Phases

| 8 Comments
moon_clouds.jpg
Nearly full moon on a typically cloudy night this time of year. 2004:10:28 20:41:46

We've noted in the past that the Moon in the Mideast is said to rise and set at times in a "horns-up" orientation. My efforts to photographically document this arrangement have been foiled, thus far, by the fact that winter is fast approaching. This can be discerned by two observations: first, that long-sleeve shirts are starting to appear after dark; and second, that it is cloudy at night.

However, the good contributors to the APOD site have provided a nice illustration of the phenomenon.

The crescent moon has had considerable significance in the Mideast since ancient times, although I don't know that I necessarily buy this article's claim that lunar supremacy is a general feature of nomadic mythologies. Anyway, Sargon was already a king of cities when he set up Enheduanna as priestess of the Moon at Ur, although there's no denying that nomadic traditions play an important role down to the present day. However, I suspect it's the continuity of Islam (and Judaism), and not a connection to ancient wanderers, that is chiefly responsible for the continued use of the lunar calendar in this part of the world. Notice, though, that the star-and-crescent symbol of Islam might be a distant derivative of the star of Inanna (itself probably Venus), and one begins to realize the essential imprecision of such distinctions.

The major significance of the crescent moon to anyone using a lunar calendar is that this phase marks the passing of the months, as in all such calendars of which I am aware, the new month begins with the new moon. Thus, to tell when the month is ending, one must rise before sunrise and watch the waning crescent moon rise just before the sun; likewise, the new month is known to have begun the first time the crescent moon can be seen in the evening, setting just after the sun. Since this transition is important to, for instance, determine the exact start of Ramadan, such sightings are now coordinated worldwide. Sure, you could just compute when the conjunction occurs (and they do this, to cross-check), but I think Islamic doctrine requires an actual, visual sighting.

So, what about this "horns-up" thing? The short version is, that's approximately what it'll look like whenever the Moon falls near a vertical line drawn from the Sun (which you'll only be able to perceive if the Sun is already below the horizon). Just imagine a lamp illuminating a ball from below (an experiment that you can try, if you're not one of the astronomers in the audience and already sick of demonstrations involving lamps and globes). When will the Moon appear to rise/set almost vertically?

orbit.gif
Schematic of the Earth's orbit about the sun, linked from Dr. Thorngren's Orbit Tutorial at Palomar Community College

The answer, approximately speaking, is when you're standing sideways in the above picture, whicn can only ever happen exactly if you're within about 23 degrees latitude of the Equator. I recommend a visit to the above-linked Tutorial if you don't see why. The Mideast isn't actually at that low latitude, but it's only off by about ten degrees, so we'll call it close enough. Even when you're close, this only happens once a day -- 12 hours later, you'll be standing as close as you ever get to vertically in the picture. So to be able to see the crescent Moon, it has to be a smidgeon before sunrise or after sunset locally.

Put this all together, and the desired configuration is visible in two broad periods each year. The waning crescent Moon can be seen rising vertically in the early mornings after the September equinox, and the waxing crescent will set vertically in the evenings leading up to the March equinox. If you need a range to go observing, let's say between the equinox and the adjacent cross-quarter day. And, of course, this will work from any location sufficiently close to the equator; there's nothing special about the Mideast's longitude. So those of you in Texas actually can try this at home.

8 Comments

Michael,

Saw the astro phenom; it was awsome. At the time did not know that, you too, was viewing from Isarel. Fantastic !!!!!! You have come a long way since the methane experiment in 4th grade.
Will see you at the annual Posada in SA. Take care, pray for your safe return, go in peace.
PAX "K"

Michael,

Saw the astro phenom; it was awsome. At the time did not know that, you too, was viewing from Isarel. Fantastic !!!!!! You have come a long way since the methane experiment in 4th grade.
Will see you at the annual Posada in SA. Take care, pray for your safe return, go in peace.
PAX "K"

This is the best reason I have ever come across to do some travelling. I could give a rat's ass about meeting new people, experiencing different cultures, visiting friends, and so on...but seeing the moon tipped on its side? Now, THAT's something to see! Maybe I will make it to Texas someday, after all.

You reminded me that when I worked at the Naval Observatory in DC we were specifically instructed never to answer any questions from the public regaurding the phases of the Moon. The reason was its religious significance in general and specifically the significance in Islam which you alluded to. Imagine, some half awake astronomer misreads the almanac and bingo, instant international incident.

Excellent explanation of this phenomenon. I'm glad to see that there is a practical explanation for it -- as there almost always is. So the crescent shaped magur boats used by the Sumerians and Akkadians on the Tigris and Euphrates probably were designed in imitation of the "moon boat" of Nanna and Ningal. I assume that similar designs in other latitudes have different origins. Maybe not. If you are a Viking in a rough sea, designing your ship after the crescent moon in its more familiar orientation might make sense. . . .

Don't forget that the moon has an inclination of 5 degrees to the ecliptic, so the phenomemon can happen at latitudes higher up than 23 degrees.

Anyone know where I can find something a bit more accurate?

The US Naval Observatory is a good place to start. Especially if you're looking for basic data like accurate rise/set times.

http://www.usno.nava.mil/

But it really depends on what you're looking for. For instance, if you want the actual orbital elements of the Moon, they're easily available. From the JPL, for instance:

http://ssd.jpl.nasa.gov/sat_elem.html

Based on the homepage it linked to, I think "and bingo" is just a spammer (I deleted the link). But it's still a good if rather vague question, so I left the post.

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This page contains a single entry by Milligan published on November 18, 2004 10:03 PM.

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