Chris Peterson wrote:
alter-ego wrote:Barely total or barely partial, we may never know.
I'm not sure about that. It depends on definitions. Normally, we would ignore the effects of the atmosphere, and simply consider the geometry. That can be done rigorously.
I suspect this was actually a total eclipse from a geometrical standpoint, but appeared partial because of atmospheric effects.
Well, I'm certainly in agreement with you about not being sure.
Interestingly, the visual umbra is counterintuitive. In fact the geometric umbra is smaller by ~2%, not larger as I believe you inferred. The official eclipse predictions we see include an average correction for atmosphere perturbation of the geometrical umbra. Prediction variations in eclipse magnitude (depth) depend on the method use to estimate umbral size (not standardized). Fred Espenak
does the eclipse calculations for the NASA Eclipse Website
. He describes the histories and descriptions of the two methods for determining the size of the umbra are here
. The accepted definition of the visual umbral edge is the point of inflection in the shadow density (steepest intensity gradient). Also, I'll add that crater timings have been and are critical for assessing umbral size and variations.
This eclipse is a rare one; whether it is barely total or deep partial depends entirely on the atmosphere and possibly 2nd order corrections to Earth's actual shape, not the geometrical umbra. Just to be clear, whether this eclipse is total or partial is fun to think about, but Fred's comment below reflects my question of whether we'll really know the true depth of this eclipse.
Fred Espenak wrote:So the small magnitude differences discussed here are only of academic interest. Still, it is important to note which shadow enlargement convention is used because it is critical in comparing predictions from different sources. In both the Thousand Year Canon of Lunar Eclipses 1501 to 2500 and the Five Millennium Catalog of Lunar Eclipses: -1999 to +3000 (NASA TP-2009-214173), Earth's penumbral and umbral shadow sizes have been calculated by using Danjon's enlargement method. Other sources using Danjon's method include Meeus and Mucke (1979), Espenak (2006) and Connaissance des Temps. Several sources using Chauvenet's method are Espenak (1989), Liu and Fiala (1992), and Astronomical Almanac.