Sun, Moon and Stars

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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. The northern hemisphere continues to tilt more and more towards the Sun, until the longest summer days in June. 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. 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 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 top half of the Earth we call the northern hemisphere, and the bottom half we call the southern hemisphere.The year is the period taken by the Earth to complete an orbit around the Sun and, again, there are a number of ways in which this can be measured. But for calculating a calendar that is to remain in step with the seasons, it is most convenient to use the tropical year, since this refers directly to the Sun’s apparent annual motion. The tropical year is defined as the interval between successive passages of the Sun through the vernal equinox (i.e., when it crosses the celestial equator late in March) and amounts to 365.242199 mean solar days. 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 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.

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 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.Venus – Kōpū – also known as Meremere-tū-ahiahi (evening star) and Tawera-i-te-atatū (morning star) 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 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. 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 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.