Braille Monitor                                                 June 2012

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A Reverse Sundial

by Father Ephraim

From the Editor: Father Ephraim is a Greek Orthodox priest and monk who has become interested in Braille and who read 7,000 Braille pages last year. Since he has come to understand the importance of effective orientation for blind travelers, he sent us the following interesting little article that those who were not born with that amazing instinct for always knowing which way north is will find useful. I had to read it several times before mastering the technique, so don’t get discouraged. This is what he says:

Knowing which way is north can be very helpful when navigating in unfamiliar places. An easy trick to determine your orientation is to point the hour hand of your watch at the sun, and south will be halfway between it and 12:00 noon on your watch (or halfway between it and 1:00 when on daylight savings time). This method works even without a watch, as long as you know roughly what time it is so that you can imagine where the hour hand of a watch would be. It is a reverse sundial because, instead of determining the time using the sun's position and a dial aligned north, north is determined using the sun's position and the time.

In the Southern hemisphere this method is inverted: Before pointing the hour hand of your watch at the sun, you need to flip your watch upside-down, so that the back side of your watch (the side usually touching your skin) is facing you. Then north (not south) will be halfway between the hour hand and 12:00 on your watch.

This method is accurate enough for most practical purposes (except near the equator or when the sun is nearly directly overhead). One way to improve its accuracy is to adjust the calculation based on your longitude. Instead of finding the halfway point between 12:00 and the hour hand of the current time, subtract from the current time four minutes for every degree longitude west you are located from the central meridian of your time zone. The central meridian in most places around the world is a multiple of fifteen (because there are twenty-four time zones in 360 degrees around the globe). For example, if you are in Tucson, Arizona, the longitude is 111 degrees west. This is six degrees west of the central meridian of the Mountain Time Zone, which is located at 105 degrees west (a multiple of fifteen). Six times four is twenty-four, so to determine south in Tucson it is necessary to imagine where the hour hand would be after subtracting twenty-four minutes from the current time, and then find the halfway point between that imaginary hour hand and 12:00. If you are east of your time zone's central meridian, you add (instead of subtract) four minutes for every degree longitude east of it you are located. In most locations in the world, however, the benefit from including this adjustment is negligible and therefore can be omitted for simplicity's sake.

A simple non-visual technique to find the sun's position with precision on a sunny day is to rotate until you feel the sun on your face. Then cover your face with your palm and gradually move it away from your face in the direction necessary to keep its cool shadow on your face. Once your arm is fully extended with your palm's shadow still on your face, you will know quite accurately where the sun is.

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