Moon Question of the Day

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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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Last 7 Days

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?

On December 26, 1978, the Moon had a star resting on its tip. What was this called and how often does it happen?

In the morning hours of December 26, 1978, Venus was totally obscured by the Moon, depending upon where you were standing. From some angles, it appeared the planet was resting on the Moon. Such a phenomena is called a "lunar occultation."

Before gravity was understood, what did the ancients think caused tides?

Some people explained the motion of the planets by assuming that the planets were being pushed by angels. The same may have been true for tides. In other words, the explanation was found in supernatural forces. Since the Mediterranean Sea is very little affected by the tides, the ancient Greeks and Romans paid very little attention to it. Plutarch and Aristotle recognized the Moon as the cause of the tides, without understand gravitational pull. Pliny believed that the Moon had an attraction which caused the tides. Other scientists, like Ptolemy, settled just for a description without searching for a cause.

What and when is the feminine side of the Moon?

The Moon has neither a feminine nor a masculine side. Your idea probably comes from the popular interpretation of the Moon's surface features, as seen with the naked eye, as the "Man in the Moon." In modern scientific English, the Sun and the Moon are each spoken of as "it"; only in poetic language is the Sun "he" and the Moon "she." As far as we know, in all other western languages, including scientific use, the Sun is masculine and the Moon is feminine. The interesting exception is German, in which the word Moon uses a masculine article and Sun uses a feminine article.

What do the phrases "Moon rides high" and "Moon runs low" in the Almanac mean?

As the equator wraps around Earth, so the celestial equator wraps around the sky. It runs in an arc from east to west, passing low in the south; the farther north one stands, the lower it passes. When the Moon runs low in the sky, it is the farthest beneath this equator. If you live sufficiently far north, it will never rise above the horizon at night. Similarly, the Moon will be unusually high in the sky when it rides high.

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.

Does the Moon rise and set as the Sun rises in the east and sets in the west? The enormous full Moon always takes me by surprise, and I'd love to get some shots of a big yellow Moon. Is it possible to predict the dates and times when the Moon will appear largest over the horizon?

Basically, the Moon rises in the east and sets in the west, but its path varies during the month because its orbit is elliptical. However, you will never see a rising Moon in the west or a setting Moon in the east. The time to see a huge Moon is in its full phase and when it is close to the horizon. If you check the right-hand calendar pages in the print Almanac, you will see the notation "Moon runs low"; this is when it is closest to the horizon. Check the date for each such notation, then check the left-hand calendar page for the Moon's phase and the time of its rising and setting for that date.

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