Date: August 13, 2009
Title: Down to My Last Quarter
Podcaster: Larry Sessions
Organization: North American Skies
NASkies Twitter http://twitter.com/NASkies
Description: Moon phases are an unexpectedly misunderstood topic for many, based on my years of teaching. It seems that full moon means something entirely different to young people today, and quarter moons are really halves instead. One phase in particular, the Third of Last Quarter, is probably the least understood of all. Find out why that is so, as well as when and where to see it.
Bio: Larry Sessions is a former director and staff astronomer at Denver’s Gates and Fort Worth’s Noble planetariums, and now is an instructor for Metropolitan State College and the Community College of Aurora, Colorado. He also is the webmaster and editor for the Southwestern Association of Planetariums, as well as his own website, North American Skies, and a contributor to both Space.com and EarthSky.org. A NASA/JPL “Solar System Ambassador,” he has every copy of the Royal Astronomical Society’s annual handbook since 1971.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Drs. James and Jeanette Hill of Austin Texas, in commemoration of their 21st wedding anniversary.
Transcript:
Down to My Last Quarter
Welcome again to the wonderful world of astronomy, where the Moon hides when it is new, where two quarters don’t make a half, and where some people actually mistake three quarters for a half. Hi, I’m Larry Sessions, an astronomy instructor and former planetarium director in Denver, as well as editor and webmaster for the North American Skies website, and my Twitter site, NASkies. Occasionally I also write for EarthSky.org.
More than many other languages, English has a funny way with words. For example, when some people see a half-illuminated Moon standing high in the sky at sunset, they call it a “Half Moon.” Commercial products, restaurants, and even cities, such as California’s beautiful Half Moon Bay, get into the act by naming themselves after this non-existent astronomical entity. Technically, however, it is a “Quarter” Moon, not a “Half” Moon.
The reason is simple. While what you see is the Moon only half illuminated, only one half of the Moon is facing us. The “farside,” which we never see, is the other half. So when half of our view of the Moon is illuminated, we really are seeing just one half of one half of the Moon’s total surface — or one quarter. You also can consider that the Moon at First Quarter phase has completed just 25 percent of its current orbit. So a quarter is just a quarter.
And by the way, I referred to the “farside” of the Moon, rather than the commonly used “dark side” of the Moon. “Dark side” is a poor choice in reference to the Moon — even though one of my favorite albums is Pink Floyd’s Dark Side of the Moon. This is because there is no one dark side of the Moon. The familiar face we see from Earth is in the dark just as often as the side we can’t see.
We have a special name for when the Moon’s face is too dark to be seen from Earth at all. It is called “New Moon.” The New Moon hides in the glare of the Sun. You could call it “No Moon” because you can’t see it. Still this is considered to mark the beginning of the Moon’s orbital cycle, which is why it is “new.”
Sometimes people see a thin Crescent Moon in the eastern dawn a day or so before New Moon, or more likely, a thin Crescent Moon in the western dusk a day or so after New Moon. These are the Waning Crescent Moon and Waxing Crescent Moon, respectively, not the New Moon. Observers around the world enjoy the challenge of viewing the “oldest” and “youngest” Moons, by which I mean the extremely thin crescents just hours before or after the moment of New Moon.
No doubt you have seen the image of a Crescent Moon with a star inside it, frequently used as a symbol of the Islamic faith. Sighting the young Crescent Moon just after New Moon has great importance in determining the times of holy days in the Muslim world. Choice of this symbol may have been related to this or some other reason, but in any event, the symbol is done with a good bit of artistic license. The crescent often extends well more than 180 degrees and the star often is depicted within the crescent, neither of which happens in Nature.
So we have New Moon, which technically cannot be seen, but which marks the beginning of a new phase cycle. About one week later, the Moon has moved 90 degrees eastward from the Sun to appear half illuminated but called a “quarter” Moon, as I have already mentioned. After all, 90 degrees is one quarter of a full 360 degree circle. It’s “First Quarter” in this case.
About another week after First Quarter, the Moon has moved another quarter turn, making it two quarters, which adds up to one half, right? …Well, not exactly.
If we start with the New Moon, halfway, or 180 degrees around its orbit is *not* the Half Moon. When the Moon is in quarter phase, we see half the visible face of the Moon, but that isn’t Half Moon. We call it “Quarter Moon” because it is a quarter of the way around in its orbit. Then when the Moon is halfway around in its orbit, we see just half of its entire surface, so you might think it reasonable to call it a Half Moon.
Nope, sorry, this is the “Full Moon.” At this point, halfway through the current phase cycle, the Moon is opposite the Sun in the sky. Thus the entire visible surface of the Moon is in sunlight, and appears Full. At this phase the Sun and Moon do a seesaw act — when the Sun is setting in the West, the Moon is rising in the East. But because the Moon is constantly moving in its orbit, this will appear to be true only a night or two before the Full Moon phase. After the Moon becomes Full, it rises slightly after sunset, even though it may visually appear to still be Full.
This brings up a couple of points. First, due to the Moon’s orbital motion, it appears 12 to 13 degrees farther East each day, regardless of phase. This is a bit more than the apparent width across the knuckles of your clenched fist (including thumb) held at arm’s length. You can easily track this from night to night (or day to day, although the lack of a stellar background makes it harder). This causes the Moon to rise about 50 minutes later each successive day, averaged through the year.
Second, the phases are way points in the Moon’s orbit around Earth. They occur at exact moments in time. While the Moon may appear roughly the same for a day or so before or after the onset of each phase, in fact the phase cycle is constantly changing. Not only does the Moon’s position change daily, as just mentioned, but its appearance smoothly transitions from one phase to the next, never appearing exactly the same from one day to the next.
About one week after Full Moon, the lunar orb has traveled another quarter of the way around its orbit. Thus it has traveled 75% of its orbital path, which brings us to Third, or Last, Quarter. As you might now expect, this three-quarters Moon really looks more like a half Moon because only half of the visible surface is illuminated. As with the First Quarter Moon, the side of the Moon turned sunward is the part lit up. But while the First Quarter Moon is in the evening sky with the right hand side lit up (as seen from the Northern Hemisphere), the Last Quarter Moon is in the morning sky with the left side lit up (again, from the Northern Hemisphere).
Probably fewer people are familiar with this late night phase of the Moon simply because it does not rise until around midnight local time. On the other hand, it may be the phase most often seen in the daytime, standing prominently in the morning sky as people scurry about to work and school. Sighting it then comes as a big surprise to many people, but it shouldn’t. In fact the Moon is in the daytime sky just as much as it is in the night sky, but the lesser contrast with the bright blue daytime sky makes it less noticeable.
Now that I have come to my last quarter, I’ve run out of change, and I’ve run out of time. But the Moon hasn’t. It’s still out there, evolving in appearance daily as it revolves around Earth. Go find it!
End of podcast:
365 Days of Astronomy
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The best argument against the phrase “dark side of the moon” does not lie in the fact that it isn’t literally dark. In fact, I think that particular argument is a bad one, as it implies the rather strange point of view that metaphors are somehow deplorable. The best argument against the phrase “dark side of the moon” lies in the fact that it isn’t metaphorically dark; that is to say, it isn’t mysterious. It used to be, when the phrase was originally used, but that was then and we know quite a lot about it now.
Scientists, and in particular physicists (of which astronomers have the unique distinction of being both the parent and the offspring) are a particularly literal lot. We, in general, expect things to be as advertised. By and large, the literal, factual side side of things is at least as important as the artistic, creative, right-brained or philosophic ways of looking at things (at least for phyiscs and other sciences). This is not to say that less precise, more general and maybe even amorphous views are not valid or important. Quite the contrary. In fact physics was originally known as “natural philosophy.” But on the other hand, science, and in particular physics, is prone to seek precision over generality, fact over possibility. We prefer “farside” over “darkside” because it is more literally true, and at least in my opinion, just as “mysterious” and philosophically satisfying. It is an opinion, and we are all entitled to our own.