Garden Sundials and beyond: How to read them and how they work

Garden Sundials and Beyond: How Most of These Instruments Work and How to Read Them – for Gardening Enthusiasts and More

December 15, 2025

Sundials are instruments capable of indicating the time with an accuracy of up to one minute.
But why does the time shown by sundials, with only a few exceptions, not coincide with that indicated by ordinary clocks? Essentially, this happens because sundials, again with few exceptions, show the true solar time of the place where they are installed.

The clocks we use in everyday life, instead, all display time zone time: an arbitrarily established convention designed to unify the time of several countries to facilitate human activities – for example transport and telecommunications. Wintertime is commonly referred to as “solar time” because it is the closest approximation to true solar time, especially when compared with summer daylight saving time.

To compare the true solar time indicated by sundials with the time kept within a time zone, two distinct corrections must be applied to the sundial reading: the mean time correction and the longitude correction. To understand why these differences arise, we must first understand the operating principles of sundials.

The map shows in red the extent of Central Europe’s 1st Eastern Time Zone, which was adopted in countries located well beyond the 15° span related to its reference meridian.

How a Sundial Works: Basic Principles and How to Read the Time

A sundial is essentially a dial – whose shape and size may vary – accurately marked and equipped with an element called a gnomon, used to cast a shadow onto the dial. The gnomon itself may take different forms and dimensions depending on the type of sundial being constructed. The shadow cast by the gnomon moves in the opposite direction to the apparent motion of the Sun.

The Sun’s apparent motion across the sky is caused by the Earth’s actual rotation on its own axis.

The principle by which the hour lines are laid out is based on the rotation of the Earth, which brings the Sun to cross the local meridian every 24 hours. Dividing the full 360° rotation on the equatorial plane by 24 hours results in 15° per hour, and 4 minutes for every 1°.

The same principle can be applied at any point on the Earth’s surface, provided the gnomon is always set parallel to the Earth’s axis and ignoring parallax error which, given the astronomical distances involved, is negligible for the accuracy required by these instruments.

Naturally, the hour lines on sundials constructed at different latitudes – whether horizontal, vertical or inclined – are no longer spaced 15° apart, but become deformed according to the angle at which the dial intersects the gnomon aligned with the Earth’s axis of rotation.

The plane of an equatorial sundial can be translated to any point on Earth, provided that its rotation axis (the gnomon) remains parallel to Earth’s rotational axis (it must point toward the North Star).

The hour lines, spaced every 15° on the equatorial plane—perpendicular to Earth’s rotational axis—become distorted when projected onto horizontal or vertical dials. This deformation varies with latitude. To be accurate, sundials must therefore be designed specifically for their correct latitude, since not only the gnomon’s inclination but the entire hour diagram changes.

The Sun, in its apparent daily motion, describes an arc from east to west, passing southwards and reaching its highest point of the day when it crosses the local meridian on which the sundial is installed. This apparent arc is never identical from one day to the next: between the winter solstice and the summer solstice it gradually rises, while from the summer solstice to the winter solstice it gradually descends. In the ten days surrounding the solstices, however, this change becomes so slight as to be imperceptible without appropriate instruments. The Sun appears almost still, and it is precisely this apparent “standstill” that gives rise to the term solstice.

Apparent arcs traced by the Sun during the solstices, equinoxes, and the dates marking the transitions between zodiac signs. Simulation created for latitude 45°. Dates represent statistical averages and may vary slightly from year to year.

In its apparent daily rotation, the Sun reaches its highest point in the sky when it crosses the celestial meridian (the reference meridian for true solar time, as indicated by a sundial), which divides the Sun’s daytime arc exactly in half.

Using the local meridian as a reference, the celestial sphere can be divided into slices by placing additional meridians spaced every 15°, marking the Sun’s apparent passage each hour.

The Sun’s height in the sky changes from one day to the next (except for the 10 days around the solstices). This variation is used to draw calendars on sundials. To do this, the tip of the gnomon—inclined parallel to Earth’s rotational axis—is used as a reference, or a “node” (a mark or a small sphere) is placed on it. Alternatively, a gnomon orthogonal to the dial can be used, such as an obelisk (painted red in the image), whose tip corresponds to the tip of a virtual gnomon.

The Sun’s varying height in the sky is used to trace calendars on sundial dials. Typically, these calendars follow the signs of the zodiac; more rarely, they show civil calendars. However, the same principle can be used to draw personalised lines marking birthdays, anniversaries and other commemorative dates.

The image shows how the southeastern sky would appear if it were possible to see the meridians marking the hours and the celestial parallels showing the Sun’s arcs on the days when zodiac signs change, along with sunrise at the solstices and equinoxes and the Sun’s apparent path.

Like the previous image, this one shows how the southwestern sky would appear if the hourly meridians and celestial parallels were visible, along with sunset at the solstices and equinoxes and the Sun’s apparent path

Since the Sun follows the same apparent arc twice a year—once while rising and once while descending—the dials display double indications. The only possible ambiguity is resolved simply by knowing the season in which the observation is made.

When we observe a properly made sundial with hour lines and a calendar, it is as if we were looking at the sky and its meridians and parallels reflected in a mirror. The shadow moving across them simply represents the Sun’s apparent motion in the sky.

In practice, the gnomon’s shadow moves across the dial like the hand of a conventional clock: its angle indicates the hour of the day, and its length reveals the month and date. Alternatively, we may think of it as the shadow of a finger tracing the diagram on the dial, showing us the day, the hour and the Sun’s position in the sky.

Correction for Mean Time and New Conventions for Measuring Time

For centuries, timekeeping remained unchanged, based exclusively on the Sun, regarded as a precise and indispensable reference. Everything changed around 1650: in the seventeenth century, with the advent of more reliable mechanical clocks, it became clear that the solar day does not always last exactly twenty-four hours. Depending on the season, it can vary daily by a few seconds, and at certain times of the year the Sun’s passage across the local meridian may occur more than 20 seconds earlier or later than the exact twenty-four-hour mark.

To understand the cause of these variations, we must remember that the Earth not only rotates on its axis, but also orbits the Sun at a speed of approximately one degree per day (since it covers 360 degrees in about 365 days). Consequently, for the Sun to cross the same meridian again on the following day, the Earth must rotate 361 degrees on its axis (360 degrees plus one) to compensate for the degree lost in its daily revolution around the Sun.

Seen from space and compared with the fixed stars, Earth rotates 360° on its axis in 23 hours, 56 minutes, and 4 seconds (annual average). This rotation is called the sidereal day.

Seen from space (sizes and angles exaggerated to highlight the differences), and compared with the Sun—the star at the centre of its yearly orbit—Earth must rotate 360° plus the additional degree travelled in its daily orbital motion for the same meridian to face the Sun again. This rotation is called the solar day.

Unfortunately, the Earth follows an elliptical orbit around the Sun, which occupies one of the foci of this ellipse, and the complex gravitational forces acting upon it (which we will not discuss in detail here for the sake of simplicity) cause it to travel along its orbit at varying speeds depending on its position. A change in orbital speed affects the angular distance covered in the approximate daily degree of revolution. Indeed, when the orbital motion is faster, the Earth travels slightly more than one degree and must rotate correspondingly further on its axis for the Sun to cross the same meridian, inevitably causing the Sun to transit the meridian later. Consequently, all the hours of that day will also be “late” compared with the previous day. Conversely, if the Earth is in a slower section of its orbit, it covers less than the approximate daily degree and the Sun crosses the meridian earlier. Even tiny fractions of a degree produce noticeable differences from one day to the next, which, when accumulated over time, result in systematic oscillations—depending on the season—of approximately 14 minutes in advance and 16 minutes in delay. In practice, the Sun’s meridian passage, and therefore all the hours it defines, are subject to regular cyclical fluctuations.

To avoid the inconvenience of having to synchronise mechanical clocks with the real Sun every day, it was decided to adopt “mean time”: a reference based on a virtual Sun which, by convention, moves at a perfectly constant rate throughout the year, thereby cancelling out the fluctuations in the true solar day. The difference between true solar time and the mean time indicated by mechanical clocks became known as the Equation of Time.

If we position a camera facing south toward the sky and take a picture every day at 12:00 according to a well-made sundial, then superimpose the images at year’s end, we obtain an image like the one above: a vertical line of solar disks.

The solar disks in the image show the portion of the celestial meridian between the summer solstice (top) and the winter solstice (bottom).

This effect of mean time can be easily observed by comparing the Sun’s passages across the local meridian, since the other astronomical events related to our star—such as sunrise and sunset—are naturally subject to seasonal variations.

If we point a camera due south and take a photograph at precisely mean noon (as indicated by an ordinary clock) on several consecutive days, then superimpose the images, we can observe the advance and delay of mean noon relative to true solar noon, represented by the meridian line. If we continue this process throughout the entire year, the resulting figure resembles an elongated figure-of-eight.

If we are located exactly on the longitude of a time-zone reference meridian (such as Greenwich or any 15° multiple), and we take a photo every day at 12:00 according to a well-synchronised clock, then superimpose the images at year’s end, we obtain an elongated figure-eight of solar disks. This is commonly called a lemniscate or analemma.

The solar disks in the image show the delay (disks to the left of the meridian) and the advance (disks to the right) of mean time throughout the year, relative to the celestial meridian.

In some well-crafted sundials, and in true meridian lines, the advance or delay of the virtual Sun for each noon of the year was often indicated along the line of true local noon. This was made possible through the depiction of the so-called lemniscate, formed by two overlapping curves. To interpret it correctly, the line needed to be drawn in such a way that each section of the figure corresponded clearly to the different months or seasons.

The Sun reaches the same apparent height twice a year (except at the solstices): once while rising toward summer and once while descending toward winter. The image above shows, on the outer ring, the zodiac signs crossed during the Sun’s ascent from the winter solstice to the summer solstice (in green), and those crossed during its descent from the summer solstice to the winter solstice (in blue). Similarly, on the lemniscate, the Sun’s ascending and descending paths between the two solstices are shown, together with the delays and advances of mean time relative to the celestial meridian.

The difference between mean solar time—kept by ordinary clocks—and true solar time—traditionally shown by sundials—is called the Equation of Time. These values are usually displayed in a specific table placed on or near the dial, and represented in a chart. In this chart, a horizontal line represents the twelve months of the year, while the perpendicular columns indicate the minutes to be added to or subtracted from the time shown by the sundial, depending on the date of observation.

Typical graph representing the equation of mean time. It is often found drawn directly on sundials or at their base. The red curve indicates the number of minutes (shown on the left) that must be added or subtracted—according to the observation date, displayed along the bottom—to the time indicated by the sundial to obtain the mean time of the time zone (assuming one is located exactly on the reference meridian, such as Greenwich or any 15° multiple, since this graph includes no correction for longitude differences).

The Adoption of Time Zones Added Another Gap Between Local Solar Time and Civil Time

Local mean time, like true local time, is strictly tied to the meridian of the place in question. It is identical for all locations lying on the same meridian but changes with longitude. For example, each degree of longitude corresponds to a chronometrically measurable difference of four minutes, meaning that, historically, a single country could have dozens of different local times simultaneously. Towards the end of the nineteenth century, with the development of railways and telecommunications, most nations recognised the need to regulate time on a scale wider than that of each town or village, and thus adopted the system of time zones. This convention involved dividing the world into twenty-four one-hour sectors, based on meridians spaced 15 degrees apart, starting from the Greenwich Prime Meridian at 0°. In principle, all territories crossed by a given reference meridian adopted the local mean time of that meridian.

How to Read a Sundial Today: Time Zones and Mean Time

Apart from sundials built exactly on the reference meridian for the time zone in use, all other sundials—designed to display true local solar time—require a correction in order to be compared with civil time. It is necessary to add or subtract four minutes for each degree of difference from the reference meridian: if the sundial lies to the east, four minutes per degree are subtracted; if it lies to the west, they are added. This is a constant correction throughout the year and is linked to longitude. For a correct modern reading, this correction must be combined with the mean-time correction, adding or subtracting the minutes corresponding to the date of observation. To simplify the reading of modern sundials, the constant longitude correction is usually incorporated directly into the mean-time chart, placed next to or superimposed on the dial.

If we are in a location not lying on the reference meridian of a time zone, the elongated figure-eight will appear shifted relative to the celestial meridian: by 4 minutes for each degree of longitude away from the reference meridian. For example, the image above shows the difference between the previous set of photos and those that would be taken in a location 4 degrees west (roughly the distance between Plymouth and the Greenwich meridian).

Example of a graph including longitude correction for reading mean time. In this graph, a correction of 16 minutes has been added for a sundial located 4° west of the reference meridian (the previous example comparing Plymouth with the Greenwich meridian).

Example of a horizontal garden sundial. The dial, made of stoneware and painted with ceramic colors in a third firing, shows the local true solar time and the classic lemniscate, drawn while accounting for the longitude difference so that it indicates the time zone’s noon.

Dials Designed to Indicate Time-Zone Time Directly

In some modern dials featuring a lemniscate, the longitude correction can be incorporated into the design of the diagram itself, so that the shadow falling on the figure indicates the time-zone’s time directly.

Stoneware dial decorated with ceramic colours (third firing) for a horizontal garden sundial, with hour lines arranged in the central part of the dial in the shape of multicoloured lemniscates, indicating the mean time of the time zone.

The Most Recent Adjustment to Local Solar Time

Wintertime is often—albeit incorrectly—referred to as “solar time” because it is the closest approximation to true solar time, especially when compared with summer daylight-saving time.

Beginning in the twentieth century, many countries introduced and adopted daylight-saving time to make better use of the increased daylight available in summer, adding yet another correction to those described above. Although some sundials include a double scale—one for winter and one for summer—this is a relatively minor issue. Generally, anyone familiar with sundials knows that during the summer period, daylight-saving time must be applied by simply adding one hour to the time indicated by the sundial, following any previous corrections.