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Go Back   SailNet Community > Contributing Publishers > Good Old Boat
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Old 08-31-2007
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Moonglow

Moonglow
The forgotten magic of our heavenly satellite
by Henry Cordova

There is no dark side of the moon, really. Matter of fact, it's all dark. -- Pink Floyd

The next time there's a new moon, go out and look for it. You won't see it. In fact, the only way you'll know the moon is new is because you looked it up in a calendar or almanac. But go out and look for it anyway: face west right after sunset and you can be assured it is somewhere very close to the sun, either about to set or perhaps already below the horizon.

The new moon is invisible because the side of the moon that is illuminated by the sun faces the sun, away from you. When you're thinking about the moon it's important to remember that, just like the earth or any other world of the solar system, it is always half-lit by the sun. It has a day and a night just like we do.

When the moon is new, we are looking at its night side. An observer on the moon looking back at us would see a "full earth," a dazzling bright blue-and-white globe dominating all of heaven.

The reason the moon looks different from night to night is that it has phases. This is because we view it from different angles as it circles the earth in its orbit. Its phases are the result of its and our relationship to the sun and the changing geometry of star, planet, and satellite. Yet the way we experience this relationship and its subtle connections to our own lives, to wildlife, and to the ocean tides touches us in a purely visual fashion.

The monthly lunar cycle has a profound influence on our natural world and, until very recently, it had a profound influence on us too. Before the advent of electric lighting, everyone knew all the time, instinctively, what the phase of the moon was: where it would appear in the sky at any given hour and how much light would be available for nocturnal activities, when, and for how long. Our ancestors used it for illumination, to tell direction and time, to divide the year, and to help plan the long evenings of their lives.

First sighting

Nomads in the Ice Age carved the first sighting of each waxing moon on a stick they always carried with them, counting the months until spring. It's not surprising that a great deal of superstition and myth also grew up around the moon, but from a purely practical point of view it was essential to understand it and its appearance in the sky if you were going to spend time out of doors. That's why, until very recently, the lunar phases were printed on every household calendar. This was vital information, an essential part of the weather report. To our ancestors, and to mariners as well, a knowledge of the moon was instinctive and automatic.

"An observer on the moon looking back at us would see a "full earth," a dazzling bright blue-and-white globe dominating all of heaven."

Go out at sunset a night or two later and look again. The moon is just visible now as a thin crescent, still very near the sun and following it to set shortly afterward in the west. The ancients marked the beginning of the month with the first sighting of the crescent moon. Some sects of Judaism and Islam do so to this day, and religious functionaries are appointed to look for this event and announce it to the faithful. As the moon circles earth, the angle between it and the sun increases so we are gradually able to see more and more of the sunlit half.

We say the moon is now one or two days old, that age being a measure of where it is on its monthly voyage around our planet. It takes our satellite about 27  1⁄3 days to go completely around the earth, what astronomers call a sidereal month. This is the time it takes for the moon to circle earth, relative to the distant fixed stars. But during that time the earth is also traveling on its own orbit about the sun. For the moon to be new again, it must go a little farther and longer to repeat the alignment. We call the time between new moons a synodic month, or about 29  1/2 days.

Takes a full year

As the earth circles the sun, the sun appears to slowly travel among the fixed stars, taking a full year to creep slowly around the bowl of night to return to its original position. The moon orbits the earth, taking about a month to circle our planet. Both of these motions are obscured by the earth's rotation on its axis, once each 24 hours. We hardly notice it today. But to our ancestors, who had clear dark skies and plenty of free time at night, these interlocking cycles were a part of daily life.

The sun's yearly journey east (about a degree per day) manifested itself by the stars seeming to rise just a little earlier each night and the fact that different stars were visible in different seasons.

View facing south, for a northern hemisphere observer.

The moon travels east as well but not as slowly. If you carefully note its position relative to the stars, you can actually see it moving among them at about 13 degrees per day, or about its own width in an hour. But these gradual motions are difficult for the casual observer to perceive because they are overwhelmed by the earth's daily rotation from west to east, causing the entire celestial sphere to roll from east to west in just a day, carrying moon, sun, and stars along with it.

If it's still clear the following night, look west again. The lunar crescent will be wider as more of the sunlit side is turned to our view. The moon is farther from the sun now in a darker part of the sky and, if you look carefully, you can sometimes see the ghostly image of the dark part, barely visible, framed by the gleaming crescent. This phenomenon is often referred to as "the new moon in the old moon's arms." The source of this dim apparition is the earth itself. Sunlight reflected from the earth's cloud tops and polar caps bathe the lunar night with enough glare that we can see it from earth.

Much brighter

An astronaut standing on the dark side of the moon would see an almost-full earth brightly shining on the gray rocks about him, much brighter than any moonlit earthscape. We can't see this glow at the time of a new moon because the moon is too close to the sun and the surrounding sky is not dark enough to provide the needed contrast. But when the moon is one, two, or three days old, depending on conditions, sometimes you can catch a glimpse of it. On good nights you can even make out features on the lunar surface, dimly lit by earthshine.

"The sailor with a sextant can shoot fixes all morning. The lit side is facing east now toward the sun. "

There are seven days in a week, about four weeks in a month. If you look at the moon when it's a week old, it will be about 90 degrees from the sun, about due south at sunset (for Northern Hemisphere observers). The moon is now far enough from the sun that it appears half illuminated, so we see a half moon, a semicircle of light pointing at the sun. Because of the angles, the half moon will rise about noon and be easily visible all afternoon -- yes, you can see the moon in broad daylight -- as it follows the sun across the sky. The moon will be at its high point at sunset; it will set around midnight.

Although a half moon is visible, this phase is called first quarter because you are now one week, or one quarter, into the lunar month. This terminology invariably causes confusion, so beware. That the moon can be seen during daylight hours is of particular importance to celestial navigators since they can derive two lines of position and a valid fix. What's more, they have plenty of time to do it, not just the few minutes around twilight. At first quarter, conditions are ideal, the lines of position will cross at right angles, and observations can be made all afternoon long, as long as moon, sun, and horizon are simultaneously visible. A similar configuration will occur another fortnight later, at the third quarter.

More illuminated

As the moon moves farther from the sun, it is said to be waxing. That is, the farther it gets from the sun, the more of the illuminated side we can see. Between first quarter and full moon, the phase is said to be gibbous (pronounced as in "'gibbous' this day our daily bread"). At full moon, about two weeks, or 14 days, into the lunar month, the sun and moon are on opposite sides of the sky, and the lunar disc is now fully illuminated. The moon rises at sunset and is visible all night until it sets at daybreak.

View facing south, for a northern hemisphere observer.

Incidentally, when the moon is full it is possible for there to be a lunar eclipse if the sun, earth, and moon are in a straight line and the moon can cross into the earth's shadow. Likewise, at new moon a solar eclipse can occur when the sun is blotted out by the moon. However, the moon's path around the sky is inclined about 5 degrees to the sun's (the ecliptic) so usually the lineup is not perfect as the moon or its shadow passes just above or below the alignment point. Eclipses are rare enough that they took even our sky-savvy forebears by surprise, although all the early civilizations soon developed the astronomer-priests necessary to accurately predict them.

Before the introduction of timepieces, a knowledge of how the moon's position in the sky was related to its phase allowed it to be used for timekeeping. In daylight, a look at the sun will give you a good idea of the time, but at night the problem is more complicated. Our ancestors could simply glance at the moon and, knowing its phase and position, could easily tell how many hours were left before daylight. Being able to tell time, even to the nearest hour, was essential for mariners to do even the roughest dead reckoning after dark. They could also derive some compass direction from these observations on those nights when stars were obscured by haze or twilight.

Essential knowledge

It is not that important today to have these skills, but to the early sailor or hunter it was essential knowledge. Even today, once aware of these celestial geometries, they quickly become a part of your instinctive navigational equipment. When I'm in a position where I cannot see the sky or when it is overcast, I become disoriented and uncomfortable. (I often wonder how I would react in the Southern Hemisphere, where I've never traveled and where everything in the sky is upside down. For example, in southern temperate latitudes, both sun and moon are primarily in the northern half of the sky. I'm sure you'd get used to it, but it must be disconcerting at first.)

After full moon, the moon's visible shape begins to wane (diminish in size). The moon goes through its gibbous, half moon, and crescent phases in reverse. The positions of the waxing moon are also reversed: when the moon is three weeks old, at third quarter, the half moon rises at midnight, is overhead at sunrise, and sets at noon. The sailor with a sextant can shoot fixes all morning. The lit side is facing east now toward the sun. The waning crescent rises just ahead of the sun, an unfamiliar sight to us now because we rarely get up that early any more . . . unless we are sailors.

It may be helpful to learn these lunar relationships by imagining how the earth would appear to an observer on the moon throughout the lunar month. The moon's rotation is gravitationally locked to the earth. That is, the moon always turns to keep the same hemisphere facing us. (There is a slight wobble, but it is not very significant.) That's why we never see the back side of the moon. To a lunar observer, the earth would always be in the same part of the sky; it would move only slightly and never stray too far from its position. At new moon, the earth would be full, with glorious navy-blue seas, blinding white clouds and icecaps, and dimly visible brown and green land masses with pastel coastlines.

Remained stationary

As the synodic month progressed, the earth would remain stationary, but it would go through all the phases. At full moon the earth would be new, a black hole in the stars next to the sun. During a lunar eclipse the lunar observer would see the new earth eclipse the sun. During a solar eclipse, the lunar shadow would sweep across the full earth. The sun would creep across the sky in a month, not a day.

For the cruising sailor, the position of the sun and moon gives an indication of the latitude. The sun never strays more than 23  1/2 degrees from the equator, and the moon is never more than about 5 degrees north or south of the sun's path. The result is that in the Northern Hemisphere lunar and solar activity are always in the southern part of the sky. This effect becomes more obvious the farther north you travel. The opposite occurs in the Southern Hemisphere. In the tropics, sun and moon spend a lot of time very high in the sky, often passing directly overhead.

I've often wondered how the ancient Polynesian voyagers interpreted this; they were very familiar with the sky and employed a sophisticated system of celestial navigation. They also traveled great distances across the equator, from New Zealand to the Hawaiian archipelago. They would have known both the Big Dipper and the Southern Cross and the constellations about both celestial poles. They must have noticed the celestial sphere slowly rocking over their heads as they sailed north and south. Perhaps the oldest and wisest even guessed the truth, that the earth was not an endless flat lake dotted with tiny islands to infinity. Perhaps a few of them, in a flash of insight on some long watch a thousand years ago, realized the world is round and even got a feeling for how big it is.

There will be a direct link from those ancient mariners to you when you are able to go out at sunset and look at where you know the new moon to be . . . without having to look it up in the almanac first.

The author:
Henry Cordova is a geographer/cartographer who has been a sailor of the military persuasion (U.S. Navy Reserve on the USS Dewey) and of the recreational variety (a San Francisco Pelican and a MacGregor 22).

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Old 09-04-2007
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Very interesting article, answered several questions I had wondered about over my many years. Thank you for sharing.....John B
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Old 09-04-2007
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Hey don't forget Sailnet is a web thingie - it's international. The full moon is clearly visible to us here in New Zealand

If you're quick, you'll get a look at the full moon while in lunar (or is that solar) eclipse from the earth the other day (28th August) - put up on a weather site I often visit...

http://www.metvuw.com/
(I'd put a pic up here, but no method I can do so)

Regards, Duncan (Christchurch, New Zealand).

Last edited by DuncanNZ; 09-04-2007 at 07:13 PM. Reason: fix
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Old 09-05-2007
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caspargutman is on a distinguished road
I wish to add this article was enhanced by the very capable illustrations of Mr Fritz Seegers.

Thanks, Fritz
Henry Cordova

Last edited by caspargutman; 09-05-2007 at 06:30 PM. Reason: spelling
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