Glossary S

Second.

Setting.

Sidereal Month.

Sidereal Year.

Sirius.

Solstice. Traditionally, a solstice occurs when the Sun reaches its maximum angular distance from the celestial equator. This happens twice per year. In modern times, solstices occur on June 20th or 21st and December 21st or 22nd. When the Sun is at its most northerly point (maximum declination) we have the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere. When the Sun is at its most southerly point (minimum declination) we have the winter solstice in the northern hemisphere and the summer solstice in the southern hemisphere. The limit of the Sun's angular distance from the celestial equator is set by the obliquity of the ecliptic (currently 23.439°). When the Sun reaches its maximum excursion it appear to stop moving (stand still) against the background of stars for a few days before revering direction.
     In some ancient cultures, the solstice was determined by looking for the most northerly or southerly rising or setting of the Sun on the horizon. This method was practised in Bronze Age Europe and South America.
     Determining the precise time of a solstice is not easy because the daily motion of the Sun near the solstice is much slower than at other times of the year. In the 24 hours on either side of the solstice, for example, the declination of the Sun varies by only around 30 seconds of arc. Nowadays, the ecliptic longitudes (90° and 270°) of the Sun are used to determine, by calculation, the dates and times of the solstices, even though this does not agree necessarily exactly with the maximum and minimum declinations of the Sun. This method does not suffer from small variations in the Sun's declination caused by irregularities in the Earth's motion.
     In ancient Greece the solstices were determined either by observing the position of the Sun on the horizon at sunrise or sunset, or by measuring the length of the shadow of a vertical gnomon at midday or the position of a shadow on an instrument such as a meridian circle or quadrant. None of these methods is easy, and in order to determine when this occurred, multiple measurements needed to be taken over at least a month in order to approximate the true date.

Star. A celestial object that consists of a luminous spheroid of plasma that is bound together through gravity and which shines through the process of thermonuclear fusion. The Sun is a star and is the nearest one to Earth. The next nearest bright star, α Centauri, is 270 thousand times further away. Several thousand stars are visible to the naked eye but the exact number depends upon the darkness of the sky, the time of year, the latitude of the observer and their eyesight.
     Ancient people had no idea what constituted stars, and indeed their true nature was not understood until the middle of the C20th. They were typically associated with deities and thought to be fiery in nature. In ancient Greece, Anaxagoras in the C5th BCE was the first to suggest that the Sun was a star. The Greek word for star is ἀστήρ or ἄστρον but this can also mean a planet, an asterism (small group of stars), or even a whole constellation. In the Iliad (4.75), it is also used for a meteor.
     From Earth, the stars appear to move as a body, once across the whole sky (360°) from east to west in 24 hours, but also once across the sky in the same direction, relative to the Sun, over the year. Most ancient civilisation recognised this twofold movement. Because the stars never appear to move relative to each other (the proper motion of stars was unknown), the Greeks called the true stars ἀπλανῆ ἄστρα ' not wandering (i.e. fixed) stars' as opposed to the ἄστρα πλανητά 'wandering stars' (i.e. the planets).
     The Greeks had no systematic naming system for stars. Mostly they were referred to by locations within their constellations. Thus β Orionis, for example, would be referred to as the 'left foot of Orion'. Only a few stars were explicitly named, amongst them: Arcturus (Ἀρκτοῦρος) 'bear–watcher', Canopus (Κάνωβος/Κάνωπος), Procyon (Προκύων) 'leading the dog' (i.e. Canis Major), Sirius (Σείριος), Spica (Στάχυς) 'ear of corn', Vindemiatrix (Προτρυγητήρ) 'harbinger of the vintage'.

Stellar Visibility.

Sun. The star about which the Earth revolves. Although the most blindingly obvious of celestial objects, the Greeks did not accord it as much religious status as did some other ancient peoples, such the Egyptians or the Persians. It is true that it was often personified as Ἥλιος (Helios, the Sun God) and envisaged as driving a four horse chariot across the sky, he was never an important part of the pantheon, although local traditions varied, and in some parts of Greece he assumed more importance, such as at Rhodes.
     The early philosophers came to the (not very surprising) conclusion that the Sun was made of fire, but this was often (for example in Anaximenes, Herakleitos and Xenophanes) not a feature that necessarily distinguished it from the other heavenly bodies. The same philosophers largely took a traditional view of the Sun travelling around the Earth during the night and being rekindled in the morning. Although Xenophanes, and possibly Herakleitos, took the view that each day gave rise to a new Sun which travelled across the sky forever disappearing over the horizon at night.

Sundial.

Sunrise / Sunset. The first sighting of the Sun as it rises above the eastern horizon in the morning and the last sighting of the Sun as it sinks below the western horizon in the evening. In modern terms, this refers to the sightings of the upper limb of the Sun and not to the centre of the Sun. Since the apparent radius of the Sun is 16 minutes of arc this means that the equatorial arc of the Sun at equinox is at least 180° 32'. In fact, it is always more than this owing to atmospheric refraction which varies but is typically taken as 34 minutes of arc at the horizon. In geometric terms this means that sunrise and sunset  occur when centre of the Sun is actually 50 minutes of arc (16' + 34') below the true horizon. In ancient Greece, informal use of the terms sunrise and sunset (which was based on direct observation) mainly corresponded to the modern use. However, the astronomers from Eudoxos onwards, who used a celestial globe to model the sky, probably intepreted the terms in their geometrical sense, when the centre of the Sun was aligned with the horizon.
     Sunrise and sunset vary through the year and according to latitude. The further away from the equator an observer is, the earlier or later sunrise or sunset will appear at any given time of the year. Beyond the arctic or antarctic circles of latitude (±66.561°) there are periods of the year when sunrise or sunset does not occur and the Sun is above the horizon for at least 24 hours once per year and below the horizon for at least 24 hours once per year.

Synodic Month. [συνοδικός 'meeting, conjunction'] The period of the Moon's orbit in relation to the Sun as seen from Earth, also known as lunation. After one synodic month the Moon will be in exactly the same phase as it was on the previous synodic month. The currently estimated mean value is 29.530589 days.
     In ancient times this was the generally the only recurring period of the Moon that mattered (and usually the only one that was known), and in ancient Greece it defined the month. It should be distinguished from other types of month such as the anomalistic month, the draconic month and the sidereal month.

Syzygy.