Method for improving the definition and accuracy of gnomon shadow, device and application thereof

Abstract

A method for improving the definition and accuracy of gnomon shadow, its device and application, including a shadow-seeking card with a slit, which is in the shape of a straight line or a cross, and a movable bracket for supporting and adjusting the shadow-seeking card. The shadow-seeking card is placed in the pseudo-umbral zone behind the umbral zone of the sundial, with one side of the card facing the sunlight. The slit of the card is parallel to the blurred shadow of the sundial. A white bright strip with a thin black line in the middle appears on the dial behind the card. The thin black line is the shadow image of the cylindrical sundial through the slit. The position of the thin black line is the projection of the central axis of the sundial and the projection of the vertical diameter of the sun on the dial. According to the position of the thin black line on the time scale, the time scale value can be accurately read. The device is used for reading large equatorial ring sundials, determining the geographical north-south direction, and measuring the shadow length of the sundial at noon on the winter solstice.

Description

Technical Field

[0001] This invention relates to the field of astronomy, and more particularly to a method, apparatus, and application for improving the clarity and accuracy of sundial shadows. Background Technology

[0002] The sundial is a great invention of humankind, utilizing the shadow cast by the sun. It mainly consists of a dial and a dial face with specific orientations and shapes. Sunlight casts a shadow (dial shadow) on the dial face, which has time markings. The time is read from the position of the shadow on the time markings. Because the sun is not a point source but a surface source with a certain angular diameter (approximately 0.5 degrees), sunlight, after being blocked by the dial, forms the umbra, penumbra, and antumbra. In the umbra, sunlight is completely blocked by the dial; if the dial face is in the umbra, the shadow appears black. In the antumbra, some sunlight is blocked by the dial, and the dial face appears grayish-black. In the penumbra, some sunlight is also blocked, and the shadow appears grayish-white. Furthermore, the more blurred the shadow is from the inside out, the farther the dial face is from the dial, and the wider and more blurred the shadow becomes. Near the gnomon, the umbra is much larger than the penumbra, and the black portion of the shadow is larger than the gray portion, making the shadow appear relatively clear. However, at a greater distance from the gnomon, the penumbra is larger than the umbra, and the gray portion is larger than the black portion, causing the shadow to become blurry. When the distance between the shadow and the gnomon exceeds the umbra and enters the antumbra, the black portion disappears, leaving only a grayish-black shadow. Furthermore, the farther the distance, the wider and blurrier the shadow becomes, sometimes even faint and indistinct. The difficulty in determining the exact position of this wide and blurry shadow on the time scale, and thus the inability to accurately read the time, has been a long-standing problem.

[0003] For example, the famous observatory in Jaipur, India, is said to house the world's largest sundial, including an equatorial ring sundial with a distance of 14.9 meters between the dial and the dial surface. The time markings on the dial are accurate to two seconds. From a certain distance, the time display appears remarkably precise. However, when people approach the time scale for close observation, they discover that its blurry shadows are tens of millimeters wide. It is impossible to determine the accurate time through such a blurry and wide shadow; even a 2-second accuracy on the time scale would be meaningless in improving the sundial's precision.

[0004] In order to improve the accuracy and overcome the difficulty of measuring the shadow, Guo Shoujing, a famous scientist in the Yuan Dynasty of China, utilized the principle of pinhole imaging and invented the "Jingfu" with a pinhole device. The sunlight passes through the pinhole on the blade of the "Jingfu", presenting the image of the crossbeam of the gnomon on the four-meter-high platform on the scale of the horizontal sundial surface (the gui). Based on the precise scale on the sundial surface, the accurate value of the length of the midday shadow on the winter solstice was successfully measured, greatly improving the measurement accuracy and obtaining the accurate value of the tropical year length (the actual solar year). Thus, the "Shoushi Calendar" was successfully compiled. However, due to the very small pinhole in the blade of the "Jingfu", the solar energy passing through the pinhole is limited. The image formed by the small hole is only as small as a grain of rice. The black line image of the gnomon in it is thin and dim, not bright enough, with a small visible range, and the shadow moves quickly. When determining the position of the thin black line shadow, it is necessary to additionally determine whether it is at noon and record immediately. The observation requirements are high and the difficulty is great. Summary of the Invention

[0005] The purpose of the present invention is to solve the above problems in the prior art, and provide a method, device and application for improving the clarity and accuracy of the sundial shadow, enabling the sundial shadow to pass through the shadow-finding card with slits, making the originally wide and blurred sundial shadow thin, black and clear, easy to identify and interpret; improving the clarity and accuracy of sundial timekeeping, and solving the long-term problem in shadow measurement technology that people are troubled by the blurred shadow and unable to accurately measure.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A device for improving the clarity and accuracy of the sundial shadow, including a shadow-finding card, on which there are slits, and the slits are in a linear shape or a cross shape.

[0008] The upper edge of the slit of the shadow-finding card is arranged in an arrow shape.

[0009] The width of the slit is 0.5 - 0.8 mm.

[0010] The shadow-finding card is an opaque card with white on both sides.

[0011] The present invention further includes a movable support for the shadow-finding card, which includes a support body, an iron long-arm support, a magnet, an iron L-shaped support, and two U-shaped hanging pieces on the shoulder of the support body; the support body is arranged in an arc shape, one end of the iron long-arm support is connected to the center of the support body, and the other end of the iron long-arm support is connected to the iron L-shaped support through a magnet; the width of the U-shaped hanging piece is the same as the width of the annular sundial surface of the sundial, one side of the U-shaped hanging piece is fixed on the support body, and the other side is buckled on the annular sundial surface of the equatorial ring type sundial; wherein, the shadow-finding card is fixed on the iron L-shaped support through a magnet.

[0012] A method for improving the clarity and accuracy of the sundial shadow is as follows:

[0013] Place the finder card in the pseudo-umbra behind the umbra of the gnomon, with one side facing the sunlight. Position the slit of the finder card parallel to the gnomon's blurred shadow. A bright white bar with a thin black line in the middle will appear on the gnomon face behind the finder card. This thin black line is the image of the cylindrical gnomon through the slit. At this point, the image of the thin black line, the center line of the slit, the central axis of the gnomon, and the vertical diameter of the apparent sun are all in the same plane. The position of the thin black line is the projection of the central axis of the gnomon and also the projection of the vertical diameter of the apparent sun onto the gnomon face. The time can be accurately read based on the position of the thin black line on the time scale.

[0014] One method to improve the clarity and accuracy of sundial shadows is as follows:

[0015] The cross-shaped finder card is used to image the shadow of the cross-shaped gnomon and determine the position of the apparent center of the sun: The cross-shaped finder card not only displays the position of the projection of the axis of the vertical or horizontal gnomon, but also displays the position of the intersection of the projection lines of the axis of the vertical and horizontal gnomons, that is, the position of the projection of the apparent center of the sun.

[0016] Application of the device for improving the clarity and accuracy of sundial shadows in this invention: for reading large equatorial ring sundials.

[0017] The application of the device for improving the clarity and accuracy of sundial shadows in this invention: for determining geographical north-south direction.

[0018] The application of the device for improving the clarity and accuracy of sundial shadows in this invention: used to measure the length of the sundial shadow at noon on the winter solstice.

[0019] Compared with the prior art, the beneficial effects achieved by the technical solution of this invention are:

[0020] The shadow cast by the gnomon on any sundial (especially a large one) is always blurry, making it difficult to indicate precise time. For small sundials, the gnomon is short, the shadow is short, and the dial surface is small, making reading the time relatively easy when accuracy requirements are not high (e.g., only the hour or quarter of a day is needed). To improve the accuracy of sundial timekeeping, larger sundials are built, and the height of the gnomon is increased to increase the shadow length and reduce relative measurement error. However, the longer the shadow, the blurrier its end, making it even more difficult to read – a seemingly insurmountable contradiction. Therefore, readings from sundials (especially large ones) can only be rough estimates and cannot be very accurate. This invention employs a slit imaging method to create a simple gnomon card. It can obtain clear gnomon images with strong black-and-white contrast, controllable size, and high accuracy, with obvious effects. It completely solves the long-standing problems of blurry gnomon images and difficult interpretation in shadow measurement technology.

[0021] In this invention, the vertical slit of the seeker card determines the projected position of the apparent sun's vertical diameter; similarly, the horizontal slit of the seeker card determines the projected position of the apparent sun's horizontal diameter. Therefore, the intersection of the two orthogonal black lines of the gnomon shadow obtained by the orthogonal slits is the projection of the intersection of the axes of the cross-shaped gnomon, and also the projection of the center of the apparent sun. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the image-finding card of the present invention;

[0023] Figure 2 A schematic diagram of the movable support for the image search card;

[0024] Figure 3 A schematic diagram illustrating the principle of slit imaging for image seekers;

[0025] Figure 4 Image of the slit in the image seeker;

[0026] Figure 5 This is a schematic diagram used to determine the geographical north-south direction;

[0027] Figure 6 This is a schematic diagram used to determine the length of the shadow cast by a gnomon at noon on the winter solstice. Detailed Implementation

[0028] To make the technical problems, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0029] 1. Shadow Search Card

[0030] In this embodiment, a flat, non-deformable, white, approximately 0.5mm thick opaque card is selected. A straight slit, 0.5-0.8mm wide and approximately 5cm long, is cut into the card. This is the I-shaped lookout card. Figure 1 As shown in a);

[0031] Another type is the cross-shaped seeker card, such as... Figure 1 As shown in b); cross-shaped orthogonal slits are engraved on the card, with the vertical slit about 6 cm long and the horizontal slit about 4 cm long. The cross-shaped lookout card is used to image the shadow of the cross-shaped gnomon. It can not only display the position of the projection of the vertical or horizontal axis of the gnomon separately (without interference), but also display the position of the intersection of the projection lines of the vertical and horizontal axes of the gnomon at the same time. That is, the cross-shaped lookout card is used to determine the position of the projection of the apparent sun center.

[0032] Cut the upper edge of the slit in the seeker card into an arrow shape to create a seeker card with an arrow, such as... Figure 1 As shown in c); the arrow clearly indicates the position of the gnomon's shadow on the scale and prevents the shadow from overlapping with the scale lines, thus improving the readability of the gnomon's position.

[0033] Finder cards are easy to make and readily available. Any material and method can be used, simply by creating a slit in the card. For example, two 0.5mm thick utility knife blades can be used to cut a slit approximately 15mm wide horizontally. These blades can then be arranged parallel to each other, spaced about 0.5mm to 1mm apart. The front of the blades will naturally form an arrow with a slit. A thin card can then be glued to the back of the blades for fixation. Both sides of the card can be sprayed with white paint (to indicate the position of the sundial's shadow for easier handling). Alternatively, a small magnet can be attached to the card (or its side) to attach it to a movable iron stand. The advantage of this method is that the blades are hard, straight, and sharp, resulting in a thin and straight slit opening and good image quality.

[0034] 2. Movable holder for the image search card

[0035] The method of this invention can be operated by holding a handheld finder card. To improve the stability of the finder card during observation and to facilitate observation, photography, and recording, a movable support for the finder card is designed. As is well known, the Earth rotates on its axis while revolving around the Sun. In addition to its diurnal apparent motion (which changes the hour angle) along the diurnal parallel circle, the apparent Sun also undergoes an annual apparent motion. Therefore, the declination of the apparent Sun changes with the date and season, and the direction and position of the sundial's shadow also change accordingly. The spatial position of the finder card must also change accordingly to ensure that the sundial's shadow can move stably on the time scale of the dial for easy reading.

[0036] like Figure 2 As shown, the sundial's movable support consists of a support body, a long iron arm support, a small magnet, a long iron L-shaped support, and two U-shaped hanging pieces on the shoulder of the body. The width of the U-shaped hanging pieces is the same as the width of the annular dial surface of the sundial. The sundial is fixed to the long iron L-shaped support by the small magnet, and the long iron L-shaped support is fixed to the long iron arm support by the magnet.

[0037] The lookout card connects to an L-shaped iron bracket via a magnet, allowing it to move up and down, left and right, and forward and backward. Adjusting the spatial position of the lookout card adjusts the thickness of the thin black line of the gnomon shadow and ensures the arrow on the lookout card aligns with the bottom of the time scale for accurate reading. One side of the U-shaped hanger is fixed to the bracket body, while the other side is attached to the annular dial face of the equatorial ring sundial. The entire lookout card's movable bracket can move flexibly left and right on the dial face.

[0038] 3. The process and characteristics of slit imaging with a seeker card

[0039] See Figures 3-4Make sure the cylindrical gnomon, the finder card slit, and the dial face are parallel to each other and perpendicular to the incident sunlight. Place the finder card in the antumbra behind the umbra of the gnomon, with the white side of the card facing the sunlight, so that the slit of the finder card is in the blurred shadow of the gnomon and parallel to it. A bright white bar with a thin black line in the middle will appear on the dial face (screen) behind the finder card. The thin black line is the image of the cylindrical gnomon formed by the slit.

[0040] Sometimes the thin black line is not parallel to or curves the white bright bar. This is because the slit of the finder card is not in the shadow of the dial and is not parallel to it. Adjusting the position of the slit will solve the problem. Generally, the thin black line is not exactly in the center of the white bright bar. In this case, simply move the finder card slightly left or right, and the thin black line will also move left or right (the direction of movement of the thin black line is always the same as the direction of movement of the finder card, and the speed of movement of the thin black line is greater than that of the white bright line). Continue until the two white edges next to the thin black line are the same width. At this point, the shadow of the thin black line, the center line of the slit, the central axis of the gnomon, and the vertical diameter of the apparent sun are all in the same plane. Clearly, the position of the thin black line is the projection position of the central axis of the dial, and also the projection position of the vertical diameter of the apparent sun on the dial surface. The time can be accurately read based on the position of the thin black line on the time scale.

[0041] Experiments have shown that even if the dial, slit, and dial face are not perpendicular to the incident sunlight, such as in classic equatorial or horizontal sundials (where the dial is perpendicular to the dial face, but the incident sunlight is oblique to the dial and dial face), the final experimental results are the same. This is because when the two white edges next to the thin black line of the shadow are the same width, they are also in the same plane.

[0042] When the plane of the cross-shaped gnomon is oblique to the sunlight, the shadow cast by the originally orthogonal cross-shaped gnomon may also be oblique. As long as the face of the finder card is adjusted to be parallel to the plane of the cross-shaped gnomon, the thin black line shadow cast by the originally orthogonal slit of the finder card will also be oblique. The intersection point is the projection of the intersection of the two central axes of the cross-shaped gnomon, which is also the projection of the apparent center of the sun.

[0043] If the seeker card is moved back and forth, the distance between the slit screens is greater (image distance is larger), and the black line is thicker and blurrier; if the distance between the slit screens is closer (image distance is smaller), the black line is thinner and clearer. The thickness of the black line of the gnomon shadow can be adjusted by changing the distance (image distance) between the seeker card and the dial surface to meet the needs of different observers. When the distance between the slit screens is smaller, the black line becomes too thin and disappears, leaving only a bright white line.

[0044] The characteristics of slit imaging are different from those of pinhole imaging. In pinhole imaging, the object and image are centrally symmetrical, and the top, bottom, left, and right are reversed. In slit imaging, the object and image are planar symmetrical, and the left and right are reversed, but the top and bottom remain unchanged.

[0045] 4. Used for reading large equatorial ring sundials

[0046] To verify the functionality of the finder card, a relatively large equatorial ring-type sundial was constructed. The gnomon was tilted at an angle of 24.5 degrees, pointing towards the North Pole and parallel to the dial face. The time scale (dial face) was always on the equatorial ring and parallel to the gnomon. To facilitate the fabrication of the time scale on the sundial face and to ensure that the time scale closely matched the International System of Units (SI) units of length, a scale value of 1 min / 1 cm (i.e., 6 s / 1 mm) was chosen. The distance between the gnomon and the dial face, i.e., the radius of the sundial, was R = 720 cm / π = 229 cm. The gnomon was a 6 mm diameter, 1 m long aluminum rod. A straight-line finder card with an arrow was used, adjustable in distance from the gnomon to the dial face at approximately 25–30 cm. The finder card's movable support moved flexibly, and the thin black line of the gnomon's shadow was accurately and clearly aligned with the time scale. Without a finder card, the gnomon's shadow is blurry, with a width of approximately 30mm, making reading difficult and resulting in significant random errors in estimation. With a finder card, the gnomon's shadow is clear, timing is accurate, and the effect is distinct.

[0047] The equatorial ring sundial has a radius of 2.29m. Due to the Earth's rotation, the shadow of the thin black line moves continuously from west to east at a rate of 1 cmy / min. Since the shadow is constantly moving during photography, multiple photos should be taken, and the value at which the thin black line intersects should be selected for line analysis. Linearity verification measurements were conducted on the morning of July 11, 2022, and the results are shown in Table 1 below.

[0048] Table 1

[0049] 1 2 3 4 5 6 7 8 9 Hourly scale / cm 0.0 5.0 9.9 16.0 19.0 27.7 29.5 35.2 35.7 Standard time t / min 15.3 20.1 25.0 31.2 34.2 43.0 44.8 52.6 51.1 <![CDATA[t n -t1(comparison value)]]> 4.8 9.7 15.9 18.9 27.7 29.5 35.3 35.8

[0050] As shown in Table 1, the standard time intervals recorded in the photographs are very close to the time intervals on the time scale, differing by only 0 to 0.2 seconds, indicating a very small error in the data table. This demonstrates that the experiment meets the requirements.

[0051] 5. Used to determine the north-south direction in geography.

[0052] (1) The north-south direction is determined by the "true sun noon shadow measurement method". That is, at the instant when the true sun crosses the local meridian (i.e., local true sun noon 12h), the direction of the shadow cast by the sun on a vertical rod or plumb line is recorded, which is the north-south direction or the direction of the local meridian. This method has the problems of blurry shadows and large errors. The problem can be solved by using a straight shadow-finding card.

[0053] For example, on March 21, 2022, the true north-south direction of a location in Xiamen (118.11E, 24.5N) was measured. By first consulting an astronomical almanac or other relevant data, such as the time difference curve for the Xiamen area, it can be found that the true solar noon of 12:00 on March 21, 2022 in Xiamen corresponds to 12:14:55 Beijing time.

[0054] See Figure 5 A suspension line is set between two supports. A straight stainless steel rod, about 2 meters long and 3 mm in diameter, is suspended horizontally from the suspension line as a counterweight, equivalent to a vertical gnomon. The slit of the linear finder is positioned in the blurred shadow near the suspension point of the plumb line, creating a shadow of the distant plumb line on the horizontal plane. The precise Beijing time can be obtained by checking a mobile phone. March 21st, 12:14:55 Beijing time corresponds to true solar noon. At this point, adjust the position of the linear finder until the two white edges next to the thin black line of the gnomon shadow are of equal width. The shadow of the plumb line on the horizontal plane will then lie on the local meridian. Immediately mark the position of a point on the distant thin black line; the line connecting this point and the plumb line's point of contact is the local meridian.

[0055] (2) Determining north-south direction using the solar altitude method

[0056] Choose point A on a horizontal surface. Erect a vertical pole approximately 1 meter long at point A. At a certain time in the morning, when the sun is at a certain altitude above the horizon, the shadow of the pole will be AB, and the shadow at the top of the pole will be the shadow tip B. In the afternoon, when the sun is at the same altitude, the shadow will be AC, and the shadow at the top of the pole will be the shadow tip C. AC equals AB (points B and C lie on concentric circles centered at point A). Draw the bisector AN of ∠BAC. The direction of AN is due north-south. This method is called the solar altitude method for determining north-south direction. However, this method results in blurry shadows at the pole's endpoints B and C, and its accuracy is not high. A cross-shaped finder card can be used to solve the problem of blurred shadows at the end of a gnomon. A cross-shaped gnomon is set at the top of the gnomon, with the horizontal gnomon pointing east-west. The surface of the cross-shaped finder card is parallel to the plane of the gnomon. The cross-shaped slit of the finder card is placed in the shadow of the gnomon. The position of the finder card is carefully adjusted. When the white edges next to the two thin black lines are of equal width, the intersection of the two intersecting thin black lines of the shadow on the plane is the projection position B and C of the center of the cross-shaped gnomon (which is also the projection position of the apparent center of the sun). Similarly, the bisector AN of ∠BAC is drawn. The direction of AN is the accurate true north-south direction (i.e., the local meridian direction).

[0057] 6. Used to determine the length of the shadow cast by a gnomon at noon on the winter solstice.

[0058] For a cross-shaped gnomon, the intersection of the two orthogonal black lines of shadow cast by the orthogonal slits of the cross-shaped finder is the projection of the intersection of the axes of the cross-shaped gnomon, and also the projection of the apparent center of the sun. Using this, the measurement of the length of the gnomon shadow at noon on the winter solstice can be made more accurate. Figure 6Find a rooftop of a building about 20 meters high with an open view to the north. Set up a cross-shaped gnomon, approximately 10cm in diameter and 1m long and wide, tilted northwards. The vertical gnomon should be tilted at a 48-degree angle to the north, and the horizontal gnomon should point east-west. Suspend a plumb line at the intersection of the cross-shaped gnomons to determine the location of the plumb line on the ground and measure the height of the center of the cross-shaped gnomon. Determine the meridian passing through the plumb line. Begin observations before noon on the winter solstice. Position the finder card towards the sun. When the cross-shaped slit of the finder card is in the faint shadow of the cross-shaped gnomon, a shadow with a white cross-shaped bar will appear on the gnomon face. Move the slit of the finder card up, down, left, and right. Thin black lines will appear among the orthogonal white bars on the gnomon face. When the width of the white edges next to both the horizontal and vertical thin black lines is equal, the position q of the two orthogonal thin black lines is the projection position of the cross-shaped gnomon (also the projection position of the apparent center of the sun). Since the sun's declination changes very little around the winter solstice, the position of the finder card can be adjusted up and down first, centering the thin black line of the horizontal white bright bar. Then, the finder card can be moved repeatedly up, down, left, and right until the vertical thin black line is also centered. When the thin black line in the vertical white bright bar coincides with the meridian (this is exactly noon), immediately mark the position of the horizontal thin black line on the meridian and measure the distance from that point to the vertical point. This is the accurate projection length of the center of the cross-shaped gnomon. The advantages of this application are: it increases the height of the gnomon (more than twice that of a 4-zhang high platform), reducing relative errors; using the cross-shaped finder card allows for more precise determination of the projection position of the apparent sun's center, improving accuracy; the orthogonal thin black line shadow formed by the slit is clearer and more distinct than the shadow formed by pinhole imaging; and the moment the vertical thin black line coincides with the meridian is noon, eliminating the need to determine the noon time separately, thus simplifying the experimental procedure.

[0059] In this invention, to observe whether the sundial is blocking the sun, it must be observed through a Baader film to ensure eye safety.

[0060] This invention significantly increases the light energy for imaging via a seeker card, resulting in a very clear gnomon image. It also provides a slit imaging method: a seeker card with an engraved slit is placed in the shadow of the pseudo-umbra of a cylindrical gnomon, with the slit parallel to the gnomon. A bright white line with a thin black line will form on the gnomon surface behind the seeker card. This thin black line is the image formed by the gnomon shadow through the slit. The position of the seeker card is adjusted until the bright white lines on both sides of the thin black line are of equal width. The position of the thin black line in the gnomon shadow is the projection position of the gnomon's axis, which is also the projection position of the apparent diameter of the sun.

Claims

1. A device for improving the clarity and accuracy of sundial shadows, characterized in that: The device includes a lookout card and a movable support for the lookout card. The lookout card is an opaque card with white sides and a slit on it, which is either straight or cross-shaped. The movable support for the lookout card includes a support body, a long iron arm support, a magnet, a long iron L-shaped support, and two U-shaped hanging pieces on the shoulder of the support body. One side of the U-shaped hanging piece is fixed to the support body, and the other side is fastened to the annular dial surface of the equatorial ring sundial. One end of the long iron arm support is connected to the center of the support body, and the other end of the long iron arm support is connected to the L-shaped iron support via a magnet. The seeker card is fixed to an iron L-shaped bracket by a magnet. The seeker card is placed in the pseudo-umbra behind the umbra of the gnomon, with one side of the seeker card facing the sunlight. The slit of the seeker card is placed in the blurred shadow of the gnomon and parallel to it. A white bright bar with a thin black line in the middle will appear on the gnomon face behind the seeker card. The thin black line is the image of the cylindrical gnomon formed by the slit. At this time, the image of the thin black line, the center line of the slit, the central axis of the gnomon rod, and the vertical diameter of the apparent sun are all in the same plane. The position of the thin black line is the projection position of the central axis of the gnomon.

2. The device for improving the clarity and accuracy of sundial shadows as described in claim 1, characterized in that: The upper edge of the slit of the image seeker is arrow-shaped.

3. The device for improving the clarity and accuracy of sundial shadows as described in claim 1, characterized in that: The width of the slit is 0.5~0.8mm.

4. The device for improving the clarity and accuracy of sundial shadows as described in claim 1, characterized in that: The support body is arc-shaped, and the width of the U-shaped hanging piece is the same as the width of the annular dial surface of the sundial.

5. A method for improving the clarity and accuracy of a sundial shadow, applied to the apparatus for improving the clarity and accuracy of a sundial shadow as described in any one of claims 1 to 4, characterized in that: The cross-shaped finder card is used to image the shadow of the cross-shaped gnomon and determine the position of the apparent center of the sun: The cross-shaped finder card not only displays the position of the projection of the axis of the vertical or horizontal gnomon, but also displays the position of the intersection of the projection lines of the axis of the vertical and horizontal gnomons, that is, the position of the projection of the apparent center of the sun.

6. A method for improving the clarity and accuracy of a sundial shadow, applied to the apparatus for improving the clarity and accuracy of a sundial shadow as described in any one of claims 1 to 4, characterized in that: Used for reading large equatorial ring sundials.

7. A method for improving the clarity and accuracy of a sundial shadow, applied to the apparatus for improving the clarity and accuracy of a sundial shadow as described in any one of claims 1 to 4, characterized in that: Used to determine the north-south direction in geography.

8. A method for improving the clarity and accuracy of a sundial shadow, applied to the apparatus for improving the clarity and accuracy of a sundial shadow as described in any one of claims 1 to 4, characterized in that: Used to determine the length of the shadow cast by a gnomon at noon on the winter solstice.

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