Time is the great arbiter of duration. It dictates continuance, succession, and simultaneity. The concept of time has defined human existence. It owes its development to quite a storied past. Before the development of instruments that abstracted such a concept into a measurable quantity, any sort of cognizance of time was reduced to rhythms set forth by the forces of nature. The rising and the setting of the Sun, the movements of celestial objects, as well as the changing seasons helped early humans grasp a certain pattern of regular change, heralding the first inklings of any perception of the passage of time.

In particular, great celestial devices such as the Sun allowed a periodic progression of day and night and established a certain reference point for marking the passage of time. In due course, the notion of the day began to emerge as the Sun was observed to reappear right after disappearing the evening prior. Shadows cast by the Sun were noted to change regularly in length and in direction as day tended towards night and as the Sun moved in its apparent path across the sky, observations which soon instituted the notions of morning, noon, and afternoon.

Early humans noted that during the morning as the Sun lay low on the eastern horizon and as light from the Sun hit the Earth at a shallow oblique angle, shadows cast from vertical objects appeared to be relatively longer. By contrast, at noon, when the Sun lay directly overhead and reached the highest point in the sky relative to the horizon, light from the Sun hit the Earth at a steeper, more direct angle, resulting in the shortest shadows. After noon, as the sun sank lower on the western horizon, shadows lengthened again. Thus, at night, just like in the morning, objects, too, were observed to cast their longest shadows. To ancient observers, the Sun only appeared to move from the eastern horizon to the western horizon. This “apparent motion of the Sun” as it is so called, is in reality, caused by the spin of the Earth from west to east, which leads to an observer on the Earth observing the Sun rising in the East and setting in the West. Ancient Greek astronomers were led to believe that celestial objects such as the Sun were attached to the inside of a sphere, a so-called “celestial sphere” that rotates around the Earth, an imaginary concept which, to this day, remains widely employed by astronomers to describe the apparent motions of celestial objects.

The revolution of the Earth, the rotation of the Earth around its axis, and the resulting apparent motion of the Sun across the celestial sphere, were thus some of nature’s great early governors of time. They allowed humans to develop the first insights of dividing the passage of time into periods of a particular convenient length.

Ancient Egyptians and ancient Babylonians were among the first to employ these notions to develop one of the first known instruments of time-keeping, the sundial. The sundial consisted of a vertical rod, known as gnomon, which cast a shadow on a plate. As the Sun moved during the day, so did the shadow cast by the gnomon onto the plate. The position of the gnomon thus determined time. About 3500 years ago, ancients Egyptians are thought to have been using obelisks as sundials to divide the day into two parts, as shadows cast by the obelisks gradually shortened from sunrise to noon, and then lengthened again from noon to sunset. Eventually sundials became sophisticated enough to incorporate calibrated markings, ones which further divided the day into specific hours, corresponding to predictable variations in lengths and in directions of shadows cast as the Sun moved along in its journey.

As their understanding developed, ancient Egyptians, Babylonians, and Sumerians are thought to have further divided the day into 12 equal parts and the night into 12 equal parts. The Bablyonians, using the (sexagesmial) base-60 number system for calculations, divided the hour into 60 minutes and those minutes again into 60 seconds. The base-60 number system provided an added advantage in that the number 60 has many divisors (2, 3, 4, 5, 6, 10, 12, 15, 20, and 30). Such a system would also, later, contribute to geometry, where degrees would be divided into 60 arcminutes, each of which would be divided into 60 arcseconds, a notion introduced in Ptolemy’s Almagest.

However, as observations of the Sun ensued, Ancient Babylonian, Egyptian, Chinese, and Greek astronomers also noted that the maximum height that the Sun attained in the Sky differed from day to day. In the Northern Hemisphere, they observed that the Sun moved northwards until it was directly at 23.5 N latitude (today known as the Tropic of Cancer). The Sun, then stopped for a day and commenced a southwards journey, whereupon it continued to travel southwards until it was directly at 23.5 S latitude (today known as the Tropic of Capricorn) in the Southern hemisphere. Again, the Sun stopped for a day before reversing direction and commencing a northwards journey. As the Sun journeyed northwards, days grew progressively longer as the Sun lay above the horizon more of the time. Yet, as the Sun changed course and moved southwards, days grew shorter and shorter as the Sun lay lower on the horizon. The day where the Sun was observed to have journeyed farthest north of the celestial equator later came to be known as the summer solstice and it represented the longest day of the year in the Northern Hemisphere. Conversely, the day where the Sun was observed to have journeyed farthest south of the celestial equator later came to be known as the winter solstice. This represented the shortest day of the year in the Northern Hemisphere. Such notions led ancient astronomers to the realization that the Sun did not just move eastwards to westwards every consecutive day but also that it moved northwards to southwards as time progressed.

The observation of such a regular cycle of the Sun’s motion helped ancient astronomers establish yet another length of time, namely the year, representing the passage of the Sun from the farthest north to the farthest south and its subsequent return to the farthest north.

As ancient astronomers continued to study the sky, they noted that other celestial objects were, too, exhibiting cyclical change. One such celestial object, the Moon, was observed to change in form. In particular, it went from New Moon to Full Moon and back again to New Moon. These changes were observed to take place the same number of days apart, at about 30 days. The Moon, as a consequence, allowed ancient astronomers to devise yet another concept of time, namely the lunar month. It became common, thereupon, in ancient parlance to employ expressions such as “two or three moons ago”. The word month, in fact, comes from the word moon.

These recurring patterns exhibited by the Sun and the Moon, in addition to having already helped establish the concept of the day, moreover allowed a larger measure of time that formed the basis of the calendar in many ancient civilizations. About 6000 years ago, the ancient Egyptians were the first to devise a solar calendar that also incorporated the rising of Sirius, the brightest star in the sky. Ancient Egyptians noted that Sirius rose next to the Sun every 365 days. In fact, the rising of Sirius was found to coincide with every successive inundation of the Nile. Since the inundation of the Nile was such an important event in Ancient Egypt, the beginning of the civil year was marked by the rising of Sirius. The Egyptian calendar, however, was 0.25 days short of a true solar year as we, now, know it, corresponding to 365.2422 days or the period of time required for the Earth to complete one full revolution around the Sun, as measured by successive arrivals of the Sun to the vernal (spring) equinox. Such a difference added up with time, causing the Egyptian calendar to “wander in time” and deviate from the seasons. In particular, the Egyptian calendar drifted from the average solar year by one day in 1460 years. Ancient Egyptians could have accounted for this by adding one day to the 365-day year every four years. Yet, the concept of a leap year did not exist at the time. Nonetheless, about four thousand years ago, ancient Egyptians started devising calendars based on the lunar cycle to plan religious ceremonies. This lunar calendar comprised alternating 29-day and 30 day months, giving a year of roughly 354 lunar days. Ancient Greeks and Babylonians are thought to have harmonized the lunar calendar with the solar year by adding a 13th month to some years.

In order to avoid the confusion of lunar calendars encompassing alternating 29-day and 30-day months to account for the lunar month averaging 29.5 days, ancient Babylonians devised a schematic solar calendar of 360 days, following their tradition of the base-60 number system. In this calendar, every month amounted to 30 days in length and each year comprised 12 months. This calendar was still 5 days too short of a solar year, a difference which added up to 30 days every 6 years. The Babylonians accounted for this by adding a 13th month once in 6 years. This was accomplished by doubling the Babylonian month of Adar.

The Babylonian calendar remained in use up to the time of the Romans. After a certain period of time, it is thought that the Romans came to abandon the lunar calendar in favour of following the solar year. In particular, the Romans devised a calendar that consisted of 355 days, with months comprising either 29 or 31 days except for February which comprised 28 days. An extra month of 22 or 23 days was added every two years to conform with the seasons. It is thought that Roman pontiffs commonly manipulated the calendar by adding or removing extra months for reasons of political gain such as extending terms of office or delaying elections, to the point that, by the time of Julius Ceasar, the Roman calendar had been losing concord with the seasons and had been drifting three months ahead. As a result, when Julius Caesar became Dictator of Rome, he commissioned the Greek-Egyptian astronomer Sosigenes of Alexandria to reform the calendar. The Julian calendar, one which was 365 days and 6 hours in length, incorporated an extra leap day, added to February, every four years to account for any drifts that a 365.25 day cycle of a solar year, as concurrently thought to have exactly been, would cause. The calendar decreed that the 1st, 3rd, 5th, 7th, 9th, and 11th months should comprise 31 days each but that the remaining months should comprise 30 days, except for February which was to comprise 29 days. Caesar also changed to name of the month Quintilis to Julius (July), after himself. Caesar’s successor Augustus would, also, fancy a month named after himself so the name of the month Sextilis would, later, change to Augustus. Furthermore, Augustus would decree that his month ought to be just as long as Julius Caesar’s month of July. He would, thus, take the 29th day from February and add it to August, leaving February with just 28 days.

The Julian calendar, incorporating changes made by Augustus, would persist for quite sometime. It, however, still erred by being 0.00781th of a day (11 minutes and 14 seconds) in excess over the true solar year. This presented a notable problem by the year 1582, where the vernal equinox (the first day of spring) occurred 10 days earlier than it should on March 11th, instead of on March 25th. Since that meant religious holidays moving earlier into the season, Pope Gregory XIII decreed for a new calendar to be established, the Gregorian calendar. He removed 10 days from that year. Hence, October 4th was followed by October 15th. Furthermore, in order to account for the 0.00781th of a day deviation exhibited by the Julian calendar amounting to 3 days every 400 years, Pope Gregory XIII introduced other changes such as the omission of leap years divisible by 100 but the retention of leap years divisible by 400. Despite this, even the Gregorian calendar is not perfect. Its 0.0003 day discrepancy from the actual tropical year will accumulate to a day in 3300 years.

Human progress in reckoning time spurred major attempts to develop instrumentation to mark intervals of time. Sundials were insufficient on their own since individual hours determined were equal to 1/12 of daylight or 1/12 darkness, and were thus unequal in duration. As previously explained, hours were observed to be longer in the summer when days were longer yet shorter in the winter when days were shorter. The fact that those “temporal hours” did not reflect equal increments of time spurred other developments in timekeeping. In the 13th century, Arab astronomers perfected the use of equinoctial hours (equal-length hours) for astronomical calculations and developed devices that helped mark a shift from a dependance on seasonal hours. The introduction of mechanical clocks in 14th Century Europe helped standardize equinoctial hours, laying foundations for modern time-keeping.