The Rarest Eclipse of Them All

"You can't exactly bake a man to your specifications... A hybrid of Einstein, Tarzan and Inge Meysel doesn't exist." -Peer Steinbruck

Of course, if you create enough chances, all the things that could eventually happen will come to pass. This is as true for human beings as it is for the physical Universe. And for our planet, our nearest major neighbor in the Universe -- the Moon -- provides us with a huge variety of sights, if only we're willing to wait.

Image credit: Doug Zubenel.

Every 27.3 days, the Moon makes a complete orbit around the Earth, traveling a full 360° through space. As it changes its position relative to the Earth and Sun, different portions of the half of the Moon illuminated at any given time become visible, resulting in the Moon's phases.

Image credit: Wikimedia Commons user Tomruen.

You may have noticed, however, that the Moon's full cycle of phases doesn't take 27.3 days, but slightly longer: 2.2 days longer, to be more precise. This is because, as the Moon orbits the Earth, the entire Earth/Moon system is in orbit around the Sun! Remember, it's the relative position between those three bodies that determines what we see when we view the Moon from Earth, and it's the fact that Earth travels some 8% of the way around the Sun during a lunar month that requires the Moon to move those extra 2.2 days to complete the cycle of its phases.

Image credit: Wikimedia Commons user Orion 8.

At the same time, there are a few more interesting things going on with the Moon. One is that its path around the Earth, although approximately circular, is in fact actually an ellipse. At its farthest point in its orbit, the Moon is more than 406,000 km away from Earth, while at closest approach, it's some 50,000 km closer.

Image credit: John Walker of http://www.fourmilab.ch/.

The variation you see in a typical lunar month means that the Moon appears about 12-13% bigger at perigee (its closest point to Earth) than at apogee (its farthest). As the Moon orbits our planet according to Kepler's laws of motion, this also means that it moves a little faster when its closest to us than when it's farther away. As a result of this, we don't always see exactly the same half of the Moon, even though it's tidally locked to us; there's a little bit of apparent "rocking," which is known as lunar libration.

Image credit: Wikimedia Commons user Tomruen.

The Earth orbits the Sun and the Moon orbits the Earth, and so you might think that every month, when the Moon is new, it would pass between the Earth and Sun, and when the Moon is full, the Earth passes between the Sun and Moon. A naïve picture of the Moon's phases would seem to indicate exactly this.

Image credit: Dr. Guy Worthey of Washington State University, via http://astro.wsu.edu/.

But you've got to remember that the Earth orbits the Sun in one plane, and the Moon orbits the Earth in a slightly different plane! These two planes are at a slight angle to one another -- just a hair over 5° -- but since the Moon and Sun each only take up around 0.5° on the sky, that means that during the vast majority of the time the Moon, Earth and Sun can not form a straight line in the sky.

Image credit: V. Charkarn / PTT Group.

It's only when -- during a new or full Moon -- these three bodies do happen to line up, that we get one of the most spectacular celestial phenomena as seen from Earth: eclipses. There are typically two opportunities for this per year: once when each of the Moon's nodes, or the places where it crosses the Earth's orbital plane around the Sun, line up during a new or full Moon.

Image credit: James Schombert / University of Oregon.

During a full Moon, the Earth's shadow blocks the Moon from the Sun, which creates a lunar eclipse, as seen by us. There are typically two of these per year (as you can see), although occasionally you'll have back-to-back months where the Moon just barely clips the Earth's shadow, once from above and once from below, as was the case in April and May of this year.

Image credit: Wikimedia Foundation, Inc., via their list of 21st Century eclipses.

You can have a total lunar eclipse, when the entire Moon plunges into Earth's shadow, a partial lunar eclipse, where a fraction of the Moon enters Earth's shadow, or a penumbral lunar eclipse, where the entire Moon still receives sunlight, but an observer on part of the Moon would see a fraction of the Sun blocked out by the Earth.

Image credit: Addison Wesley.

That same variety -- and same frequency -- occurs for the other possible alignment: where the new Moon passes between the Earth and the Sun.

Image credit: John McFarland, Armagh Observatory, via http://www.arm.ac.uk/.

These are solar eclipses, and we typically think of three varieties for them as well: partial eclipses, where the Moon appears to partially block out the Sun as seen from Earth, and then the other two (much more interesting) types. You see, the Moon and Sun happen to be almost the same angular size as viewed from Earth: about half-a-degree. Depending on where the Sun, Moon and Earth are in their relative orbits, sometimes the Moon appears larger, while at other times the Sun appears larger as viewed from our planet.

Image credit: NASA / Solar Dynamics Observatory.

When the Moon appears larger than the Sun, it can block the Sun's entire disk, casting a shadow on Earth and plunging that shadowy region into temporary darkness. That's what a total solar eclipse is. When the opposite is true -- when the Sun appears larger than the Moon -- you get an annular eclipse, where the Moon passes in front of the Sun but is insufficient to block the entire disk, creating a ring of sunlight, or an annulus, as seen from Earth. (Longtime readers may even remember I went and hunted the one that happened last year!)

Image credit: Me, taken during my 2012 Annular Eclipse expedition!

About a third of all solar eclipses are partial, where no place on Earth will see either a total or annular eclipse, about a third are annular and about another third are total. The Moon, on average, appears larger than the Sun around 50% of the time and smaller the other 50%, which accounts for that distribution. When the Moon appears larger, a shadow is cast on the Earth; a shadow that's been captured from space!

Image credit: Mir 27 Crew; Copyright: CNES, of http://www.cnes.fr/.

But think about this for a second. During a solar eclipse -- either a total or an annular eclipse -- the Moon's path needs to cut across the entire Earth. Even though this is a relatively quick process, lasting around five-to-six hours, it isn't instantaneous. You might wonder whether there's ever a time where an eclipse will start off as an annular eclipse (but just barely), and then during that five-to-six hour timeframe, the Moon moves slightly closer to Earth, just enough that an observer can experience a total eclipse (again, just barely) by the end? Or whether the opposite is true: that an eclipse can start off total and then -- as the Moon moves away from Earth -- transition into annular for an observer on a different part of Earth?

Image credit: Left: Fred Espenak (Mr. Eclipse); Right: Stephan Heinsius.

Yes you can! That's known as a hybrid eclipse, and it turns out to be the rarest of all eclipses, making up just over 2% of all (solar+lunar) eclipses! It turns out that not only the orbits of these three bodies, but also the fact that the Earth is of a finite size (and the Moon's shadow has a little longer to go to reach Earth's edges) helps make this as common as it actually is!

Image credit: Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay.

The 21st Century will see only 7 of them, including the one in 2005 (shown above) and -- believe it or not -- the very next one, on November 3rd of this year! A lucky (and very, very dedicated) observer located a few hundred miles off the coast of Liberia could actually watch the transition, experiencing a little of both the total and annular phases in the same eclipse, something that (as far as I know) has never been photographed or video captured!^{[* -- see comment 7 for more detail on this]}

Image credit: Fred Espenak, NASA's GSFC, Official work for NASA.

Although a hybrid eclipse typically occurs, on average, about once per decade, there's actually a huge variety in their frequencies. The 17th Century saw a whopping 24 hybrid eclipses, while the 23rd Century will see a mere three. If you miss the one this year, you'll have to wait until 2023 for your next chance at a hybrid, the rarest of all eclipses, where observers in Indonesia will actually have a chance to experience the shadow and the annulus at the same location!