The northern hemisphere continues to tilt more and more towards the Sun, until the longest summer days in June. 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 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). From March to September, the Sun’s path appears to be north of the celestial equator. From September to March, it appears to be south of the celestial equator. The Sun crosses the celestial equator at spring and autumn. The Sun’s rising and setting points change through the yearThe planets (initially known as wandering stars) appear to move among the fixed stars – at too fast a pace to be really useful in navigation. Also, they don’t follow consistent orbits, so they aren’t reliable for finding direction. However, they can be helpful in holding a direction because we know they rise broadly in the east and set broadly in the west and are easy to recognise. Venus is particularly bright and recognisable. 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. 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. 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

At this point in the orbit, the Earth’s tilt means that the southern hemisphere is facing more towards the Sun. This means that the light and heat from the Sun is more direct and stronger. The days are the longest in the year and the nights, the shortest. This is summer in the southern hemisphere. Up here on the International Space Station I don’t get affected by the seasons but on Earth the seasons are always changing: Spring, Summer, Autumn and Winter. Hipparchus, who flourished in Rhodes about 150 bce and was probably the greatest observational astronomer of antiquity, discovered from his own observations and those of others made over the previous 150 years that the equinoxes, where the ecliptic (the Sun’s apparent path) crosses the celestial equator (the celestial equivalent of the terrestrial Equator), were not fixed in space but moved slowly in a westerly direction. The movement is small, amounting to no more than 2° in 150 years, and it is known now as the precession of the equinoxes. Calendrically, it was an important discovery because the tropical year is measured with reference to the equinoxes, and precession reduced the value accepted by Callippus. Hipparchus calculated the tropical year to have a length of 365.242 days, which was very close to the present calculation of 365.242199 days; he also computed the precise length of a lunation, using a “great year” of four Callippic cycles. He arrived at the value of 29.53058 days for a lunation, which, again, is comparable with the present-day figure, 29.53059 days. 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. After summer it starts tilting away from the Sun again. The days get shorter and colder as we move into Autumn.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 top half of the Earth we call the northern hemisphere, and the bottom half we call the southern hemisphere. 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. The tropical year and the synodic month are incommensurable, 12 synodic months amounting to 354.36706 days, almost 11 days shorter than the tropical year. Moreover, neither is composed of a complete number of days, so that to compile any calendar that keeps in step with the Moon’s phases or with the seasons it is necessary to insert days at appropriate intervals; such additions are known as intercalations.

Venus – Kōpū – also known as Meremere-tū-ahiahi (evening star) and Tawera-i-te-atatū (morning star)The Earth’s tilt means we experience four seasons as we orbit the Sun. So, starting with winter in the northern hemisphere, the Earth moves round and the days get longer and warmer until it becomes spring. 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 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. Another early and important cycle was the saros, essentially an eclipse cycle. There has been some confusion over its precise nature because the name is derived from the Babylonian word shār or shāru, which could mean either “universe” or the number 3,600 (i.e., 60 × 60). In the latter sense it was used by Berosus ( c. 290 bce) and a few later authors to refer to a period of 3,600 years. What is now known as the saros and appears as such in astronomical textbooks (still usually credited to the Babylonians) is a period of 18 years 11 1/ 3 days (or with one day more or less, depending on how many leap years are involved), after which a series of eclipses is repeated. 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.