by Dr. Tim, O'Brien, author of 'LIGHT YEARS AGO'
The Earth, like all the other planets in the solar system, orbits the sun (Fig. 1). Throughout this orbit, the Earth is continually spinning about a north-south axis which is tilted about 23.5 degrees in relation to the plane of its orbit around the sun (Fig. 2). This tilt of the Earth's axis is referred to as the Obliquity of the Ecliptic. Each complete spin about this north-south axis is referred to as a day. From the Earth, the Sun appears to rise on the eastern horizon (dawn), spend some time traversing the sky travelling westward, and finally set in the west (sunset). If the spinning axis of the Earth was not tilted in relation to its orbit about the sun, then each day and night would always be of equal length.
The effect of the Obliquity of the Ecliptic
The tilt in the Earth's axis has remained relatively constant through the ages. It has changed by only .5 of a degree over the past 5,000 years, but this factor must be taken into account when considering present day alignments of Neolithic structures. This tilt therefore results in seasonal variations and accounts for the difference in the length of daylight hours in Ireland between summer and winter (Fig. 3). To people living at or near the Equator this seasonal variation is virtually unnoticeable. In contrast, all areas above Latitude 66.5 degrees experience days of total darkness in the winter and days when the sun never sets in the summer. The daily change in the rising position of the sun is smallest at the limits of movement when the direction of change reverses (Fig. 4). The day on which this reversal occurs is referred to as the solstice. There are two solstices each year. The winter solstice occurs around December 22nd and is the shortest day of the year. The summer solstice occurs around June 21st and is the longest day of the year (Fig. 5). The date of the solstices is variable because every fourth (leap) year we add an extra day to the Calendar which changes the solstice date when compared to the previous year. The day which lies midway between the solstices is known as the equinox, or that day when daylight hours are exactly equal to darkness hours. Consequently, there are two Equinoxes each year, a spring (Vernal) and an autumnal equinox. While the sun appears to be at standstill around the solstices, the greatest change in its position occurs at the equinoxes.
The basis of the modern Calendar:
The basic unit of time-keeping is the day, but for longer periods we need a larger cycle and therefore use the period it takes the Earth to circle the sun as the basis for our present Calendar. This period or cycle we refer to as the tropical year and it is relatively constant. This cycle has been broken into smaller units of hours, minutes and seconds which can now be defined in very accurate terms, independent of the orbit of the Earth around the sun. The cycle of the Earth's orbit around the sun was adopted as the unit of time when it became obvious that it was the most constant and easily measurable period which predicted the regular occurrence of natural phenomena. Such Lunar Calendars have a recurring 29 and 30 day monthly cycle. Twelve of these months resulted in a 354-355 day year, a system which is still observed by the Muslim world. The ancient Egyptians were reputed to have based their original Calendar on the yearly regular flooding of the Nile and, as it was a yearly event, it corresponded well with the tropical or solar year.
Our calendrical year, based on the sun's cycle as observed from the Earth, is usually counted as 365 days and also encompasses the simpler lunar Calendar by including 12 months, although these no longer correspond to the lunar cycle. As it actually takes the Earth 365.2422 days to complete the full orbit small adjustments in our 365 day Calendar have to be made. The addition of an extra day incorporated into the leap year every four years almost corrects the discrepancy.
Julius Caesar is credited with the introduction of the modern Calendar based on the tropical year. Prior to the Julian reform the ancient Roman Calendar was loosely based on a 355 day lunar cycle.
The year commenced around March 25th, which coincided with the spring (Vernal) equinox. Adjustments were made from time to time to keep the Calendar in step with the sun's movement. These adjustments were apparently made on the advice of the astronomers, but were often used to lengthen the reign of popular Consuls by the addition of many days and shorten the reign of those less favoured. Julius Caesar introduced a more regular Calendar based on a 365 day solar year, abolishing the idea of adding extra days at irregular intervals. This reform of the Calendar was based on the advice of Egyptian scholars, who were more advanced in the art of astronomy than their Roman counterparts. He also introduced the concept of a leap year, adding an extra day every four 365 day solar years, to keep the new Calendar in step with the solar year calculated at 365.25 days. For this reform he honoured himself by naming one of the new months in his memory. It has been suggested that Julius Caesar was aware of an accurate Calendar, the Coligny, in use by the Celtic peoples at the same time. The Coligny Calendar records this time measuring ability of the Celts, which some have considered to be a very precise long term Calendar. It has not been explained how the Celts could have devised such an accurate Calendar. It was probably due to many years of observation.!!!
