19-QT2. How long does twilight last on the moon? (Hint: The moon has no atmosphere.)
19-QT4. What would be the color of the sky if air molecules scattered the longest wavelengths of visible light and passed the shorter wavelengths straight through? (Use a diagram to help explain your answer.)Since the moon has no atmosphere and therefore no scattering of incident sunlight, darkness occurs instantly upon sunset. There is no twilight.
| Under these circumstances the red end of the visible spectrum would be scattered out of sunlight while the blue end would persist in rays coming directly from the sun. Hence the sun would appear blue-white and the sky would be colored red as shown in the accompanying diagram. |
19-QT7. Why are rainbows seldom observed at noon?
19-QT9. During a lunar eclipse, the earth, sun and moon are aligned as shown in Fig. 19.37. The earth blocks sunlight from directly reaching the moon's surface, yet the surface of the moon will often appear a pale red color during a lunar eclipse. How can you account for this phenomenon?A rainbow is a circle of 42° radius centered on the anti-solar point. At noon the sun is usually more than 42° above the horizon and therefore a noontime rainbow is entirely beneath the horizon and unseen (since only those few drops of rain between the observer and the ground would contribute to the rainbow, resulting in an unnoticeably feeble rainbow). Only in those months including and near the winter solstice is the noontime sun low enough that part of the noon rainbow is above the horizon and therefore visible.
19-QT11 During Ernest Shackleton's last expedition to Antarctica, on May 8, 1915, seven days after the sun had set for the winter, he saw the sun reappear. Explain how this event (called the Novaya Zemlya effect) can occur.The earth's atmosphere refracts sunlight about its outer edge, thus the moon is bathed in light similar to the sunset light on earth. A person on the moon at such a time would see the earth as a skinny, red "doughnut". The red light of the "doughnut" would be the light which produces the ruddy illumination of the moon.
19-QT13. Explain why it is easier to get sunburned on a high mountain than in the valley below. (The answer is not that you are closer to the sun on top of the mountain.)Shackleton witnessed a superior mirage elevating the apparent position of the sun from its true position below the horizon to an apparent position above the horizon. Such an effect can occur when a cold layer of surface air has a considerably warmer layer of air suspended above it.
A sunburn is produced by the action of near ultraviolet rays on the skin. Near ultraviolet rays are partially absorbed by air, hence the more air they pass through the weaker they are. Therefore, on a mountain top, the overhead air mass is smaller, the absorption of ultraviolet radiation is less, the surviving ultraviolet radiation is stronger and the sunburning produced by that radiation is greater.
