SciTech

Why does a ‘first-quarter’ moon show half of it?

The half-illuminated moon, often called a half moon, is a quarter of the way along its cycle from new moon (dark) to crescent moon to full moon (fully illuminated) and back again.
The half-illuminated moon, often called a half moon, is a quarter of the way along its cycle from new moon (dark) to crescent moon to full moon (fully illuminated) and back again. NASA

Q: On the weather page, I saw a “first-quarter moon” depicted as a moon with a vertical slice off the left side. I had always thought of this as a half moon. If it is a quarter moon, is there such a thing as a half moon?

A: The half-illuminated disk of the moon that we see and often refer to informally as a half moon is a quarter of the way along its journey in time from new moon (dark) to crescent moon to full moon (fully illuminated) and back again. It is also a quarter of the way along the moon’s journey in space as it orbits Earth. For that reason, modern astronomers refer to this phase as the first-quarter moon. They also refer to a later “half moon” phase, seen with the vertical slice off the right side, as the third-quarter moon. (The sides are reversed when viewed from the Southern Hemisphere.)

“Half moon” was a familiar term to poets like Shakespeare and to the Dutchmen who called Henry Hudson’s ship the Halve Maen when it sailed into New York Harbor in 1609. But to confuse matters further, such older sources were often referring to the crescent moon, either waxing or waning.

In reality, of course, exactly half the surface of the moon is illuminated by the sun at all times. What waxes and wanes is not the moon itself but the part of the illuminated side that can be seen by observers on Earth, owing to the changing angles formed by the sun, Earth and the moon.

When an eardrum doesn’t work

Q: Can you hear without an intact eardrum?

A: “When the eardrum is not intact, there is usually some degree of hearing loss until it heals,” said Dr. Ashutosh Kacker, an ear, nose and throat specialist at NewYork-Presbyterian Hospital and a professor at Weill Cornell Medical College, “but depending on the size of the hole, you may still be able to hear almost normally.”

Typically, Kacker said, the larger an eardrum perforation is, the more severe the hearing loss it will cause.

The eardrum, or tympanic membrane, is a thin, cone-shaped, pearly gray tissue separating the outer and middle ear canals, he explained. Soundwaves hit the eardrum, which in turn vibrates the bones of the middle ear. The bones pass the vibration to the cochlea, which leads to a signal cascade culminating in the sound being processed by the brain and being heard.

There are several ways an eardrum can be ruptured, Kacker said, including trauma, exposure to sudden or very loud noises, foreign objects inserted deeply into the ear canal, and middle-ear infection.

“Usually, the hole will heal by itself and hearing will improve within about two weeks to a few months, especially in cases where the hole is small,” he said. Sometimes, when the hole is larger or does not heal well, surgery will be required to repair the eardrum. Most such operations are done by placing a patch over the hole to allow it to heal, and the surgery is usually very successful in restoring hearing, Kacker said.

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