Answer
Thin air. It lengthens the day.
Work Step by Step
Light travels fastest in vacuum, and thus travels faster in thin air than in thick (higher-pressure) air. In thin air, there are fewer particles for light to interact with, and light travels quickly.
This speed difference produces atmospheric refraction, which makes the sun appear to be higher in the sky when it is near the horizon, which lengthens daylight hours.
This is discussed on page 525, and shown in Figure 28.19.
