David Francis Website: AskANerd: When is a full moon the fullest?

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AskANerd: October 18, 2000

Dolores of Goshen, IN asks:

If the moon is full today, was the moon I saw going down as I went in to work this morning full, or does it wait until the moon comes up before it is full? I don't know if that is an askanerd question or a religious/philisophical question! I do know that when Moslems celebrate Ramadam (sp?), it ends when the full moon comes up over the horizon. The dj on the radio this evening called this a hunter's moon, but I always thought October's was the harvest moon and November's the hunter's moon.

The Nerd Responds:

An interesting question! The answer might be subjective depending on what you consider to be a truly full moon. If we look at it from an entirely astronomical perspective, a full moon occurs at percisely the moment when the moon, earth, and sun line up exactly with the earth between the other two. At exactly this point in time, observers on earth can see the full half of the moon which is being illuminated by the sun. Since the earth and moon are always in motion, this only occurs for an instant and then they are out of synch again. The charts I have seen say that the moon and earth were in line for this full moon (10/13/2000) at exactly 8:54am Greenwich Time, so I think you could make a good argument that the moon you saw setting that morning was "fuller" than the one that rose in the early evening! After all, if it was at its fullest at 9am UTC (that is 3am Goshen time), then at 6am as it was setting, it was only 3 hours waning from full, whereas at 6pm when it rose again, it had been waning from full for 12 hours! So, although the visual difference in fullness is probably very slight, the moon you saw going down was the "real" full moon,

Figure 1

This might surprise people, but you should remember that moon-rise and moon-set have nothing to do with the actual "fullness" of the moon. That is determined purely by the orbits of the earth and moon. On the other hand, when we see the moon rise and set is almost entirely based on the rotation of the earth. Take a look at Figure 1 which is the sun, earth, moon system looking down on the north pole from high above the solar system. As you can see the earth, moon, and sun are lined up so that we can see all of the hemisphere of the moon that faces us. Click the button below the figure to move your way through the phases. The orange section of the moon are areas that are illuminated by the sun, but because of the moon's curvature, we can't see. Now clearly when the sun is on one side of our planet, and the moon on the other is when we will see the moon as full. From the view of an observer on earth, this is when the sun is 180 degrees away from the moon in the sky. Take a look at Figure 2 to see the earth view of a full moon rising above the horizon in the east. As you can see the the sun is 180 degrees across the sky setting. For this reason, full moons always spend the entire night in the sky and then set just as the sun is rising. A quarter moon on the other hand rises at noon and sets at midnight, so it is really only visible for half the night.

Figure 2

A full moon is registered on a calendar by figuring out on what night the moon stays in the evening sky the longest. So even though you technically saw a full moon setting that morning, the moon that came up that evening stays in the sky longer than the moon did the night before, and so it gets called the full moon.

As for the difference between the Harvest Moon and the Hunter's Moon, it is pretty easy to remember. The Harvest Moon is the full moon occuring closest to the autuminal equinox. This year, there was a full moon on September 13th, ten days before the equinox so that was the Harvest Moon. The Hunter's Moon is the next full moon occuring after the Harvest moon. This year that was October 13th, so the DJ you heard was correct.

If you have ever wondered why these moons seem so impressive, it is because of two main factors.

On the average, the moon rises each night about 50 minutes later than it did the night before. That is the average, but around the autuminal equinox, the moon rises only about 10 minutes later each night (as opposed to around the vernal equinox when it rises almost 90 minutes later than the previous night). So that means that full moons around this time spend more time in the sky than any other full moon.
During this time of year, the moon rises at a very shallow angle. That means that it spends much more time low in the sky. Close to the horizon, where we can compare it to objects on the ground, the moon appears much bigger than it normally does when it is high in the sky. Also, since the moon's light travels through more atmosphere, refraction caused by particles in the air can cause the moon to seem bigger than it normally does.

The combined effects of these two factors make for bright evenings in the autumn good for harvesting and hunting--or anything else you might want to do on a crisp October evening.