The photo to the right shows a specialized 24-hour watch optimized for finding directions using the sun. With the watch set to indicate local time, the hour hand is pointed directly at the sun. South is indicated by the local noon position (in the northern hemisphere

Northern Hemisphere

The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

).

In the southern hemisphere

Southern Hemisphere

The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

, one must either use a left-handed watch (where the hands run counterclockwise), or the watch must be positioned so the sun is in the direction of the reflection

Reflection (mathematics)

In mathematics, a reflection is a mapping from a Euclidean space to itself that is an isometry with a hyperplane as set of fixed points; this set is called the axis or plane of reflection. The image of a figure by a reflection is its mirror image in the axis or plane of reflection...

 of the hour hand against the dial's vertical (12-24) axis. Holding the watch upside down, while awkward, will achieve the same result. The noon position then indicates north, not south.

Technically, any 24-hour watch will do; there is nothing special about the direction-finding watch except that the 6 o'clock, 12 o'clock, 18 o'clock and 24 o'clock positions are labeled east

East

East is a noun, adjective, or adverb indicating direction or geography.East is one of the four cardinal directions or compass points. It is the opposite of west and is perpendicular to north and south.By convention, the right side of a map is east....

, south

South

South is a noun, adjective, or adverb indicating direction or geography.South is one of the four cardinal directions or compass points. It is the opposite of north and is perpendicular to east and west.By convention, the bottom side of a map is south....

, west

West

West is a noun, adjective, or adverb indicating direction or geography.West is one of the four cardinal directions or compass points. It is the opposite of east and is perpendicular to north and south.By convention, the left side of a map is west....

 and north

North

North is a noun, adjective, or adverb indicating direction or geography.North is one of the four cardinal directions or compass points. It is the opposite of south and is perpendicular to east and west.By convention, the top side of a map is north....

, respectively.

Finding South With a 12-Hour Watch

The approximate south (or north, in the southern hemisphere) can be found fairly easily with any wrist watch or pocket watch

Pocket watch

A pocket watch is a watch that is made to be carried in a pocket, as opposed to a wristwatch, which is strapped to the wrist. They were the most common type of watch from their development in the 16th century until wristwatches became popular after World War I during which a transitional design,...

 with a 12-hour

12-hour clock

The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

 analog face.

In the northern hemisphere, the watch is held or laid flat so that the direction of the sun bisects the acute angle between the hour hand and 12 o'clock. For instance, at eight in the morning, the sun should be in the 10 o'clock position, and at two in the afternoon, it should be in the 1 o'clock position. The 12 o'clock position on the dial indicates south.

In the southern hemisphere, the watch is held or laid flat so that the direction of the sun bisects the reflex angle between the hour hand and 12 o'clock. For instance, at eight in the morning, the sun should be in the 4 o'clock position, and at two in the afternoon, it should be in the 7 o'clock position. The 12 o'clock position on the dial indicates north.

Accuracy

With both methods, the accuracy depends on a number of factors:

• Accurate determination of the position of the sun, which will vary with weather conditions and visual obstructions (landscape, buildings).
• Accurate positioning of the watch, which amongst other factors depends on the size of the dial, or rather the length of the hour hand, and the surface on which the watch is placed.
• In most parts of the world that observe Daylight saving time
  Daylight saving time
  Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...
  
  , the sun's position at noon will be offset by 15 degrees in the summer.
• The watch's accuracy relative to local solar time
  Solar time
  Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...
  
  , which may differ significantly from local apparent time. For instance, the easternmost parts of Norway
  Norway
  Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
  
   and the westernmost parts of Spain
  Spain
  Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
  
   are both in the same time zone
  Time zone
  A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
  
   (Central European Time
  Central European Time
  Central European Time , used in most parts of the European Union, is a standard time that is 1 hour ahead of Coordinated Universal Time . The time offset from UTC can be written as +01:00...
  
   or Central European Summer Time
  Central European Summer Time
  Central European Summer Time is one of the names of the Daylight saving time offset using the UTC offset of UTC+02:00, 2 hours ahead of Coordinated Universal Time. It is used as a summer daylight saving time in most European countries. During the winter, Central European Time is used...
  
  , depending on the time of year), despite being nearly 45 degrees of longitude (three hours) apart, and China
  China
  Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
  
   uses a single time zone across the country, which spans more than 60 degrees of longitude (four hours).

These factors can be mitigated to a certain extent by different means:

• Placing the watch on a perfectly horizontal surface and using the shadow of a plumb line to determine the direction of the sun will, to a great extent, mitigate the first two factors.
• The watch can easily be adjusted to compensate for daylight saving time.
• If the approximate longitude
  Longitude
  Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
  
   is known, it can also (somewhat less easily) be set to the approximate local mean time
  Local mean time
  Local mean time is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude...
  
  .
• Setting the watch to local apparent time, however, requires more advanced computations involving the equation of time
  Equation of time
  The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...
  
  .

The angular diameter

Angular diameter

The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 of the sun is so small (approximately 30 arcminutes) that it makes no difference unless all other sources of inaccuracy are eliminated. The visual angle of the plumb line, if one is used, is likely to be larger.