sunrise, where the geographic latitude of the sub-satellite point (and thus also the geolocation of the tangent point at which the atmosphere is probed) changes slowly over a year, the latitude of the sub-satellite point at moonrise varies significantly within a monthly period. Since the brightness of the lunar disk can be insufficient to exceed that of an illuminated cloudy atmosphere, moon occultation measurements are only possible when the moon rises on the night side beyond the terminator. In addition refraction has to be taken into account. Its impact is similar as in the case of sunrise due to the same apparent size of 31.5 arcmin for both solar and lunar disk (fig. 2-14).

 

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fig. 2-13:

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SCIAMACHY's monthly lunar visibility occurs between 1 and 2 over the southern hemisphere (lunar phase > 0.5). The hatched area illustrates the limb TCFoV of 88?. Visibility at smaller lunar phases over the northern hemisphere between 3 and 4 is not used because it coincides with solar occultation. (graphics: DLR-IMF)
 
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fig. 2-14:

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The rising moon seen from a spacecraft in a low-Earth orbit. Differential refraction distorts the lunar disk. (photo: NASA)
 

 

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