Every find yourself caught off guard by the late hour in the day, typically in the changing of the seasons when the sun starts going down early. In these times predicting sunset can be important in deciding to settle down for the evening, or charge ahead to the next campsite.
By using your fingers at arms length, one can roughly guess at the time until sunset. At arms length, and your fingers together. See how many fingers the sun is above the horizon. Each finger represents roughly 15 minutes. So if you can put 5 finger widths between the horizon and the sun you have about an hour and fifteen minutes until the sun goes down.
Below you see the imminent arrival of sunset, not even a finger width will fit, meaning less than 15 minutes away.