Sun Clock Basics and Types of Sundials

Sun clocks, also known as sundials, have been around for thousands of years, with the earliest known sundial dating back to ancient Egypt around 3500 BC.

Sundials work by casting a shadow as the sun moves across the sky, indicating time. The shadow is typically cast by a vertical stick or object, known as a gnomon.

There are several types of sundials, including horizontal sundials, vertical sundials, and polar sundials. Each type has its own unique characteristics and is suited to different environments and purposes.

A horizontal sundial is the most common type, consisting of a flat surface with hour lines marked on it, which are used to indicate time based on the shadow's position.

The history of sun clocks dates back to ancient civilizations in Egypt and Babylon, with the earliest known sundials appearing around 1500 BC. These early sundials were known as shadow clocks and were used for astronomical observations.

Curious to learn more? Check out: List of Clocks

The ancient Egyptians and Babylonians used sundials to estimate the circumference of the world and chart the latitude of cities. The people of Kush created sundials using geometry, and the Roman writer Vitruvius listed dials and shadow clocks known at that time in his De architectura.

Sundials have been in common use since the 16th century, and were used by religious orders from the 7th to the 14th centuries to indicate the canonical hours of liturgical acts. A canonical sundial is one that indicates the canonical hours of liturgical acts.

The functioning of a sundial is based on the shadow or light falling on the dial face, which is usually inscribed with hour lines. The hour lines may be straight or curved, depending on the design of the sundial. In some designs, it is possible to determine the date of the year, or it may be required to know the date to find the correct time.

Here are some common features of sundials:

The earliest sundials known from the archaeological record are shadow clocks from ancient Egyptian and Babylonian astronomy, dating back to around 1500 BC.

These early sundials were used for timekeeping and astronomical observations, and they paved the way for more sophisticated sundials to come.

By 240 BC, Eratosthenes had estimated the circumference of the world using an obelisk and a water well, showing just how advanced the knowledge of astronomy was at the time.

The people of Kush created sundials through geometry, demonstrating their understanding of mathematical principles.

The Tower of the Winds in Athens included both a sundial and a water clock for telling time, highlighting the importance of timekeeping in ancient civilizations.

A canonical sundial is one that indicates the canonical hours of liturgical acts, and these were used from the 7th to the 14th centuries by religious orders.

The Italian astronomer Giovanni Padovani published a treatise on the sundial in 1570, which included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials.

For more insights, see: Water Clock

Here are some notable Arabic manuscripts related to sundials:

Sundials work by using the shadow or light that falls on the dial face, which is usually inscribed with hour lines. These hour lines may be straight or curved, depending on the sundial design.

The dial face may also display additional information, such as the horizon, equator, and tropics, which are collectively known as the dial furniture.

The object that casts a shadow or light onto the dial face is called the gnomon, but it's usually only the edge of the gnomon that casts the shadow used to determine the time, known as the style.

The style is typically aligned parallel to the axis of the celestial sphere and is aligned with the local geographical meridian. In some designs, only a point-like feature, such as the tip of the style, is used to determine the time and date, known as the nodus.

A fresh viewpoint: Sun Dial Clock

Some sundials use both a style and a nodus to determine the time and date. The gnomon is usually fixed relative to the dial face, but not always; in some designs, the style is moved according to the month, like in the analemmatic sundial.

The line on the dial plate perpendicularly beneath the style is called the substyle, and the angle the style makes with the plane of the dial plate is called the substyle height.

A sundial cannon is a specialized sundial that creates an audible noonmark by automatically igniting gunpowder at noon. These novelties were installed in parks in Europe, mainly in the late 18th or early 19th centuries.

They typically consist of a horizontal sundial with a gnomon and a mounted lens, set to focus sun rays at exactly noon on the firing pan of a miniature cannon loaded with gunpowder.

To make your sundial accurate, it's essential to adjust it correctly. The sundial must be designed for your local geographical latitude, and its style must be parallel to the Earth's rotational axis.

First, ensure the sundial's style is aligned with true north. You can do this by using a compass or by observing the direction of shadows at noon. The style's height, or angle with the horizontal, must also equal the local latitude.

To adjust the style height, you can tilt the sundial slightly up or down while maintaining the style's north-south alignment. This will help you get accurate time readings from your sundial.

To get your sundial accurate, you need to adjust it for the Equation of Time. This correction accounts for the Earth's slightly elliptical orbit around the sun, which affects the timing of solar noon.

The most common reason for a sundial to differ greatly from clock time is that it hasn't been oriented correctly or its hour lines haven't been drawn correctly. You can often adjust the style height of a horizontal sundial by tilting it slightly up or down while maintaining its north-south alignment.

Some areas of the world practice daylight saving time, which changes the official time, usually by one hour.

This shift must be added to the sundial's time to make it agree with the official time.

If you live in an area that observes daylight saving time, you'll need to adjust your sundial's time accordingly.

The shift usually occurs during the summer months, which means you'll need to make the correction during this time of year.

A standard time zone covers roughly 15° of longitude, so any point within that zone which is not on the reference longitude will experience a difference from standard time.

If a sundial is located at a longitude 5° west of the reference longitude, then its time will read 20 minutes slow, since the Sun appears to revolve around the Earth at 15° per hour.

This correction can be made by rotating the dial surface by an angle equaling the difference in longitude for equiangular dials such as equatorial, spherical or Lambert dials.

However, for other dials like a horizontal dial, the correction must be applied by the viewer.

For political and practical reasons, time-zone boundaries have been skewed, causing official noon to occur up to three hours early in some areas.

This occurs in the far west of Alaska, China, and Spain, where the Sun is actually on the meridian at official clock time of 3 PM.

Reclining sundials are a bit more complicated than their vertical counterparts, but don't worry, it's still easy to understand.

The angle of reclining, denoted by R, is relative to the local vertical. To calculate the hour-lines for a reclining sundial, you need to consider the sundial's geographical latitude, L, and the angle between a given hour-line and the noon hour-line, HRV.

The formula for calculating HRV is slightly different for reclining sundials. It involves the arctangent of cos(L + R), which gives you the angle of the hour-line relative to the noon hour-line.

For example, if you're trying to calculate the angle of the 3pm hour-line, you would use the formula HRV = arctangent(cos(L + R)), since tan 45° = 1. This will give you the correct angle for the hour-line.

Some authors use the terms reclining, proclining, and inclining to describe the orientation of the shadow-receiving plane. If the plane's face points downwards towards the ground, it's said to be proclining or inclining, while a dial is said to be reclining when the dial face points away from the ground.

The length of a human shadow can be used to measure the sun's elevation and, thence, the time. This method is based on the assumption that a person's shadow length will vary with their geographical latitude and the time of year.

A monk's height is often assumed to be six times the length of their foot, which is used to estimate the time from the length of their shadow in feet. This is a useful trick to know, especially when traveling or outdoors.

The shadow length at noon is short in summer months and long in winter months. This is a result of the changing angle of the sun throughout the year.

Chaucer, an English poet, evokes this method a few times in his Canterbury Tales, as in his Parson's Tale.

Reflection sundials are a great way to make the most of unused space, and they're surprisingly accurate. Isaac Newton developed a design that uses a small mirror on a south-facing window sill to cast a single spot of light on the ceiling.

The mirror acts like a nodus, following the sun's movement and tracing a conic section, such as a hyperbola, on the ceiling. This is similar to the way a nodus-based sundial works, where the rays of light trace out a cone over the course of a day.

If the mirror is parallel to the Earth's equator and the ceiling is horizontal, the resulting angles are those of a conventional horizontal sundial. This means you can use the ceiling as a sundial surface, making it a very accurate time-keeping tool.

The antiborean sundial, a spherical nodus-based sundial, is another example of a sundial that uses a single ray of light to tell time. It faces true north and uses a small hole at the sphere's pole to let sunlight in and fall on the hour and date lines inscribed within the sphere.

In a reflection sundial, the mirror's position is critical to getting accurate readings. If it's not parallel to the equator, the angles will be off, and the sundial won't work correctly.

Meridian lines were used to accurately determine the length of the solar year. They were a crucial tool for historical astronomers.

A horizontal line aligned on a meridian with a gnomon facing the noon-sun is called a meridian line. This type of line doesn't indicate the time, but instead, it shows the day of the year.

Historical examples of meridian lines include the Bianchini meridian line in Santa Maria degli Angeli e dei Martiri in Rome. The Cassini line in San Petronio Basilica at Bologna is another notable example.

Meridian lines were used to make precise measurements of the solar year. This was a significant contribution to the field of astronomy.

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There are many types of sundials, each with its own unique characteristics and advantages. One popular type is the equatorial sundial, which has a planar surface that receives the shadow, exactly perpendicular to the gnomon's style.

The equatorial sundial is easy to construct, with hour-lines spaced 15° apart, making it a great choice for public display. It's also easy to make equation of time and daylight saving time corrections by simply rotating the dial plate by the appropriate angle each day.

The universal equinoctial ring dial is another type of sundial that's inspired by the mariner's astrolabe. It has a thin slit that allows the Sun's rays to fall on the hour-lines of an equatorial ring, and can be made self-aligning with the addition of a more complicated central bar.

Shepherd's dials, also known as timesticks, are portable cylindrical sundials with a knife-like gnomon that juts out perpendicularly. They can be twisted to correct for the equation of time, and the entire sundial can be twisted on its string so that the gnomon aims towards the Sun.

Here's a list of some of the types of sundials mentioned:

Each of these types of sundials has its own unique features and advantages, and can be a great way to learn about the sun and its movement.

The equatorial sundial is a type of sundial that's easy to construct, thanks to its uniformly spaced hour-lines.

These hour-lines are spaced 15° apart, which is a result of the Earth's rotation of 360° in 24 hours.

You can make an equatorial sundial with an opaque dial plate, but you'll need to mark both sides of the dial since the shadow will be cast from below in winter and from above in summer.

If you use a translucent dial plate, like glass, you only need to mark the hour angles on the sun-facing side.

Equatorial sundials are also great because you can make equation of time and daylight saving time corrections by simply rotating the dial plate by the right angle each day.

This is because the hour angles are equally spaced around the dial, making it easy to adjust for these time changes.

One drawback of equatorial sundials is that they don't work well near the equinoxes in spring and autumn, when the sun moves on a circle that's nearly the same as the equatorial plane, resulting in no clear shadow being produced.

To make up for this, some equatorial sundials have a nodus added, which allows you to tell the time of year by observing the shadow of the nodus moving on a circle on the equatorial plane.

Horizontal sundials are a type of sundial that is mounted on a flat surface, such as a wall or a rock.

They work by casting a shadow on a horizontal surface, indicating the time of day. The shadow of the gnomon, or vertical stick, falls on a calibrated surface, showing the hour.

Horizontal sundials can be as simple as a stick stuck in the ground or as complex as a intricately carved stone. They are often used in gardens or public spaces.

The shadow of the gnomon moves across the surface as the sun moves across the sky, creating a line that indicates the time. This line is usually marked with hour lines, allowing the user to tell the time.

Some horizontal sundials are designed to be more decorative than functional, featuring intricate carvings or sculptures.

Polar dials are a type of sundial where the shadow-receiving plane is aligned parallel to the gnomon-style. This causes the shadow to slide sideways over the surface, moving perpendicularly to itself as the Sun rotates about the style.

The hour-lines on a polar dial are all aligned with the Earth's rotational axis. As the Sun's rays become nearly parallel to the plane, the shadow moves very quickly and the hour lines are spaced far apart.

The face of a polar dial need not be vertical; it need only be parallel to the gnomon. This means a plane inclined at the angle of latitude under the similarly inclined gnomon will be a polar dial.

The perpendicular spacing X of the hour-lines in the plane is described by the formula X = H * (1 + tan(90 - φ)) * t, where H is the height of the style above the plane, φ is the latitude, and t is the time (in hours) before or after the center-time.

For an East-facing dial, the center time will be 6 A.M., and for a West-facing dial, this will be 6 P.M. The center time is the time when the style's shadow falls directly down on the plane.

Vertical declining dials are a type of sundial that's a bit more complicated than the vertical south-facing dials we've discussed. They're used in areas where the sundial doesn't face a cardinal direction, such as south.

The formula for calculating the time on a vertical declining dial involves several variables, including the sundial's geographical latitude, the time before or after noon, the angle of declination from true south, and a switch integer for the dial orientation. This formula is: L + t - D + s_o = 90°.

A partly south-facing dial has a switch integer value of +1, while those partly north-facing have a value of -1. The angle B between the substyle and the noon hour-line is given by the formula B = arctan((s_o * sin(D)) / (cos(L) * cos(D - L))).

If a vertical sundial faces true south or north, the angle B is 0° and the substyle is aligned with the noon hour-line.

Cylindrical sundials are a unique and elegant alternative to traditional sundials. They feature a style located on the circumference of a cylinder or sphere, rather than at its central axis of symmetry.

The hour lines on a cylindrical sundial are spaced equally, but at twice the usual angle, due to the geometrical inscribed angle theorem. This is the same principle used in ancient times.

Benoy dials are a specific type of cylindrical sundial that use a long, slim mirror to reflect the Sun's rays and create an equivalent sheet of light. Examples of Benoy dials can be found in various locations in the United Kingdom.

Some modern sundials also use cylindrical or helical surfaces, with a thin gnomon located along its center or at its periphery. This design allows for a more compact and space-efficient sundial.

Here are some examples of non-planar sundials that can be found in the United Kingdom:

Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern.

The time of day is read on the ellipse, not on traditional hour lines found on other sundials. This unique feature makes analemmatic sundials stand out from other types of sundials.

Analemmatic sundials require the gnomon to change position daily to accurately indicate the time of day. This is because the shadow of the gnomon must fall in a specific spot on the ellipse to display the correct time.

Human gnomon analemmatic sundials are a creative twist, where a person's shadow is used as the gnomon. However, these sundials are not practical at lower latitudes where a human shadow is quite short during the summer months.

The Foster-Lambert dial is a type of movable-gnomon sundial that's circular in shape with evenly spaced hour lines, making it an equiangular sundial.

One key feature of the Foster-Lambert dial is that its gnomon is tilted northwards by an angle α = 45° - (Φ/2), where Φ is the geographical latitude. This tilt is crucial for accurate timekeeping.

At a latitude of 40°, the gnomon would be tilted away from vertical by 25° in a northerly direction.

To read the correct time on a Foster-Lambert dial, the gnomon must also be moved northwards by a distance involving the radius of the dial and the Sun's declination for that time of year.

Altitude-based sundials are a type of sundial that measures the height of the Sun in the sky.

They are not oriented towards true north, but rather towards the Sun and generally held vertically.

The Sun's elevation is indicated by the position of a nodus, either the shadow-tip of a gnomon, or a spot of light.

Altitude dials are read from where the nodus falls on a set of hour-curves that vary with the time of year.

These dials are often portable and simple to use, but many are designed for only one latitude.

The capuchin dial has a version that's adjustable for latitude, making it a more versatile option.

Altitude dials are less accurate near noon because the sun's altitude is not changing rapidly then.

At times equally spaced about noon, the user had to know whether it was morning or afternoon, as the Sun's altitude is the same at these times.

The bifilar sundial is an interesting type of sundial that uses two non-intersecting threads parallel to the dial. One thread is usually parallel to the other, while the second thread is orthogonal to the first.

The intersection of the two threads' shadows gives the local solar time, making it a unique and accurate way to tell time. This is especially useful for travelers who need to know the local time.

Invented by the German mathematician Hugo Michnik in 1922, the bifilar sundial is a relatively modern innovation in sundial design. It's a testament to the ongoing evolution of sundial technology.

By using two threads, the bifilar sundial eliminates the need for a gnomon, making it a simple and elegant design.

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Ring dials are a type of sundial that's hung vertically and oriented sideways towards the sun.

A beam of light passes through a small hole in the ring and falls on hour-curves inscribed on the inside of the ring.

To adjust for the equation of time, the hole is usually on a loose ring within the ring, so that the hole can be adjusted to reflect the current month.

The ring itself is typically perforated, which is why it's also known as an Aquitaine or a perforated ring dial.

Ring dials rely on the sun's position in the sky to tell time, making them a simple yet effective way to keep track of the hours.

Card (Capuchin) sundials are a type of altitude dial that use a card aligned edge-on with the sun to determine the sun's altitude.

They work by having a ray of light pass through an aperture onto a specified spot, which then indicates the time based on the position of a weighted string and bead.

A weighted string hangs vertically downwards from a hole in the card, and the position of the bead on the hour-lines of the card gives the time.

For another approach, see: Clock Position

In more sophisticated versions, such as the Capuchin dial, there is only one set of hour-lines, and the position of the hole from which the weighted string hangs is varied according to the season.

This allows for a more universal application of the Capuchin dial, which can be used without needing to adjust the hour lines for different seasons.

The Capuchin sundials are constructed and used graphically, as opposed to direct hour-angle measurements.

A diptych sundial is a type of sundial that consists of two small flat faces joined by a hinge.

These dials usually fold into a flat box and are made of materials like white ivory, inlaid with black lacquer markings.

The gnomon of a diptych sundial is a string between the two faces, made of materials like black braided silk, linen, or hemp.

The string gnomon is continuous, which means that the shadows must meet at the hinge, resulting in the same time being shown on both dials regardless of their orientation.

Diptych sundials can keep a calendar well-enough to plant crops, thanks to the correct markings and a knot or bead on the string as a nodus.

A common error is that diptych dials are self-aligning, but this is not correct for diptych dials using a string gnomon between faces.

Additional reading: List of Largest Clock Faces

The globe dial is a unique type of sundial that uses a sphere aligned with the Earth's rotational axis. It's equipped with a spherical vane that helps determine the time.

To use a globe dial, you simply need to rotate the vane to give the smallest shadow. This angle is critical in determining the time.

The globe dial works similarly to sundials with a fixed axial style, relying on the Sun's azimuthal angle in its apparent rotation about the earth. This angle is what gives the globe dial its accuracy.

By aligning the globe dial with the Earth's rotational axis, you can get a more accurate reading of the time. This is especially useful for precise timekeeping.

Additional reading: Earth Clock

Sundials have been designed with mottoes that reflect on the transience of time and life. These mottoes often serve as a memento mori, encouraging the observer to think about their own mortality.

Some sundial mottoes are quite serious, like "Do not kill time, for it will surely kill thee." Others are more lighthearted, such as "I count only the sunny hours."

Sundial mottoes can be found in various collections that have been published over the centuries. These collections showcase the creativity and wit of sundial designers.

Locations near the Sun's zenith position in the sky have a unique characteristic. The Sun is nearly overhead at these locations.

The Sun's zenith position is an important factor in sundial design. For example, in Yopal, the Sun is near its zenith at 11:48 am, with the Sun approximately 40 km away.

Some locations near the Sun's zenith position in the sky include Yopal, Tunja, and Garagoa. These locations all have the Sun near its zenith at the same local time, 11:48 am.

Here's a table showing the distance of the Sun from these locations:

These locations have varying distances from the Sun, with Yopal being the closest at 40 km.

To accurately measure and observe the sun's movement, you can use an empirical method to mark the positions of the hour-lines on a sundial. This involves marking the shadow of the gnomon at hourly intervals using a clock set to "sundial time", which is standard time plus the equation of time for the day.

The equation of time must be taken into account to ensure that the positions of the hour-lines are independent of the time of year. This is done by adjusting the clock every day or two to account for the variation of the equation of time.

By marking the positions of the shadow of the style on a real sundial at hourly intervals, you can easily locate the hour lines empirically, making it a simple and effective method for sundial design.

For your interest: Equation Clock

The empirical method is a practical approach to finding the positions of the hour-lines on a sundial.

This method involves marking the positions of the shadow of a style on a real sundial at hourly intervals as shown by a clock.

To ensure accuracy, the equation of time must be taken into account, which can be done by setting a clock to "sundial time" - standard time plus the equation of time for the day in question.

Mark the shadow of the style when the clock reads whole numbers of hours, and label it with the corresponding number of hours.

For example, when the clock reads 5:00, mark the shadow and label it "5" (or "V" in Roman numerals).

If you can't mark all the hour-lines in a single day, adjust the clock every day or two to account for the variation in the equation of time.

This method is simpler than the complex calculations involved in other methods, and is a great way to get accurate results without the hassle of complicated math.

A sundial can be used as a compass if it's made for the latitude it's being used in and mounted correctly with its plate horizontal and gnomon pointing to the celestial pole.

If the directions of the cardinal points are unknown, but the sundial is aligned to show the correct apparent solar time, its gnomon will point to True north or south. This method is much more accurate than using a watch as a compass.

The sundial can be placed on a horizontal surface and rotated about a vertical axis until it shows the correct time, and its gnomon will then be pointing to the north in the northern hemisphere or to the south in the southern hemisphere.

An alternative method uses two sundials of different designs, attached to and aligned with each other, and oriented so they show the same time. This allows the directions of the cardinal points and the apparent solar time to be determined simultaneously.

Daylight is a fascinating topic, and as someone who appreciates the beauty of the sun, I'm excited to share some interesting facts with you.

Sunrise and sunset times can be precisely displayed for any location and date, allowing you to plan your day around the sun's schedule.

I recall a trip to the mountains where I was able to witness a stunning sunrise over the peaks. It's amazing how the timing of the sun's appearance can vary depending on your location.

Sunrise and sunset times can be precisely displayed for any location and date, allowing you to plan your day around the sun's schedule.

The duration of daylight can also be calculated and visualized, giving you a better understanding of how much time you have to enjoy the outdoors.

Here are some of the different types of twilight phases that can be shown separately:

These twilight phases are essential for photographers and outdoor enthusiasts to plan their activities around the best lighting conditions.

The golden and blue hour, which occur when the sun is low on the horizon, can also be visually represented, making it easier to plan your photography sessions or outdoor activities.

Solar altitude angle, which shows the current sun elevation in the sky, can be displayed, giving you a better understanding of the sun's position.

Solar time, which shows true solar time alongside standard time, can be displayed, helping you to keep track of the sun's time and the clock time.

The equation of time hand, which visualizes the difference between solar and clock time, can also be shown, making it easier to understand the relationship between the two times.

In some regions, like Germany, there are specific rules regarding night hunting, and an optional marker can be added to show the legal hunting times based on local sunset/sunrise.

Suggestion: Solar Alarm Clock

The design and construction of a sun clock is a fascinating process. The sundial's face is typically made of a flat surface, often stone or metal, with hour lines radiating from a central gnomon, which casts a shadow as the sun moves.

The gnomon's shape and size are crucial in determining the accuracy of the sundial. A vertical gnomon is often used in sun clocks, allowing for a more precise measurement of time.

The dial plate's material and finish can also impact the sundial's performance. A smooth, flat surface allows for a more accurate shadow to be cast, while a textured or rough surface can introduce errors.

Empirical hour-line marking is a simple method of finding the positions of the hour-lines on a sundial, which can save a lot of work in cases where the calculations are complex.

This method involves marking the position of the shadow of the gnomon of a real sundial at hourly intervals. The equation of time must be taken into account to ensure that the positions of the hour-lines are independent of the time of year when they are marked.

To do this, set a clock or watch to show "sundial time" which is standard time, plus the equation of time on the day in question. This will give you an accurate reading of the time.

Mark the hour-lines on the sundial to show the positions of the shadow of the style when the clock shows whole numbers of hours. Label each hour-line with the corresponding number of hours.

If you can't mark all the hour-lines in a single day, you'll need to adjust the clock every day or two to take account of the variation of the equation of time.

Vertical sundials are a bit more complex than their horizontal counterparts, but they're still a great option for small spaces or unique designs.

The formula for calculating the time on a vertical sundial is given by t = (L - D) * 4, where L is the sundial's geographical latitude, t is the time before or after noon, and D is the angle of declination from true south.

This formula assumes the sundial is facing south, which is the most straightforward orientation.

If the sundial is not aligned with a cardinal direction, the substyle of its gnomon is not aligned with the noon hour-line, and the angle B between the substyle and the noon hour-line is given by the formula B = arctan(2 * D / (1 + cos(L))).

The height of the gnomon, or the angle the style makes to the plate, G, is given by G = arctan(2 * D / (1 + cos(L))) for vertical sundials that face true south or north, where D=0° or D=180°, respectively.

The angle between the gnomon style and the dial plate, B, in this type of sundial is given by the same formula as above, which is B = arctan(2 * D / (1 + cos(L))).

Spherical sundials are a great example of how sundial design can get creative. The surface receiving the shadow can be any shape, but it's convenient to use a spherical shape when the style is aligned with the Earth's rotational axis.

This is because the hour-lines are equally spaced on a spherical shape, making it an equiangular sundial. The armillary sphere and the equatorial bow sundial are two types of equiangular sundials that use this principle.

The equatorial bow sundial is particularly interesting, with a gnomon that's a bar, slot, or stretched wire parallel to the celestial axis. The face of the sundial is a semicircle, corresponding to the equator of the sphere, with markings on the inner surface.

Some equatorial bow sundials are built to be incredibly precise, like the ones made of marble found in Yantra mandir. One of these sundials, the Samrat Yantra, is 27 meters tall and has a shadow that moves at an incredible 1 mm per second.