What was the total lunar eclipse of 2008 February 21 like?


Unique Light Pattern. The exact observed illumination pattern displayed by the Moon during totality is unique for each eclipse and cannot be accurately predicted in detail. However, it can be better understood by recalling a simplified description of the event as it would be seen from the Moon, a very sensitive screen that can be used as remote detector of volcanic aerosols in the stratosphere.


Distorted Sun. As the shadow of Earth would approach, a hypothetical observer on the Moon, standing in the penumbra, would see the partially occulted solar disk gradually disappear behind the much larger disk of our planet. The Sun would then look more and more distorted due to refraction of sunlight grazing Earth`s atmosphere and would eventually become a very bright protuberance on top of a segment of arc as it gradually hid behind Earth`s disk.


Fuzzy Penumbra-Umbra Border. Our atmosphere would be acting like a powerful lens severely scattering, filtering and deviating sunlight from the penumbra into the shadow cone cast by our planet.  When that protuberance would merge with the segment of arc, the observer would be standing on the fuzzy penumbra-umbra border. That edge is neutral in color and corresponds to the maximum gradient of light or the line along which the level of sunlight would be falling off most sharply. It is also the frontier that astronomers try to pinpoint as they time mid-crater contacts.


Bluish Cap and Outer Umbra. A couple of minutes later, selective stratospheric absorption of red light by ozone molecules would be causing the residual sunlight to acquire the bluish color frequently noticed shortly after the onset of totality.  At that point, a small fraction of sunlight illuminating the currently aerosol-free upper layers of Earth`s atmosphere would be forming a bright bluish cap as seen from the Moon.


Segments of  Arc and Intermediate colors.  During the several minutes that would follow, the Sun would further hide behind Earth`s disk and its refracted image would become a series of annular segments that would continue to fade, becoming thinner with time and gradually acquiring tints of green, yellow and orange. During that brief period of time, observers could notice intermediate colors in changing combinations of patterns and hues.


Mostly Red and Orange Mid-Totality. For the several minutes centered at mid-totality, selective scattering of sunlight by air molecules and aerosols (that is stronger towards shorter wavelengths, according to Rayleigh`s l-4 extinction law),  mostly in the lower layers of the atmosphere, would have removed the blue color from the light path. Thus the combination of that mechanism and refraction would have turned the image of the Sun into a very thin segmented arc surrounding Earth`s limb,  mostly brick red and orange in color, that  would be  shining  at magnitude -17. From mid-eclipse on, the whole sequence described above would then develop backwards.