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The San Diego Union-Tribune

Questions answered

November 9, 2006

QUESTION: What is the feedback mechanism that keeps one face of the moon pointed at the Earth? Somebody told me it's the ocean tides.

– George Marr,

ANSWER: The moon rotates once on its axis in the time it takes to revolve once around Earth. Therefore, the same face of the moon is always pointed toward Earth. It is not a strange coincidence that the moon's rotation and revolution are synchronized. Most of the other moons in the solar system are likewise synchronized with their planets.

The synchrony has to do with tides, or at least the same forces that cause tides. The gravitational tug of the Earth warps the moon, creating a tidal bulge on the side facing Earth. Billions of years ago, when the moon rotated more quickly than it does now, the tidal bulge would get ahead of the Earth-moon line. But the Earth would tug back on the offset bulge, slowing the moon's rotation.

Over millions of years, these forces synchronized the moon's revolution and rotation so that now one side is always facing in our direction and one side is always facing away from us. The side facing away from us has sometimes been dubbed “the dark side of the moon,” but that is a misnomer. It gets the same amount of sunlight as the side facing Earth. When we have a dark “new moon,” the other side of the moon is fully illuminated.

Over time, we actually see slightly more than half of the moon's surface because rotation and revolution are not constantly synchronous. The moon's revolution speed changes depending on where it is in its elliptical orbit. Revolution slows when the moon is farthest from Earth and speeds up when the moon is closest to Earth. So rotation gets a bit behind or ahead, causing a slight change in what sections of the sphere face our direction.

How is it that hair gets bleached by the sun?

– Emily Kloth,
El Cajon

The pigment melanin gives hair its color. It is deposited within the hair by cells called melanocytes, which are found in the root of the hair. The two main types of melanin are eumelanin (brown-black) and pheomelanin (yellow-red). On average, 3 percent of a hair's weight is melanin.

Sunlight bleaches hair because it degrades melanin. Melanin absorbs some sunlight and also captures free radicals produced when UV radiation falls on other proteins in hair. In doing so, melanin helps protect other hair proteins, but melanin itself is damaged and loses its color.

Sherry Seethaler has a doctorate in science education and is a UCSD science writer and educator. Send your scientific questions to her at Quest, San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail Please include your name, city of residence and phone number.

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