June 2, 2021 12:51 PM
A sundial consists of two parts. The flat surface or platform has numerical marks in a circle to indicate the hours of the day. In the center of this circle sits a gnomon, which sticks straight up. As Earth rotates, the sun casts a shadow with the gnomon, and this shadow moves as the angle of the sun changes. Wherever the shadow falls on the marks of the sundial is the time. Both the shadow length and its direction can combine to indicate the time on a sundial.
Earth rotates on an axis, which makes it seem like the sun is moving across the sky. The changing position of Earth relative to the sun makes objects cast shadows that move throughout the day. The gnomon shadow changes as Earth rotates, which enables people to use the shadow to tell the time. Because Earth's axis is tilted, these shadows shift slightly during the year, but sundials can be adjusted to account for these changes. Some sundial platforms need to be aligned with the latitude where the sundial is located, keeping the gnomon perpendicular to the platform. Other sundial platforms remain horizontal, with the gnomon being adjustable to compensate for the changing angle of the sun during the year. Sundials also need to be calibrated for the specific time zone with a reference longitude and then additional degrees of longitude depending on the distance away from this reference point.
Historians date the sundial back to about 3,500 B.C. In 1,500 B.C., the ancient Egyptians were able to tell time by driving sticks into the ground and monitoring the shadows as time passed throughout the day. Eventually, the Egyptians created a more portable version of the sundial, and they called this timepiece a shadow clock. Other ancient civilizations also discovered that they could use the sun and shadows to mark the passage of time. Sundials gradually became more accurate and complex, and people also realized that they needed to make mathematical adjustments to compensate for the tilt of Earth's axis and the shape of Earth's orbit around the sun. People used the sun to set their clocks well into the 19th century.
It's possible to make a basic sundial right on the ground using a stick as the gnomon and some stones to mark the hours in a circle around the stick. Choose a sunny spot and drive the stick into the ground so it's pointing just slightly to the north. Watch the time throughout the morning and afternoon hours, and at the top of each hour, note where the shadow is pointing. Place a stone at each point to indicate the hours.
Humans and many other living creatures have an innate ability to sense time and the passage of time. Days and nights have specific rhythms, as do the seasons of the year. Humans have finer abilities to not only perceive the passage of time but to mark it and attempt to harness it. Humans rely on the ability to mark time, making plans and goals and remembering past events. Some cultures see time as a linear concept, while other cultures look at time as a cyclical concept.