### Find a Sundial

Babylonians and Egyptians built obelisks which moving shadows formed a kind of sundial, enabling citizens to divide the day in two parts by indicating noon.

The oldest known sundial was found in Egypt and dates from the time of Thutmose III, about 1,500 years BC. There were two strips of stone, one that did the needle and another where the hours were marked.

After this first known sundial, we must advance to the 750 BC to have references from another sundial, and is found in several Old Testament passages that describe a sundial, that of Ahaz. A biblical reference tells how Yahweh did the shadow go back ten degrees on the dial. However, we are sure that there were other much earlier among almost all peoples of antiquity, although there is no evidence so clear as in this case.

Moreover, the earliest description and design of a concave sundial is attributed to the Babylonian Berossus in the IV century BC.

With the Greeks, sundials are studied thoroughly and for the first time, the gnomon stops of being installed vertically and passes the correct position, parallel to the Earth's axis. They developed and constructed complex sundials using their knowledge of geometry.

The watch Greek is called "scaphoid" (bowl) and consisted of a block in which a cavity was emptied hemispheres, at whose end is fixed the needle bar serving.

Put the gnomon parallel to the direction Earth's axis allowed the clock signal throughout the year the hours of a constant duration, making measuring instruments, really. In the previous vertical needle had clocks where summer hours were different from those of winter (as we have already commented above). It should also be mentioned that the scaphoid were also the first sundial that measured time by the direction of the shadow and not, as heretofore, by its length.

In fact, almost all posterior cultures, at least, those who had direct or indirect contact with the Greeks used for their design sundials Greeks: the Romans, Arabs, Indians, Afghans and so on. The Greeks sundials used refinements like the orientation of the object that casts the shadow or gnomon, which did not have to be perpendicular to the ground, and the geometric shape of the surface on which the shadow cast, which did not have to be flat, and they got excellent precision for the time, precision of a few minutes that would not be surpassed for centuries.

In picture 2 we can see a splendid Greek sundial called Horologion or Tower of the Winds. It consists of an octagonal marble building oriented according to the cardinal points and topped with a conical dome. This building was entrusted to Andronicus Cirrus that he did in 50 BC. With the Roman domination the ancient Agora of Athens was too small for their duties and it was decided to build a new one to move their business activities of the city. This new place was endowed with this advanced sundial: the Horologion.

The Romans copied the Greek scaphoid, which he called hemispherium. The ancient Romans, from the scientific point of view, did not add anything new with regard to measuring time, continued to use sundials developed by the Greeks.

Pliny the Elder in his Natural History relates the history of the sundial that Emperor Augustus ordered to build in the Campus Martius, using an Egyptian obelisk of Pharaoh Psamtik II, called the Solar Clock Augustus or Augustus Meridian.

On the astronomical content found in the architecture of the Pantheon in Rome, built by Agrippa in the first century BC, there is no doubt. But now some researchers argue that the Roman building acts as a huge sundial (Picture 4).

According to Roman architect and engineer Vitruvius, were used at least thirteen different types of sundials. Vitruvius wrote a book about gnomonics in which he describes a geometric method for designing sundials called analemma.

The Roman Empire's decline and fall because of the barbarian invasions, led in Occident a long period of intellectual darkness.

In the early centuries of the Christian era, the gnomonic, dimly lighted by studies of Hellenistic astronomy is entering a decline that characterizes the entire science of European medieval cultural and economic. There are few items (mostly archaeological) we can find; there are just written to show further progress. Although in this period to the general public cared little time measurement, there are no precise scientific descriptions. However, as oddities at the time, there were two surveyors: the Venerable Bede and Higinio Gromat (II century).

You need to wait until feudalism assist the dissemination of sundials on the European continent. It was the religious order Benedictine (529 AD) and his dedication to comply with the schedule dictated by its founder, what encouraged these monks to study the construction of sundials.

Since its origin, the Catholic Church wanted to sanctify certain times of the day with a common prayer. The gnomonic of these centuries led to the construction of Mass clocks or watches of canonical hours and in them the hours of prayer were indicated. These watches are generally located in the southern facades of churches or monasteries.

First sundials carved on the stone facades of churches and cathedrals are starting to appear early VIII century. In the year 1000 horizontal sundials were constructed for which holes were used in the vaults of cathedrals.

In the IX century Arabic astronomy comes in. The caliphate of Al Mamun marks the beginning of an intense cultural activity would continue in later centuries with writers such as Averroes, Ibn Thabit Qurraa (826-901) and Al-Biruni (973-1048) as example. While Christian Europe at the time followed the works of the Venerable Bede, the Arabs had a hectic continued intellectual activity from the destruction of the Alexandria Library. It is only from the X century when Europe begins to look timidly vast compilation of ancient knowledge work done by the Arabs.

The majority of Arabs watches were flat at that medieval times, constructed of marble or copper plates. They all have an indication of the direction of the Kaaba in Mecca because of the religious precept of praying with the face turned to that place regardless of where they are located.

The XI century, a German mathematician who knows the Arabic language, wrote a treatise on the astrolabe retaining some Arabic terminology. In this treaty are some indications for the shepherd's sundial. The translation of two Arabic manuscripts gnomonic was most important cultural advance of the time in this field.

In the XIII century in Spain, King of Castile Alfonso X the Wise put together in the city of Toledo a large group of Christians, Greek, Hebrew and Arabic astronomers to translate into Latin many of the works written in Arabic. Thus the Arabic knowledge spread throughout Europe to leave behind all the cultural obscurantism in which it was immersed. Also the gnomonic was developed, like all sciences.

In the XIV century, the first mechanical clock is made. It is a large iron-framed structure, driven by weights. The function of the first European clocks was not to indicate the time on a dial, but to drive dials that give astronomical indications, and to sound the hour. They are located in monasteries and public bell towers. The earliest surviving example, constructed in 1386, is in Salisbury Cathedral, England. Mechanical clocks utilize equal hours.

In Spain during the reign of Enrique III, in 1400, the first mechanical watch with bells was installed in the tower of the church of Santa Maria de Sevilla.

The following centuries were the great age of the European sundial. In the XV century a great effort was made in Europe by the divulgation of the Gnomonic. Sundials with equal hours gradually come into use.

In the American colonies were built many sundials, some of which are still preserved. In the tropics you have to build a double disc with time. The south-facing disk is used for part of the year, from August to April, and the disk on the other side facing north would use the rest of the year. Two days a year, when the Sun passes directly above the site hours can be seen on both sides.

By the mid-XVI century the first mechanical clocks appear. It is in the XVII when these devices are refined and slowly getting more accurate operation.

The onset of the Renaissance saw an explosion of new designs. Giovanni Padovani published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. Also, Giuseppe Biancani published (1620) other treatise where discusses how to make a perfect sundial with accompanying illustrations.

In the XVIII century clocks and watches begin to replace sundials. They have advantage of not requiring sunny skies. There are, however, often unreliable and depend upon sundials to set the true time.

In the early 1800’s mechanical clocks become accurate enough and inexpensive to displace sundials as timepiece of choice.

At present, although the accuracy of mechanical clocks outweighed sundials, they continue be built, primarily as a decoration on buildings, monuments and public places. They are constructed of many types with precision and beautiful designs. The support of the computer to calculation and design of the sundial has been fundamental. As a result of this technological support, are living the revival of this ancient instrument for measuring time in recent years, but as mentioned above, its function is currently not precisely what sundial was born but only as a decoration.

In any case, I welcome the resurgence of the sundial!!