Thursday, November 26, 2009

Gibbous Moon

Note: There will be no December 3 post. Enjoy the November 26 post or browse my older posts. I'll be back with a new post on December 10.

The night sky is dominated right now by a waxing gibbous Moon, that is, a Moon that is more than half illuminated and that is growing in percent illuminated. Put another way, it is the phase of the Moon that occurs between First Quarter (also known as “Half Moon”) and Full Moon.

Once Full Moon occurs, our nearest celestial neighbor will enter its waning gibbous phase, when it is more than half illuminated and shrinking in percent illuminated. In other words, the waning gibbous Moon is the phase that occurs between Full Moon and Last Quarter (the other “Half Moon” phase).

If you’re curious about the etymology of these terms, as I was, you’ll find the following of interest.

“Wax” means to increase in size or intensity, and comes from the Old English weaxan, meaning to increase. “Wane” means to diminish in size or intensity, and comes down to us from the Old English wan, meaning deficient, and the Latin vanus, meaning empty. Finally, “gibbous” means marked by swelling, or humpbacked, and comes from the Latin gibbus, meaning hump.

Phases of the Moon

So if the Moon appears humpbacked, it is gibbous. If the western--or right--side of the humpbacked Moon is illuminated, it’s waxing gibbous. If the eastern--or left--side of the swollen Moon is illuminated, it’s waning gibbous.

And if the slithy toves gyre and gimble in the wabe, it's plain gibberish.

Astronomy Essential: Stars do not twinkle.

Stars sometimes appear to blink on and off rapidly, dim and brighten wildly, and scintillate in a variety of colors. This “twinkling” is merely an optical distortion caused by the turbulence of Earth’s atmosphere, through which starlight must travel to reach our eyes. The starlight is “bent” in many random directions as it travels through atmospheric layers and pockets of different density and temperature. Our eyes interpret this bending or refraction of the light as twinkling.

This effect is usually most pronounced near the horizon, because the light from a star near the horizon must travel a longer path through the atmosphere to reach our position than it would if the star were positioned directly above us.

The next time you notice stars near the horizon twinkling, check the stars directly above you to see if they are in fact twinkling less or not twinkling at all.

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