Observation Sundials

Sundials were one of the first ways used to read time. A stick first was used, called a 'gnomon' as it was driven in soil. Egyptians then divided day and night each into 12 hours using 'decans' or stars groups. That allowed to a type of sundial called 'temporary -or unequal, or seasonal- hour sundial' as length of hours read on the sundail was dependent upon each season. It was in the Middle Ages only that the West began to used 24-hour sundials with a tilted gnomon, or 'style.' That had been taken from the Arabs, with each hour of equal duration whatever the season. That type of 'equinoxial hour' sundial had been already known by Hellenistic astronomers and likely too by Babylonians. A gnomon, or style, is the feature which projects its shadow upon the sundial, allowing to determine the hour of the day. As they were used together with clocks, sundials reached their apogee in the 18th century A.D. and gave birth to the science of gnomonics and sundial-makers. A large variety of sundials existed since Antiquity as some of those were very elaborated, and some portable. Some sundials called 'analemmatic' have the specificity that the observer self's shadow, as the observer stands at some point of the sundial, serves like the gnomon. Ancient sundials often had mottos written on as they often were in Latin and of diverse inspiration. Most known of those is that famed one: 'Vulnerant omnes, ultima necat,' which translates 'Every (hour) is injuring man, The last one is killing him'

The time read on a sundial is the local solar time. Such a reading was enough as long modern and fast means of transport like railways did not appear. Those necessitated then the establishment of a system of legal time with timezones. To translate the solar time as read on a sundial to civil time, a series of corrections are needed. That is due to what is called the 'Equation of Time' (check more at our tutorial 'Time Systems'), the longitude of the sundial's location and the timezone of the location, with a possible existence of a ST/DST system

• function of the date, a correction is applied to the time read on a sundial through the use of a Equation of Time's diagram
• as far as the sundial location's longitude is concerned, as the Earth is rotating upon itself by 1 degree in 4 minutes, the correction equals 4 times the location's longitude in degrees. The result is in minutes. Correction is negative for eastern, and positive for western longitudes respectively
• the last correction is the one for the sundial's location timezone relative to the Greenwich Meridian and then a possible ST/DST correction

click to a diagram for the Equation of Time

some elaborated sundials may feature such or such, or all of the corrections mentionned above. Some sundials are deviced in such a way they straightly give the U.T. time. Any sundial, on a other hand, allows to read solar time at night using the shadow provided by Moon. That needs a correction relative to the Moon's age (which is not available with our text)

A simple, unequal-12-hour, horizontal or vertical sundial, reading from 6 a.m. to 6 p.m., is easy to build

• both the horizontal and vertical sundial require that the place, either horizontal, or vertical -which is ckecked with a plain or a plumb line respectivelt=y- faces due South and that, as much as possible, it has a plain horizon from East to West and even beyond. The Sun by summer indeed is rising beyond East and setting beyond West. As far as a vertical sundial is concerned, you will have to have a wall, or a base, facing due South at your disposal, which is checked with a compass relative to the East-West axis of that wall of base
• you will then find a rectangular-shaped plank, or any suitable element, of wished dimensions
• you will draw a rectangle upon that support and draw the median line which divides that rectangle in two halves along its larger length
• then draw 12 hour lines through a protractor beginning at either mid-point of each smaller size of the rectangle. Each time line is spaced from the other by 15 degree. The Sun indeed, grossly, is scanning 360 degrees a day related to the sundial's location, which means that one solar hour is worth 15 degree
• as far as the style, or gnomon, is concerned, the theory is that its angle relative to the sundial must equals the latitude of the sundial's location. That value is easy to check with a map, a software like Google, or a GPS. The style has to attached to the point of the rectangle whence time lines are originating
• from there, you need now to number each time line and install the style, both that steps depending upon whether you are building a horizontal, or vertical sundial. In the case of a horizontal one, the style will be directed towards due South. Numbering time lines thus, from 6 (a.m.) to 6 (p.m.) will have to be done from the left to the right of the lines' point of origin (left and right as determined with the sundial seen from above and the point of origin to the bottom (or South)). In the case of a vertical sundial, the style will hang at the top of the sundial and numbering will be done from the left to the right of time lines' point of origin, (left and right determined facing the sundial and the point of origin to the top). The attachment of the style and the making of the style's angle are left to each one DIY skills!
• thence, you will have to install -permanently or not- your sundial at the planed location (which also depends upon your DIY skills). As far a horizontal sundial is concerned, first check the plane of the location with a plain and then direct, with a compass, the sundial's main axis into a North-South position. The style attachment point must lie South. As far as a vertical sundial is concerned, it will have to be attached to a vertical wall, and checked with a plumb line, due South (which is compass-checked). In that case, the style attachment must lie at the top

click to a view of a simple, horizontal or vertical sundial. The style of a horizontal sundial must be directed due South

To read time at your sundial just follow the theoretical steps already described. The projected shadow of the style is the solar time for the sundial's location. It has to be correcte with the Equation of Time for the date, the longitude negative or positive correction (4 minutes of time for each degree East of West of Greenwich Meridian), and the one for your location's timezone with a possible ST/DST system. That correction process may be simplified provided one draws a diagram encompassing all of values for a whole year as such a diagram will be used when reading time on the sundial