Estimating The Time Till Sunset ©

 

From Bushcraft, Scouting & Woodlore: Notes, Vol. I, Figure 351

For the first installment of the Oelslager Files, I decided to discuss how to estimate how many minutes of day light you have remaining until the sun goes down behind the local or apparent horizon.  This is a well-known tip and I have used this wrinkle often over the years.  It is a good tip to know; not only because the time of sunset varies throughout the year, but also because the amount of daylight you have remaining, depends mostly on the terrain that you are in, wooded, hilly or flat.

The science behind this wrinkle is; the Sun stays still and does not move across the sky, the Earth rotates, and the Sun only appears to move or set. Since, the Earth rotates through 360° in every 24-hour period; this apparent movement equals 15° per hour.  Additionally, your hands and fingers are remarkably accurate measuring tools, and when you hold your hand at arm’s length, your four fingers together measure about 6^o or about 60 minutes of time and therefore each finger equals about 15 minutes. 

 

From Brian Ventrudo, “Measuring The Sky”

Dr. R. W. Oeslager wrote that you should hold your hands at arm’s length and count how many fingers fit between the bottom of the sun and the edge of the local horizon, be it a hill, ridge, or edge of a forest.  He also wrote that you should allow 10 minutes of daylight remaining for each finger; other experts calculate approximately 15 minutes per finger.  I believe that the reason for Dr. Oelslager’s estimate of 10 minutes per finger, was that he was primarily writing for Boy Scouts, who would have had smaller fingers than an adult would.  Since each person's finger-size varies, to make sure that this tip produces accurate results for you, experiment at home with a watch, to see if you have 10- or 15-minute fingers.  My fingers give a very good estimate of time, at 15 minutes per finger.

This technique will give you a good estimate of the time remaining until local sunset, particularly if you are near the equator.  However, nearer the poles, you might have more time before sun goes down, below the local horizon, than you would estimate if you were closer to the equator. 

In any case, you should always set up or start back to camp, or if you are misplaced, start building a shelter and a fire, before what woodsman call the “big dark”, when there are eight fingers or about two hours of daylight remaining.  This is similar to Emerson Hough’s First Strict Camp Rule for hunters who were out during the late fall or winter, which states, “…start home at half past two or three in the afternoon.”  By returning to or building you camp, while you still have two hours of daylight remaining, you will reduce your chance of having an accidental overnight adventure or injuring yourself as you make an overnight trek through the wilderness.

Sources:

“Calculating Sunset Time With Your Fingers”, http://awesci.com/calculating-sunset-time-with-your-fingers/, (accessed 11/27/18)

Loring Chien, “How fast does the sun move across the sky?”, January 15, 2018, https://www.quora.com/How-fast-does-the-sun-move-across-the-sky, (accessed 11/27/18)

Emerson Hough, Out Of Doors, (D. Appleton and Company, New York, New York [1915]) p. 269 - 282, reprinted in https://books.googleusercontent.com/books/content?req=AKW5QadnZNQ1xrsbkS3T4VJlC_uZQ8t4yj2UNH5POGbNio-1SDyFM8TAqUfl7ciDY5CuNZxvnnE0Pcxf1j6Thu_zOs5kz8rgR9MOtsbTxVgRVa3bwaKo0-D5YX2T4JaBO_z6G8vgwFNgLiJ9XCfBVnn9K4qR7ZFa0faNe3zmvfWDfmEo049sqXQlw697XIHDWNblRJj9niiabZoBB7aQHl82tf4y03cpU2hMdLVADF-7P2pvJKsZbLDnJN8ncLZ9IftMudapmn8E, (accessed 11/8/18)

Dr. Robert. W. Oelslager, Bushcraft, Scouting & Woodlore: Notes, Vol. I, Privately Published

Susan Newquist, “Let Your Fingers Tell Time”, July 31, 2000, https://www.backpacker.com/skills/let-your-fingers-tell-time, (accessed 11/27/18)

Brian Ventrudo, “Measuring The Sky”, April 19, 2009, https://oneminuteastronomer.com/860/measuring-sky/, (accessed 11/27/18)