Calendar. A system whereby we measure the course of the year day by day. Astronomically speaking, calendars can be of three types: lunar, lunisolar or solar. Our modern calendar, the Gregorian Calendar (which began on 15th October 1582 CE) is a solar calendar of 365.2425 days, meaning that its period is linked the orbit of the Earth around the Sun: on the first day of the new year (1st January) the Sun appears to be in the same place in the sky as it was on the previous new year, barring periodic minor corrections. The months are a matter of historical convention and do not synchronise with the Moon.
The Islamic religious calendar (the Hijri calendar, which began on 16th July 622 CE), by contrast, is a lunar calendar and is linked only with the movement of the Moon and consists of 12 lunations giving a year of 354 or 355 days. The first day of the first month (al-Muḥarram) falls 10 or 11 days earlier with respect to the Sun each year. The cycle repeats every 33 years approximately.
Apart from the solar calendars of ancient Egypt (which, via the Julian Calendar, is the ultimate source of our own) and ancient Persia, most ancient civilisations (such as the Babylonian, Chinese, Greek and republican Roman) used lunisolar calendars where the months are determined by the Moon, and the Year is determined by the Sun. Since the cycles of the Moon (29.530589 days) and the Sun (365.242188 days) do not synchronise, a lunisolar calendar always involves some sort of kludge, usually in the form of extra (intercalary) months, in order to make it work. In many cases these intercalary months were inserted at irregular intervals, a tradition which persisted long after astronomers had worked out schemes to make the intercalations more systematic.
Calendar Year.
Celestial Coordinate Systems.
Celestial Equator. The imagined projection of the Earth's equator into space. It forms the zero point of declination (latitude) in the equatorial coordinate system. It divides the sky into the northern celestial hemisphere and the southern celestial hemisphere.
Classical Era. A conventional period of Greek history which runs from the end of the Persian Wars in 479 BCE to the death of Alexander the Great in 323 BCE. It followed the Archaic Era (c. 800 – 479 BCE) and was followed by the Hellenistic Era (323 – 146 BCE). During the Classical Era Greek astronomy turned from being a qualitative study to being a quantitative science.
According to Hipparchos (CPAE 1.11.9), the term (κόλουρος κύκλος) appears to have been coined by Eudoxos, and it was certainly used by Hipparchos, Geminos and Ptolemy in this sense. It means 'truncated', referring to the fact the imagined circle in the sky is cut off by the observer's horizon.
Constellation. A major grouping of stars which serve to identify a part of the night sky. Modern astronomy recognises 88 constellations which cover the whole sky. In ancient Greece, before the constellations of the southern sky were added in early modern times, some 48 constellations were recognised up to the time of Ptolemy (C2nd CE). The Greek list of constellations was largely codified by Eudoxos (C4th BCE) and drew on earlier Greek star lore, but ultimately it was a fusion of Mediterranean and Mesopotamian traditions.
Cosmical Setting. The first visible morning setting of a celestial object on the western horizon before sunrise. On the previous day, the object had not reached the western horizon before sunrise. On the following day the object will be more easily visible but will have reached the horizon before sunrise.
Culmination. A celestial object (the Sun, Moon, planets, stars or deep sky objects) is said to culminate when it crosses the observer's local meridian. A culmination is also known as a meridian transit. An upper culmination occurs when the object reaches its highest altitude in the sky, a lower culmination occurs when it reaches its lowest altitude. Used on its own, culmination usually means upper culmination. Hesiod around 700 BCE in Works and Days (417–21) noted that the upper culmination of Sirius —which happened in late September— marked the time for the felling of wood.
