# Are sundials operational around the clock?

This plane is named equatorial because it runs parallel to the Earth's and celestial sphere's equators. The hour-lines on an equatorial dial are all spaced 15 degrees apart (360/24), because the sun revolves 360 degrees in 24 hours. Because of the consistency of their spacing, this style of sundial is simple to build. An equatorial dial can be used for daytime readings only, as it does not reveal the time during a new or full moon.

Equatorials were popular instruments in Europe and America from about 1600 to 1800. They are now regarded as historical artifacts because modern clocks function according to the standard time scale established by Greenwich Observatory in England. However, solar-powered clocks that use the solstices and equinoxes to reset themselves each day have been built since then.

In addition to being useful for telling time during daylight hours, an equatorial reveals the time during eclipses. Because the lines on an equatorial dial are evenly spaced, they also serve as convenient markers for marking lunar phases. For example, when the first line on the dial is pointing north, the Moon is in its first quarter. If you follow the path the pointer takes as it moves across the next few lines, you will see that it ends up back where it started, indicating that the Moon is still in its first quarter.

The word "sundial" comes from Latin meaning "sun's disk".

## How many degrees does a sundial have?

To adapt to the local time zone, a sundial can be turned around its axis (which must stay pointing towards the celestial pole). In most circumstances, a rotation of 7.5 degrees east to 23 degrees west is sufficient. A clockwise rotation brings the instrument back into alignment with the sun at midday.

The angle that the shadow makes with the equator determines how many degrees the sundial will show. For example, if the shadow is thrown north then south each day, then the sundial shows 5 degrees of northness or southness over the course of a year. If the shadow never moves from directly overhead at noon every day during the summer solstice, for example, then the sundial shows 10 degrees of northness or southness over the course of a year.

In practice, however, we usually need only consider degrees of northness or southness relative to some fixed point on the earth. If we fix our attention on the north star as we look up at the sky at midnight, for example, then the shadow will appear to move 5 degrees north each day in mid-summer and 2.5 degrees south in mid-winter.

Thus the sundial shows 20 degrees of northness or southness over the course of a year.

Sundials have been used since ancient times, but they were made from wood or stone.

## How is a sundial a clock, compass, and calendar?

A Sundial that functions as a clock, compass, and calendar Our coordinate system of latitude and longitude is oriented by the Earth's axis of rotation. The Earth is a spinning sphere that revolves around the Sun, our source of light. The latitude and longitude of Earth are linked to the Sun's lighting of the planet. At the equator, all points on the surface experience equal amounts of daylight and darkness each day. As you move away from the equator, there is more night than day. Longitude is the angle between any point on Earth and a fixed point outside its body (the North Pole). Latitude is the angle between a point on Earth and the equatorial line.

At the north pole, all hours are dark during the winter months and all hours are light during the summer months. During mid-latitudes, hours are neither light nor dark but vary with the season. Sundials have been used since ancient times for telling time. They work by using the sun's energy to determine the time of day. During mid-latitudes, sunlight falls on a flat surface such as concrete or glass covered in some sort of device which detects this light intensity variation and indicates the time accordingly. Modern electronic versions use silicon sensors instead.

In addition to telling time, sundials serve as compasses. The shadow cast by the dial on a clear day follows the shape of Earth, allowing us to read the direction we are facing even if we cannot see the sky.

## Does a sundial work at the North Pole?

It is perpendicular to the dial plate and parallel to the polar axis of the Earth. During the spring and summer months of March to September at the North Pole and September to March at the South Pole, the sun will illuminate this dial 24 hours a day. For the rest of the year, the sundial will be dark.

This means that a sundial at the North Pole can tell time during the dark days of winter! The sun does not rise at the North Pole in the morning or set at the South Pole in the evening. Instead, it stays above the horizon for 24 hours straight, which is why these poles are called "pole stars".

At the North Pole, the sun is directly over the center of the earth during mid-winter because the axis of the earth is completely frozen in position. At the South Pole, the sun is directly over the south pole for half of each year because the south pole is in darkness for half of the year.

During the other half of the year, when the south pole is not in darkness, the sun is below the horizon at the south pole.

In summary, a sundial at the north or south pole tells time during all parts of the year, but only in certain seasons at lower latitudes.

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