Most sources say April 4th's lunar eclipse will be total, though only barely so. However, those calculations have overlooked a subtle factor that might render the event only "partial."
All week our emails (and, yes, our phones) have been buzzing with over-the-top stories about how tomorrow's total lunar eclipse will be the "shortest of the century." Well, maybe.
As we've pointed out, it all depends on how one define's the outer edge of Earth central shadow, the umbra. The editors at Sky & Telescope have long used predictions from the U.S. Naval Observatory, which in turn are ground out at Her Majesty's Nautical Almanac Office (HMNAO) in England. Those state that tomorrow's totality will last 12.2 minutes. Eclipse veteran Fred Espenak is countering with 4.7 minutes, as is NASA's eclipse website (which is no longer being updated but still utilizes calculations made by Espenak before he retired from NASA).
This morning, while perusing the comments posted about S&T.com's various postings — yes, we do read them — I found a provocative entry from Chicago-based amateur Curt Renz. "Actually, there will be only a partial (nearly total) lunar eclipse during Saturday morning," Renz writes. "The Earth’s umbra will only cover 99.5% of the Moon’s diameter." He goes on to explain that USNO, NASA, and Espenak all assume Earth’s umbra to be perfectly circular, whereas they really should be taking into account Earth's oblate shape and, therefore, the elliptically-shaped umbra that results.
Is Renz right? I'll keep you in suspense — first, let's get some background.
If our planet had no atmosphere, then (aside for the ramifications of an airless Earth to life) it would cast a shadow with an exact edge that's geometrically proportionate to our planet's shape. However, in reality, the size of the umbral shadow that falls on the Moon is bigger than it should be, thanks to an enlargement caused by our atmosphere. We know this from decades of careful timings, no doubt made by some of you, of exactly when the umbra sweeps over particular lunar craters during an eclipse.
Moreover, the edge of the umbra is fuzzy, and the meteorological conditions on Earth vary from eclipse to eclipse. Erich Karkoschka, a University of Arizona researcher, explored these variables at great length in Sky & Telescope's September 1996 issue. It's not just volcanic ash in the stratosphere (which isn't all that common anyway), but also the presence of cirrus and stratus clouds around Earth's day-night terminator and the ozone concentration.
So the enlargement values can vary. USNO public-affairs specialist Geoffrey Chester explains: "HMNAO (and thus USNO) use the Chauvenet convention, effectively increasing the size of the umbral shadow at the Moon's distance by a factor of 2%. NASA uses the more recent Danjon model, which is based on crater timings of past eclipses and gives a figure of about 1%." Roger Sinnott, a longtime S&T staffer (now a senior contributing editor) who painstakingly compiled crater timings for decades, notes that the Danjon correction is closer to 1.7%. "Both Chauvenet and Danjon used crater timings to determine the size of the umbra, he adds, "so they are both observationally based."
Earth is not a perfect sphere — its polar diameter is only 99.67% of its equatorial diameter, a difference of about 21 km (13 miles) in radius. And Renz is correct that the most common models assume Earth to be a perfect sphere, though there's a partial compensation for oblateness by taking the radius at a latitude of 45°. But still . . .
Earlier today I contacted both Espenak and Meeus, and both find Renz's assertion intriguing. Meeus apparently conceded to Renz that, if Earth's true shape is considered, the eclipse should be deemed partial. However, "it's more complicated than just the polar radius," counters Espenak. "You've got to calculate the ellipticity of Earth projected onto the fundamental plane at the time of the eclipse, and [include Earth's] inclination toward or away from the Sun." He hinted that he might tackle those details — but not until this eclipse has come and gone. (It's supposed to be clear in southern Arizona, and he's readying his cameras.)
So, let's throw it out to you — the observers! If you're in western North America (where totality will be observable) and your skies are clear, use binoculars or a telescope to judge whether that last little sliver of the lunar disk dives into the Umbra or not, and for how long. (No cheating using a camera — there'll be too much variation in exposures from one setup to the next to make the results comparable.) Then let us know, via the comment section below, what you find.
According to the experts, it'll be very difficult to judge. "I have almost no hope that observation will indicate whether the eclipse will be just total or only partial," says Meeus, "because the edge of the shadow is too diffuse." Espenak agrees: "I do not think observers will be able to time the contacts very well nor to distinguish between an 'umbral magnitude = 1.001' total eclipse and a 'umbral magnitude = 0.999' partial eclipse."
One thing's for certain: whether "barely total" or "extreme partial," tomorrow morning's view will be unusual. With the Moon passing so far north within the umbra, at maximum eclipse its northern limb should look much brighter than the southern side.