Fall Fallacies

By Larry Sessions*

Each season has its own myths and misperceptions, and Fall is no exception. Here are just a few:

The Balancing Egg

A novel way of balancing an egg.
Here's one that never seems to go away. It comes up every Equinox (Spring and Fall), and people swear that it is true.

There is no question that with the right egg and enough talent, some people can balance an egg on one end at the time of the Equinox. Although difficult, it can be done, usually with the larger end of the egg.

So what? The ability to balance an egg on one end is not dependent on the date, or season, or any natural arrangement of gravitational forces. Any inability to do the same thing on any other day -- say November 19 or May 7 or July 4 -- is the result of a mental block, not physics.

The gravitational force of the Sun, at the distance of the Earth, varies by only about three percent throughout the entire year. The strength of that force on a two-ounce egg is equivalent to only about one-thousandth of an ounce. Although the Moon’s gravitational influence varies more because of its changing distance, it amounts to the equivalent of only about six millionths of an ounce!

The Earth’s pull on the egg is nearly 300,000 times greater than that of the Moon, and more than 1600 times that of the Sun. The very minor variations in the gravitational pull of either the Sun or Moon will not affect the ability to balance the egg. Instead, the ability to balance an egg depends on the physical characteristics of the particular egg and the talent of the human attempting the trick, not on any unusual astronomical circumstance.

Is it Fall or Spring?

At the time of the Equinoxes, the Sun shines down directly on the Equator and, theoretically at least, every location on Earth receives 12 hours of sunshine and 12 hours of night. By the beginning of Northern Winter, however, the Sun shines obliquely on the Northern Hemisphere and more directly on the Southern.

The September Equinox marks the beginning of Fall, or Spring, depending on where you live. Technically, the Equinox simply marks a point in the Earth's orbit. At the time of the Equinox, the Earth tilts neither toward nor away from the Sun, and thus every location on Earth theoretically receives 12 hours of sunshine and 12 hours of dark.

After the Equinox, the Northern Hemisphere tilts farther and farther away from the Sun. At the same time, the Southern Hemisphere tilts ever sunward with each succeeding day. As a result, the Northern Hemisphere while the Southern Hemisphere heats up. It's a bit like a seesaw, when it is Spring (or Summer) in one hemisphere, it is Fall (or Winter) in the other.

Some people believe that the September Equinox must begin each year on the same day. But that isn't true. Sometimes it is on September 22, sometimes on September 23, and on very rare occasions on September 21 or 24. These changes are due to the fact that the Earth's orbit very slowly changes over time, and because the year is not exactly 365 days long. There is an extra one-quarter day each orbit, which adds up to an extra day we observe every four years as leap day. This causes the exact moment of the Equinoxes to vary slightly from year to year.

You may also find that some calendars report one day for the Equinox, some the next day. This is simply because of time zones. The time of the Equinox usually is reported in Universal Time (UT, Greenwich, England), so if it occurs shortly after midnight UT, the event will be on the preceding day in North America. If it occurs shortly before midnight UT, it will be the following day in Australia and Japan. This year the Equinox is at 11:56 p.m. (UT) or about 5:56 p.m. (MDT) in Denver on September 22. So if you live in Australia, the Equinox will be on September 23, and Spring will begin in the Southern Hemisphere!

The Harvest Moon

The Harvest Moon is defined as the Full Moon that occurs nearest the September Equinox. That's for the Northern Hemisphere. However, in the Southern Hemisphere, a similar situation occurs near the March Equinox. (I'd be happy to hear from any Australian reader who can tell me if there is a special name for this March or April Full Moon Down Under.)

The Sun, Moon and Planets seem to move in similar areas across the sky. The line along which they move is called the Ecliptic. The Sun moves exactly along the ecliptic, whereas the Moon is just slightly off. The angle of the ecliptic in the sky is different at different times of year. That is why the Sun is high in the Summer and low in the Winter. At about the time of the Harvest Moon, the ecliptic makes a shallow angle with the eastern horizon, and the Moon appears to rise very slowly each evening for several days. In addition, it rises only about 25 minutes later each evening for several days. (Through the course of the year, on average the Moon rises about 50 minutes later each evening.)

Thus, at the time of the Harvest Moon, the Moon rises at or just after sunset, and appears to be hanging near the eastern horizon longer than usual. However, the Moon is really moving at its regular rate, and the Harvest Moon effect is really just a result of the Earth's motion around the Sun. The Harvest Moon really isn't there to help farmers bring in their crops, although that was the old story. A similar, though less pronounced effect occurs the following month during Hunter's Moon.

The Full Moon Effect

It is an undeniable fact that the rising Full Moon looks larger than the Full Moon high overhead. This so-called Full Moon Effect can happen during any Full Moon, but perhaps because more people are likely to observe it, this effect is especially prominent during the Harvest Moon in the Fall.

But it also is an undeniable fact that the Moon is not any larger near the horizon -- it just looks that way! Astronomers have measured it in both positions, and there is no easily perceptible difference. There is a slight difference however, but it is the other way around. A quick look at the geometry of the situation shows that the Moon is several thousand miles closer to an observer when it is overhead versus when it is rising. It can be up to 1/60th larger when it is overhead! Yet it does, indeed, look smaller overhead.

When the Moon is on the horizon it is almost 4,000 miles farther away than when it is high overhead. Thus, it should look larger overhead rather than when rising. But our eyes tell us otherwise.

There have been many attempts to explain this phenomenon. Some have suggested that the refractive or bending effects of the Earth's atmosphere, which does bend light around the horizon slightly, might somehow magnify the Moon's image. But the truth is that it doesn't.

The most widely accepted solution is that the Full Moon Effect is a psycho physiological "slight of eye." In other words, the brain is misinterpreting what the eye sees.

When the Moon is near the horizon, there are trees, landmarks and buildings with which to compare it. Because these early objects are typically far away, they look small compared to the Moon. Thus, by contrast, the Moon looks large.

On the other hand, when the Moon is high overhead there is nothing with which to compare it. It appears small and lost in the blackness of the sky. It would be hard, however, to convince some people of this!

Larry Sessions is an astronomer and editor of two online journals, Family Explorer and North American Skies. His previous article for shunpiking was "Every breath you take, every drop you drink: reflections on our interconnections", June/July, 1998. He lives in Denver, Colorado.