After a month, a sundial is no longer accurate. This is due to the Earth's obliquity, which causes the "path" of the Sun to shift throughout time. A sundial cannot be used in two locations at the same time....
A sundial is a device that uses the sun to tell time. Because the shadow formed by the sun on the gnomon is not crisp, precise time is limited to two minutes. The sun is 1/2 degree wide when viewed from Earth, rendering shadows near the periphery hazy. A sundial's actual construction can be exceedingly precise. It may use a gnomon made of wood, metal, or stone. The handle and shaft are usually made of steel or brass, with a glass or plastic disk at the top. The dial itself is painted black so that its shape is visible even under cloudy skies or at night.
The word "gnomon" comes from a Greek word meaning "a needle." This refers to the early sundials which were actually needles stuck into the ground to mark the hours.
The first recorded use of the term "sundial" was in 1665 by Christian Huygens who called it a "gnomonic instrument." The first known patent for a modern sundial was granted to Thomas Graham in 1816. He called his invention a "sun-dial." Modern sundials are based on this design.
Here is how a traditional wooden sundial works. The handle is rotated which moves the gnomon up or down. The closer the gnomon is to the top, the earlier it is in the morning. When it reaches the top, the hand stops and the hour is marked on the dial.
A sundial is a device that uses the apparent location of the sun in the sky to determine the time of day when there is sunlight. For the sundial to be accurate throughout the year, the style must be parallel to the axis of the Earth's rotation. If it is not, then the shadow will appear to move across the surface of the dial at an inconsistent rate, which could lead to errors in determining solar noon and thus the time of day.
The dial can be made out of any material that is transparent to the sun's rays, such as glass or plastic. The hand that displays the current time would therefore have to be made from opaque material so that its shadow does not affect the accuracy of the instrument.
Sundials have been used since ancient times for telling time at sunrise and sunset when clocks were impractical or unavailable. Modern sundials are generally based on the same design elements employed by the ancient version but some modern designs also incorporate electric lights in place of the original hand-operated models.
Electric lights are more efficient than candles and provide better visibility during nighttime conditions. They are also capable of displaying greater detail on the face of the clock through the use of luminous paint or lasers.
Lasers are now used almost exclusively for timing events during daylight hours because they are too expensive to maintain at night.
Even if the gnomon was perfect, the clock hands would have to be moved every two minutes to keep time.
The accuracy of a sundial depends on how well it can compensate for atmospheric refraction. At the equator, where there is no atmosphere, a perfectly vertical gnomon would show true east-west lines of latitude. In reality, however, the shadow cast by the gnomon is slightly curved due to the effect of atmospheric refraction.