tipping the moon

I can tell you exactly when I first noticed it.

I was on a cruise from San Juan, Puerto Rico to Acapulco through the Panama canal on the evening of 31 December 1999. The date should sound familiar. It was Y2K* eve, and we were sailing through the Caribbean.

The world's breath was bated as the minutes ticked off the seconds when we would all discover if everything electronic would come to a crash that would forever erase "Black Thursday" as the very definition of disaster -- or if life would go on as we knew it.

As is my wont, I was in the optimist camp. Like most of the things we fear, I suspected this one would turn out to be just another bust.

Just as the ship's horn signaled the new year, the horn abruptly fell silent, all the lights on the ship failed, and we sailed right into an iceberg. All passengers and hands were lost at sea.

That, of course, might have been the tale someone would be telling you now, if I had been wrong about my prediction. Instead, the horn blasted, everybody cheered the new year, and we all hugged and kissed a stranger or two.

Earlier in the evening, I happened to look up at the Moon. It was a waning crescent, but in the south Caribbean, it still lit up the ocean. 

Rather than waxing poetic with romantic reveries, it appeared to me something was wrong with the Moon. It was tilted. Severely tilted. It was on its back. Or, at least, it seemed that way to my Oregonian eyes. I was accustomed to the crescent Moon on the Turkish flag. This looked like the Moon on the flag of Mauritania.

When I returned to work the next week, I cornered Dennis Martin, a fellow amateur astronomer, and asked him if he knew why. He didn't.

The internet back then was just a shadow of what it is today, so, we made a trip to the library. The answer was extremely obvious. Had we given it much thought, we would undoubtedly have answered the question without research.

Everyone knows the basic facts. While Earth revolves around the Sun, the Moon revolves around Earth. The Moon is visible in the night sky only because it reflects light from the Sun. That means that the portion of the Moon that is lit is always facing the Sun giving us the phases of the moon.

There is an additional fact we all know. Earth is tilted on its axis. As Earth revolves around the Sun, sometimes its northern axis is tilted toward the Sun and six months later, its southern axis is tilted toward the Sun. That gives us our seasons and the optical illusion that the sun moves north and south on the horizon during the year. But, the moon's orbit around the Earth is constant (despite what Shakespeare caused Juliet to recite).

The result of this combination of a tilted Earth and a constant moon means that to we Earth-bound astronomers, the Moon appears to change its path across the night sky. Sometimes it travels at an angle toward the horizon and sometimes it travels straight down toward the horizon.

That means that from January through March, for viewers who live from 25° to 50° north latitude (the northern temperate latitudes)in the northern temperate latitudes, the changing angle of the lunar orbit will result in the "U"-shaped crescent. From July to September in the southern temperate latitudes, the same is true.

You have undoubtedly already jumped to the conclusion that the emphasis in my label as an amateur astronomer should be on "amateur," because when I lived in Oregon, that "U" Moon was in the sky for three months every year, and I did not notice it. Of course, those are the winter months, and because of cloud cover in the Pacific Northwest, we could never attest that the heavens had not been emptied of its bounty. At least, that is going to be my excuse for not being very observant for fifty years.

For the remainder of the year, the crescent appears sideways. And we get to see what I have always thought was the crescent Moon's natural shape -- as a "C."

But, I have not answered my original question, have I? I was curious why the moon looked as it did in the Caribbean.

At the time of the Y2K calendar click-over, we were just off the coast of South America, preparing for a port-of-call in Aruba the next day. At 12° north, we were deep in the tropics. And that is where the research Dennis and I conducted paid off.

For those of us who currently live in the tropical latitudes, there is not a "C" crescent Moon to be seen. What we get to see is a year-round "U" crescent Moon. When I visited Peru three years ago, one of our connecting flights put us on the tarmac of Lima's airport just as the crescent Moon was rising. And, sure enough, it was a "U." Lima is located on 12° south latitude. Just as deep in the tropics as Arbua is. 

I was amongst a group of Norwegians who had never experienced the phenomenon. That is because they live above the 50° north latitude, where the crescent always shows up as a "C." Maybe it is my Scottish and English genes, rather than my lack of observation, that made me miss the "U" Moon in Oregon.

So, there you have it. The mystery is solved why the crescent Moon appears to change -- in part of the world. If you live in Mexico or Canada, you only get one variety.

And I am quite happy with the Mexican version that looks a bit like a jagged-tooth grin.  

* -- The phrase itself is a perfect example of how the once-dreaded end-of-the world hype for 1January 2000 has simply faded. I had to look up the phrase. Of course, that may say more about my aging memory than the hype.