The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

First, they use the sun. Just getting rough direction from the sun is easy. It rises somewhere toward the east and sets somewhere toward the west. Getting accurate direction from the sun takes more care. You need to pay attention to the time of the year. Then you need to watch the path of the sun closely at each hour of the day. To tell direction accurately from the sun, a person needs to know the exact time.
To （） directions by the sun.