“And everything under the sun is in tune

But the sun is eclipsed by the moon.” -Pink Floyd

If you’ve been paying attention to the sky at night, you’ve probably noticed the Moon doing its thing, waxing and waning with regularity. Just a little over a week ago, we experienced a new Moon, and now it waxes towards full, appearing brighter in the night sky but also progressively farther away from the Sun.

Image credit: Gail Foster of http://gailfoster.ca/2013/09/beauty-within-the-ugly/.

The Moon goes through phases like this because it orbits around the Earth, which in turn is orbiting the Sun. It likely won’t surprise any of you that the phases of the Moon are due to the simple geometry of how the Sun illuminates the Moon, and then whatever portion of it is visible from Earth is what we actually see.

Think about it: the Moon is a sphere and the Sun is very far away, hundreds of times the Earth-Moon distance. Wherever the Moon is, the Sun lights up one half of it and leaves the other half in darkness.

Image credit: the Lunar Phases, via http://educatoral.com/moon_phases.html.

But what might surprise you — when you think about this picture — are two facts that I’m about to hit you with:

The Moon makes a complete, 360° orbit around the Earth every 27.3 days, but the phases go in a cycle that lasts longer. There are a total of around 25 “new” and “full” Moons (combined) every year, but only an average of four eclipses per year.

Let’s take on these two mysteries one-at-a-time. First, it’s true 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.

Image credit: Wikimedia Commons user Orion 8.

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.

In other words, it isn’t enough for the Moon to return to its original position relative to the Earth and the fixed stars, it needs to return to its original position relative to the Earth and the Sun.

Image credit: Encyclopædia Britannica, Inc.

But what about that second mystery? Shouldn’t new Moons lead to solar eclipses, and shouldn’t full Moons lead to lunar eclipses? After all, the reason we have a “new” phase is because the half of the Moon that we see is completely not illuminated by the Sun, and the reason we have a “full” phase is when we see the half that is completely lit up.

But if that were exactly the case, wouldn’t the Sun, Earth and Moon line up during each of those events? In the “new” case, shouldn’t the new Moon block the Sun as seen from Earth, and in the “full” case, shouldn’t the Earth’s shadow fall on the Moon, blocking it out?

Image credit and diagram: Gary Osborn.

It would be, if the Moon’s orbit around the Earth weren’t inclined to the plane that the Earth orbits in around the Sun! We think of the Earth as somewhat big, at around 6,371 km in radius, but that’s tiny compared to the Earth-Moon distance of around 400,000 km!

As the Moon moves around the Earth over time, it bobs up-and-down, crossing the Earth-Sun plane only twice per lunar orbit.

Image credit: Royal Museums of Greenwich.

Most months, the new Moon occurs when this alignment isn’t perfect, and the Moon’s shadow never falls on the Earth. Similarly, the full Moon typically occurs when the alignment is imperfect, too, and the Earth’s shadow never falls on the Moon.

But as the Earth/Moon system orbits the Sun, we typically get two fortuitous times each year, when the times that the Moon crosses the Earth-Sun orbital plane lines up with “new” and “full” Moons!

Image credit: Chaisson & McMillan, Astronomy Today.

Typically, whether a lunar eclipse is penumbral, partial or total is seemingly random, dependent on nothing more than coincidences of orbital motion.

Image credit: Addison Wesley.

Similarly, solar eclipses can be partial, annular or total, depending on the Moon’s distance from Earth and the exactness of the alignment between the Earth, Moon and Sun.

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

Well, next week — on April 15th — something unusual is about to begin. You see, we’re having a total lunar eclipse. In and of itself, this isn’t such an uncommon thing; as a long-term average we get about one of these a year.

Image credit: Wikimedia commons user SockPuppetForTomruen.

The Earth’s umbral shadow — the part that blocks sunlight from shining on the Moon — will fall on the Moon starting at around 5:58 Universal Time (10:58 PM Pacific Time on April 14th, for those on the west coast of the USA), and will last until 9:33 UT (2:33 AM Pacific Time on the 15th), perfect for viewing from the Americas.

Image credit: timeanddate.com, via http://www.timeanddate.com/eclipse/lunar/2014-april-15.

Because of the ways orbital dynamics work, we typically get strings of a few total lunar eclipses in a row (separated by 6-month intervals), followed by multiple years of not having any total eclipses at all. The eclipse of April, 2014 is a little bit special because it kicks off a string of four consecutive total lunar eclipses, something that only happens about once a decade in the 21st century.

But by time 14-to-15 days passes from a total lunar eclipse, the Earth has moved an additional 4% of the way around the Sun, and the Earth-Moon-Sun alignment will have shifted.

Image credit: James Schombert / University of Oregon.

And normally — under practically all circumstances — that shift-of-nodes will be enough to prevent a total (or annular) solar eclipse from immediately following or preceding a total lunar eclipse. If you compare the list of lunar eclipses with the list of total-and-annular solar eclipses, you’ll find that having a total lunar eclipse just two weeks away from a total-or-annular solar eclipse happens just a few times a century.

But it’s happening this month!

Image credit: timeanddate.com, via http://www.timeanddate.com/eclipse/solar/2014-april-29.

On April 29th, an annular solar eclipse will fall on the continent of Antarctica, with a partial solar eclipse visible to pretty much all of Australia. And though it might not get the spectacular reviews of a total solar eclipse, an annular eclipse is a wondrous sight to behold in its own right!

Image credit: my former student Destiny Fox. Thanks, Destiny!

But don’t fret if you don’t get to see the back-to-back wonder this year; it’s happening again next year, too, when March 20, 2015 brings a total solar eclipse to the Earth’s North Pole and April 4, 2015 brings a total lunar eclipse to half the world!

That total-total back-to-back combination that next year brings is a real rarity that won’t happen again until 2033, so make sure you appreciate what we’ve got!

Image credit: Fred Espenak of http://www.mreclipse.com/Special/LEprimer.html.

There are all sorts of doom-and-gloom scams that are floating around, but this is just the great cosmic dance between the Sun, Moon and Earth that’s been going on for billions of years. It’s wonderful, it’s beautiful, and it’s for all of us to be aware of and appreciate, if only we so choose it.

So enjoy this month’s back-to-back eclipses starting next week, and keep enjoying all the natural beauty and wonder this Universe has to offer!