In June 1885, ghostly, silver-blue clouds began to appear early at night in the Northern Hemisphere. They were seen several more times in the following decades. However, in recent decades they have appeared frequently, apparently indicating a major change in Earth’s atmosphere.

How can these clouds still be lit up by sunlight after sunset?

The answer is in this chapter.

1-1  What Is Physics?

Science and engineering are based on measurements and comparisons. Thus, we need rules about how things are measured and compared, and we need experiments to establish the units for those measurements and comparisons. One purpose of physics (and engineering) is to design and conduct those experiments.

For example, physicists strive to develop clocks of extreme accuracy so that any time or time interval can be precisely determined and compared. You may wonder whether such accuracy is actually needed or worth the effort. Here is one example of the worth: Without clocks of extreme accuracy, the Global Positioning System (GPS) that is now vital to worldwide navigation would be useless.

1-2  Measuring Things

We discover physics by learning how to measure the quantities involved in physics. Among these quantities are length, time, mass, temperature, pressure, and electric current.

We measure each ...

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