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Review: The A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna

You may know the moon best as a big rock that hangs in the night sky, but far from being a useless pebble, it actually offers a symbiotic relationship not only with the Earth itself, but the people on it. It’s an ancient, fascinating phenomenon that has drawn minds of all types to it since the dawn of man, a focus of both science and religion, and it’s going to be around for a while. Nowhere better is the moon’s pull more aptly demonstrated than in the A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna, which will be our guide today to uncover three facts about this planetary tagalong you didn’t know.

Moon Phases Don’t Work How You Think They Do

I’m going to give you the benefit of the doubt in understanding that the moon is not a source of light by itself. Its glow comes from the sun, which is reflected off the lunar surface and back to our eyes. Of course, the moon orbits the Earth and the Earth orbits the sun, which means the moon not only appears in different places in the night sky, but the portion that’s lit also changes. We know this as the phases of the moon. That’s pretty common knowledge. What’s less common knowledge is how these phases actually work.

Take the full moon, as demonstrated by the A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna’s lunarium, which accurately depicts not only the location of the moon in the sky, but the phases as well. Here, the balance wheel represents the sun. Although we’re all aware that the positioning of the moon and the sun relative to the Earth yields the different shapes we see—the crescent, quarter, gibbous and full moons—understanding why those happen is a little trickier.

It’s commonly misunderstood that the Earth being between the moon and the sun causes the invisible new moon, because it makes sense that, with the Earth being much bigger than the moon, the moon falls completely into shadow—but that’s wrong. As the Terraluna demonstrates, this orientation actually yields a full moon, one lit in its entirety. But how? Considering a lunar eclipse is caused by the Earth blocking the sun’s light, this seems like a contradiction.

The A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna was first introduced in 2014

The A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna was first introduced in 2014

There are three factors to consider here: one, as is demonstrated by the Terraluna, the moon is very, very small compared to the Earth, and two, not so accurately depicted, is that it is very, very far away from the Earth, some hundred times further away, in fact. Third is the clincher: the moon’s orbit is off axis compared to our own. Not by much—around five degrees.

But it’s enough that all those factors together mean that the moon can still be lit by the sun when the Earth is between them, the moon effectively peeking over the top of the Earth. So how does a lunar eclipse happen? The solar system is not a precisely programmed mechanical device so regularity isn’t guaranteed, however most years will see the off-axis orbits of the Earth and moon align well enough to see two lunar eclipses, when the alignment really does block out all of the sun’s light.

The Moon’s Light Isn’t Blue

It’s a staple of the classic horror film, the hard shadows and blue hue caused by the single source of light in the night sky: the moon. We all know that movies aren’t real, yet we’re so familiar with the colour of the moon’s light that surely making it completely wrong would just feel … off? Yet somehow it doesn’t. You’re probably wondering at this point what colour the moon actually is, and to know that, we need to understand a little bit more about light itself.

The visible spectrum of light makes up a skinny little band of wavelengths between infrared and ultraviolet that, when we view them collectively, makes us see white. Not all light is made up of every part of the visible spectrum; how much a source is able to accurately depict the full range is known as its colour rendering index, or CRI.

Ever taken a picture of someone under fluorescent lights and wondered why they look so unwell? This is because most commercial grade fluorescents have a poor CRI, spiking in the green and missing a lot of the colours typical of skin tones. This leaves skin looking pale and sickly. The sun, however—the moon’s source of light—has a very full and even CRI, which is why daylight produces the most natural looking photograph. It’s bias towards blue, which sounds like it might be a clue for our conundrum—but it’s actually a red herring.

The A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna has a power reserve of two weeks

The A. Lange & Söhne Richard Lange Perpetual Calendar Terraluna has a power reserve of two weeks

Take a picture of the full moon and what you’ll see will be something of a surprise: it’s red. It varies from noticeably so to barely perceptible, but it’s there. Because the sun’s light skims the Earth to reach a full moon, as the Terraluna demonstrates, the atmosphere actually scatters a lot of the blue from it, leaving just the reds, oranges and yellows behind. It’s the same phenomenon behind our red-orange sunsets.

But this is even more confusing, because—unless it’s a rare blood moon—the lunar glow does indeed appear to have a blue hue to the naked eye. That’s because a camera sensor and your eyeballs function in different ways. First, your perception of what’s known as colour temperature—that’s where the bias of light is in its CRI, with daylight having a bluer bias, or colour temperature—is automatically calibrated by your brain, so you don’t really notice it. This means that if the moon really were a little blue and it was the only source, you probably wouldn’t notice.

Secondly, the perception of colour in your eyes is only half the story. When things get dark, the cones—the light detectors in your eyes that determine colour—aren’t sensitive enough to work well, and so the much more sensitive, but colour insensitive, rods taken over. And not only can the rods not determine colour, but they also barely respond to red light at all. That means as things get dimmer, what little light exists is perceived to shift towards blue. This is known as the Purkinje effect, something that’s been exploited by filmmakers for generations.

The Moon Is Leaving

Despite being our astronomical pal for over 4.5 billion years, our partnership with the moon is nearing an end. Although the origins of the friendship between our home and it are unknown, it’s commonly believed that it started with something of a bang. Like any good romcom, the story begins when two worlds collided—quite literally—a Mars-sized body known as Theia impacting our then-teenage Earth.

The resulting debris, some of Theia’s and some of our own planet’s, combined over time to form a new rocky body that we know as the moon. If it were entirely of the Earth it would be identical in makeup—if it were a completely alien body, it would be very different. Turns out some bits are the same and some bits are different, and so the giant-impact theory remains solid.

This gravitational bond formed between the young Earth and moon not only caused each to form the oblique spheroids that we all know and love today, but also a symbiotic relationship that has been critical for the formation of life on Earth. It moderates Earth’s axis wobble, maintaining a temperate climate; without it, the Earth could shift its axis to greater than forty-five degrees, essentially putting it on its side. Planets like Mars, that have much smaller moons, exhibit shifting axes ten times our own.

The tides may have even helped life form on a young Earth back when the sun was a hundred times more active, back when the moon still had a magnetic field. The combined forces of the moon’s and our own magnetic fields interacting with the shifting tidal forces may have created a protective magnetosphere that kept Earth’s atmosphere protected from the sun, which would have otherwise stripped it bare.

A perpetual calendar is accurate until the year 2100 (The next time we will skip a leap year)

A perpetual calendar is accurate until the year 2100 (The next time we will skip a leap year)

But this partnership isn’t to last. In astronomical terms, it’s right about the corner. A billion years or so and it will be gone, leaving Earth alone on its interstellar journey. To you and I that equates to a growing distance of some four centimetres a year—about the speed your fingernails grow—so you won’t have to throw out your Terraluna just yet, but for scientists it presents an interesting and concerning question about the Earth’s fate when the moon finally moves out.

Earth could tilt more, days could become longer, the tides would be all but gone—the sun will still have a marginal pull. Our planet could stop spinning altogether. But I’m sure by then the relevance of your Richard Lange Perpetual Calendar Terraluna will be the least of your concerns.

A big thanks to A. Lange & Söhne for letting us use the incredible Richard Lange Perpetual Calendar Terraluna to illustrate our equally incredible heavenly neighbour. As well as a full perpetual calendar and a constant force remontoir system to maintain the accuracy of the fourteen-day mainspring, this beautiful moon phase display and lunarium is a symbiosis of astronomy and watchmaking as close as that of the Earth and the moon itself.

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