In Scotland we experience winter at the beginning of the year. Six months later the Earth hastravelled halfway around its orbit. The southern hemisphere is now tilted away from the Sun so it is winter.At the same time it is summer in the northern hemisphere because it is now tilted more towards the Sun.

The Julian period is a cycle of 7,980 years. It is based on the Metonic cycle of 19 years, a “solar cycle” of 28 years, and the Indiction cycle of 15 years. The so-called solar cycle was a period after which the days of the seven-day week repeated on the same dates. Since one year contains 52 weeks of seven days, plus one day, the days of the week would repeat every seven years were no leap year to intervene. A Julian calendar leap year cycle is four years, therefore the days of the week repeat on the same dates every 4 × 7 = 28 years. The cycle of the Indiction was a fiscal, not astronomical, period. It first appears in tax receipts for Egypt in 303 ce and probably took its origin in a periodic 15-year taxation census that followed Diocletian’s reconquest of Egypt in 297 ce. By multiplying the Metonic, solar, and Indiction cycles together, Scaliger obtained his cycle of 7,980 years (19 × 28 × 15 = 7,980), a period of sufficient length to cover most previous and future historical dates required at any one time. The Metonic cycle was improved by both Callippus and Hipparchus. Callippus of Cyzicus ( c. 370–300 bce) was perhaps the foremost astronomer of his day. He formed what has been called the Callippic period, essentially a cycle of four Metonic periods. It was more accurate than the original Metonic cycle and made use of the fact that 365.25 days is a more precise value for the tropical year than 365 days. The Callippic period consisted of 4 × 235, or 940 lunar months, but its distribution of hollow and full months was different from Meton’s. Instead of having totals of 440 hollow and 500 full months, Callippus adopted 441 hollow and 499 full, thus reducing the length of four Metonic cycles by one day. The total days involved therefore became (441 × 29) + (499 × 30), or 27,759, and 27,759 ÷ (19 × 4) gives 365.25 days exactly. Thus the Callippic cycle fitted 940 lunar months precisely to 76 tropical years of 365.25 days. Meanwhile, the northern hemisphere is tilted away from the Sun. The light and heat from the Sun is less direct, and it is spread over a wider area so it brings less warmth. The tilt means that nights are longer, days are shorter. This is winter in the northern hemisphere. The most famous of these is Stonehenge in Wiltshire, Eng., where the original structure appears to have been built about 2000 bce and additions made at intervals several centuries later. It is composed of a series of holes, stones, and archways arranged mostly in circles, the outermost ring of holes having 56 marked positions, the inner ones 30 and 29, respectively. In addition, there is a large stone—the heel stone—set to the northeast, as well as some smaller stone markers. Observations were made by lining up holes or stones with the heel stone or one of the other markers and watching for the appearance of the Sun or Moon against that point on the horizon that lay in the same straight line. The extreme north and south positions on the horizon of the Sun—the summer and winter solstices—were particularly noted, while the inner circles, with their 29 and 30 marked positions, allowed “hollow” and “full” (29- or 30-day) lunar months to be counted off. More than 600 contemporaneous structures of an analogous but simpler kind have been discovered in Britain, in Brittany, and elsewhere in Europe and the Americas. It appears, then, that astronomical observation for calendrical purposes was a widespread practice in some temperate countries three to four millennia ago. The Earth spins three hundred and sixty five times in one year. That’s why we have three hundred and sixty five days in a year.After summer it starts tilting away from the Sun again. The days get shorter and colder as we move into Autumn.

Today a solar calendar is kept in step with the seasons by a fixed rule of intercalation. But although the Egyptians, who used the heliacal rising of Sirius to determine the annual inundation of the Nile, knew that the tropical year was about 365.25 days in length, they still used a 365-day year without intercalation. This meant that the calendar date of Sirius’ rising became increasingly out of step with the original dates as the years progressed. In consequence, while the agricultural seasons were regulated by the heliacal rising of Sirius, the civil calendar ran its own separate course. It was not until well into Roman times that an intercalary day once every four years was instituted to retain coincidence. Complex cycles

Navigating with the Moon

While the Earth is spinning to give us day and night, it is also moving around the Sun. This movement is called an orbit. The Sun is a star, a giant ball of burning gas. The heat and light that it gives off helps to keep everything on our planet alive. When we see the Sun moving across the sky during the day it’s because the Earth is spinning, not the Sun. Let’s put a marker on Scotland. When this part of the Earth is facing the Sun it’s day time, when it’s facing away from the Sun that's night time.

The main use of cycles was to try to find some commensurable basis for lunar and solar calendars, and the best known of all the early attempts was the octaëteris, usually attributed to Cleostratus of Tenedos ( c. 500 bce) and Eudoxus of Cnidus (390– c. 340 bce). The cycle covered eight years, as its name implies, and so the octaëteris amounted to 8 × 365, or 2,920 days. This was very close to the total of 99 lunations (99 × 29.5 = 2,920.5 days), so this cycle gave a worthwhile link between lunar and solar calendars. When in the 4th century bce the accepted length of the year became 365.25 days, the total number of solar calendar days involved became 2,922, and it was then realized that the octaëteris was not as satisfactory a cycle as supposed. The line around which something spins is called an axis. The Earth's axis is tilted at an angle. The Earth’s tilt is the reason for the changing seasons. Determining the Moon’s rising and setting points along with the rising and setting points of the fixed stars allows the Moon to be used to give direction during the night. The line separating light and dark in the Moon points approximately north and south since the Moon is positioned east or west of the Sun as it arcs through the night sky. The planets The calendar dating of historical events and the determination of how many days have elapsed since some astronomical or other occurrence are difficult for a number of reasons. Leap years have to be inserted, but, not always regularly, months have changed their lengths and new ones have been added from time to time and years have commenced on varying dates and their lengths have been computed in various ways. Since historical dating must take all these factors into account, it occurred to the 16th-century French classicist and literary scholar Joseph Justus Scaliger (1540–1609) that a consecutive numbering system could be of inestimable help. This he thought should be arranged as a cyclic period of great length, and he worked out the system that is known as the Julian period. He published his proposals in Paris in 1583 under the title Opus de emendatione temporum.Observation of the Sun is done at sunrise and sunset. When the Sun is low on the horizon, its path is narrow and obvious, but as it rises, it gets wider and wider. When it’s too high, you can’t tell where it has risen from and have to use other clues for navigation, such as the shape and direction of the waves. Phases of the Moon Because the position of the Sun in relation to the celestial equator changes over the year, so do its rising and setting points on the horizon. At the spring and autumn equinoxes, the Sun rises due east and sets due west.

The month is determined by the Moon’s passage around the Earth, and, as in the case of the day, there are several ways in which it can be defined. In essence, these are of two kinds: first, the period taken by the Moon to complete an orbit of the Earth and, second, the time taken by the Moon to complete a cycle of phases. Among primitive societies, the month was determined from the phases; this interval, the synodic month, is now known to be 29.53059 days. The synodic month grew to be the basis of the calendar month.Winter is when the northern hemisphere (where we live) is tilted away from the Sun. Sunlight hits the northern hemisphere at a shallow angle. This spreads sunlight over a wide area so it is weaker and less warm. Winter has the coldest weather and the longest nights of the year. The fact that neither months nor years occupied a whole number of days was recognized quite early in all the great civilizations. Some observers also realized that the difference between calendar dates and the celestial phenomena due to occur on them would first increase and then diminish until the two were once more in coincidence. The succession of differences and coincidences would be cyclic, recurring time and again as the years passed. An early recognition of this phenomenon was the Egyptian Sothic cycle, based on the star Sirius (called Sothis by the ancient Egyptians). The error with respect to the 365-day year and the heliacal risings of Sirius amounted to one day every four tropical years, or one whole Egyptian calendar year every 1,460 tropical years (4 × 365), which was equivalent to 1,461 Egyptian calendar years. After this period the heliacal rising and setting of Sothis would again coincide with the calendar dates ( see below The Egyptian calendar).