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  3. For 150 years scientists have tried to determine the solar constant, the amount of solar energy that reaches the Earth. Yet, eve

For 150 years scientists have tried to determine the solar constant, the amount of solar energy that reaches the Earth. Yet, eve

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问题    For 150 years scientists have tried to determine the solar constant, the amount of solar energy that reaches the Earth. Yet, even in the most cloud-free regions of the planet, the solar constant cannot be measured precisely. Gas molecules and dust particles in the atmosphere absorb and scatter sunlight and prevent some wavelengths of the light from ever reaching the ground.
   With the advent of satellites, however, scientists have finally been able to measure the Sun’s output without being impeded by the Earth’s atmosphere. Solar Max, a satellite from the National Aeronautics and Space Administration (NASA), has been measuring the Sun’s output since February 1980. Although a malfunction in the satellite’s control system limited its observation for a few years, the satellite was repaired in orbit by astronauts from the space shuffle in 1984. Max’s observations indicate that the solar constant is not really constant after all.
   The satellite’s instruments have detected frequent, small variations in the Sun’s energy output, generally amounting to no more than 0. 05 percent of the Sun’s mean energy output and lasting from a few days to a few weeks. Scientists believe these fluctuations coincide with the appearance and disappearance of large groups of sunspots on the Sun’s disk. Sunspots are relatively dark regions on the Sun’s surface that have strong magnetic fields and a temperature about 2, 000 degrees Fahrenheit cooler than the rest of the Sun’s surface. Particularly large fluctuations in the solar constant have coincided with sightings of large sunspot groups. In 1980, for example, Solar Max’s instruments registered a 0. 3 percent drop in the solar energy reaching the Earth. At that time a sunspot group covered about 0. 6 percent of the solar disk, an area 20 times larger than the Earth’s surface.
   Long-term variations in the solar constant are more difficult to determine. Although Solar Max’s data have indicated a slow and steady decline in the Sun’s output. Some scientists have thought that the satellite’s aging detectors might have become less sensitive over the years, thus falsely indicating a drop in the solar constant. This possibility was dismissed, however, by comparing solar Max’s observations with data from a similar instrument operating on NASA’s Nimbus 7 weather satellite since 1978.
Why is it not possible to measure the solar constant accurately without a satellite?
选项 A、The Earth is too far from the Sun.
B、Some areas on Earth receive more solar energy than others.
C、There is not enough sunlight during the day.
D、The Earth’s atmosphere interferes with the sunlight.
答案D
解析 答案在第一段最后一句话“Gas molecules and dust particles in the atmosphere absorb and scatter sunlight and prevent some wavelengths of the light from over reaching the ground．”即便是在地球上云层最少的地方，也很难测量准确，因为除了云层，还有气分子和灰尘粒子会影响阳光到达地球的量，因而影响常数测量。所以是地球大气层干扰了阳光的传播，答案选D。
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