Moon Question of the Day

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Has the Moon always been the same distance from the Earth?

No. It is drifting away from us, but it is doing so very slowly. It will not be appreciably farther away for hundreds of millions of years.

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My mother told me that on a summer night between 1956 and 1960, she and many others witnessed a full Moon with what appeared to be a crack or stair steps in it. She recalls that it caused mass hysteria. Is there somewhere I could find information on this occurrence?

Outside of your mother's story, there is no report of such a sighting in the general literature. Our guess is that the "crack" was due to some atmospheric phenomenon, either the contrail of an airplane or some unusual clouds crossing the Moon. A local newspaper may have reported the event, but to find that article, the exact location and a much closer determination of time than "between 1956 and 1960" would be necessary.

What exactly is a waning or waxing gibbous or crescent Moon?

First, gibbous refers to the shape you can see when the lighted surface of the Moon is bigger than a crescent shape; the crescent shape is defined by the distinct points on the lighted sliver. The Moon moves in these phases: new Moon, waxing crescent, first quarter Moon, waxing gibbous, full Moon, waning gibbous, last quarter Moon, waning crescent, back to new Moon. So waxing means the moon is on its way to being full; waning means the Moon is on its way to being new (the phase you really can't see).

Is the distance between Earth and the Moon increasing?

Yes. Because Earth spins slower every day (that's just what spinning objects do), the Moon moves not quite two inches away from it each year. The rate decreases by 1-1/2 thousandths of a second every 100 years -- not enough so that you'd notice, but enough for the Moon's gravitational pull to lessen. This allows its orbit to increase, which results in a greater distance from Earth. Got that?

In south Texas, I saw the largest ring around the Moon I've ever seen. Why so big and does it predict weather?

The ring around the Moon (and sometimes the Sun) is called a halo. The most common halos are caused by hexagonal ice prisms where the light enters and exits through one of the side faces and the halos have a diameter of 44 degrees. A quite rare form of halo is produced by cubic ice crystal or by hexagonal ice prisms where the light goes through the end faces of the prism and this halo has an diameter of 92 degrees. I am sure you saw this 92 degree halo in that clear Texan night-a rare event. In 60 years of observing the nigh sky, I have only seen this twice. As far as I know, it has no weather prognosticating indications.

What causes the distance from Earth to the Moon to vary?

The celestial mechanics of the Moon's motion are very complex. When the distance to the Moon is measured at different times of the month, it is found to vary by more than 10 percent because the Moon's orbit is basically an ellipse, with Earth at one focus. The Moon may come as close as 356,334 kilometers (220,927 miles) to Earth's center and then move as far away from it as 406,610 kilometers (252,098 miles). The dates when the Moon is at apogee (the point in its orbit farthest from Earth) and perigee (the point in its orbit closest to Earth) can be found on each month's calendar page in The Old Farmer's Almanac. However, to understand the "mechanics," as mentioned above, we recommend consulting a basic astronomy textbook.

Is there such a thing as "Moon weather"?

Country wisdom says that the full Moon brings frosts in spring and fall and periods of extreme cold in winter. Researchers have found a striking correlation between the full Moon and cloudiness, rainfall and thunderstorms.

Why do stars twinkle, while the visible planets (and our Moon) shine constantly when viewed?

First, the stars themselves do not twinkle. We are seeing the effect of Earth's atmosphere on the light they produce. The starlight is bent by moving volumes of air in our atmosphere. The bending effect makes the stars appear to be larger than points, to dance around slightly, and to change in intensity, which is what is commonly called twinkling. Planets don't usually appear to twinkle because they are close enough to Earth that they appear as tiny disks of light. The total intensity doesn't seem to change -- hence no twinkling is apparent to the naked eye.

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