On Sept. 22 at 9:55 p.m., the autumnal equinox, the sun crosses the equator on its way south for the winter. It's the time of year when several silly customs and myths resurface.

Despite pronouncements that the equinox is the ``official'' beginning of autumn, this is only an astronomical event and does not necessarily mean the end of warm weather or the beginning of the rainy season. In fact, the warmest month in San Francisco is September; the summer months of June, July and August are kept cool by low clouds and fog there.

In San Jose, the warmest months are July and August, followed by September -- not June.

And, to the best of my knowledge, there is no agency or government that can officially declare a season.

It has been a while since I've seen it publicly attempted, but the notion persists that you can balance an egg on its end only on the equinox. However, there is no scientific basis for this myth; all you need to balance a raw egg is a hard, flat surface and a steady hand. It will work any day of the year. A good explanation is on the Bad Astronomy Page, www.badastronomy.com/bad/misc/ egg_spin.html.

Twice a year around the equinoxes, I get questions about why there are not exactly 12 hours between sunrise and sunset.

The answer has to do with the fact that the equinox is an astronomical event, calculated relative to the geometric center of the sun. In contrast, sunrise is calculated from the time the upper edge of the sun becomes visible, and sunset is when the upper edge disappears. The date when day and night are of equal length is several days after the autumnal equinox and several days before the spring equinox.

So all things being equinox, don't worry about finding a raw egg or checking the sunrise and sunset tables a week from Sunday.

Q On the first days of spring and fall, do people all over the Earth see the sun come up due east and set due west, regardless of their latitude? I think they do, but a friend says no way. Bob Phares - Denver, Colo.

A The sun does indeed rise from due east and set at due west on the equinoxes. You can prove this to your friend using the following Web site of the U.S. Naval Observatory at http://aa.usno.navy.mil/data/docs/AltAz.html#formb.

Plug in the date of this year's fall equinox, Sept. 22, and check various latitudes. The Web site will give you the altitude of the sun and its direction in the sky, or azimuth. You will see that when the altitude is zero -- that is, when the sun is at the horizon -- its azimuth is 90 degrees for sunrise and 270 degrees for sunset regardless of latitude.

Q I have heard the phrase ``greenhouse gases'' frequently -- usually in some kind of political speech regarding auto emissions -- but I have never heard or read a definition of it. I know that carbon dioxide is one of the gases. Can you tell your readers what ``greenhouse gases'' really are? George Fraine - San Jose

A Actually, the term ``greenhouse gases'' covers a broad spectrum of gases including water vapor, carbon dioxide, ozone, methane, nitrous oxide and chlorofluorocarbons. These gases are fairly transparent to the short wavelengths of incoming solar energy, but efficiently absorb outgoing, long-wave radiation. This means they trap warmth that is radiated by the Earth out into space, warming the atmosphere. If all other factors remain the same, an increase in the levels of greenhouse gases means a warmer atmosphere.

Q I have wondered for a long time what holds clouds up. They are often flat on the bottom and fluffy on top. They just seem to float at a particular elevation. I'm guessing they are at an interface between temperature and dew point: The air above is cold enough to precipitate moisture and the air below is warm enough to hold it in the air as humidity. What makes them fluffy on top? John Herren - Gilroy

A You are on the right track in thinking that dew point is a factor. This is the temperature at which air is cool enough that the water vapor in it condenses into tiny liquid particles. Clouds form as air rises and cools to the dew point, which is also known as the Lifting Condensation Level or LCL. The LCL is normally at a fairly uniform level, thus the flat bottoms that clouds typically have. However, the updrafts that help create clouds are not uniform, and consequently the areas with stronger updrafts will have higher ``towers.'' As a cloud grows in height, it draws in drier air from its surroundings and the edges of the cloud erode into a ``fluffy'' look.