S

Samhain: an ancient Celtic festival held on the 1st of November. It is one of the cross-quarter days.

saros cycle

A cycle of 18 years 11 days 8 hours (223 lunations) between repetitions of eclipses.

scales: see dialling scales.

sciagraphy

The art or science of shading and shadows. From sciaterics or scioterics – the name for gnomonics in ancient Greece.

seasons

The seasons are defined astronomically as follows:

Spring: from the vernal

equinox

to the summer

solstice

Summer: from the summer solstice to the autumnal equinox

Autumn: from the autumnal equinox to the winter solstice

Winter: from the winter solstice to the vernal equinox

In popular parlance, the seasons of the northern hemisphere comprise the following months:

Spring March, April, May

Summer June, July, August

Autumn September, October, November

Winter December, January, February

The signs of the zodiac for the seasons are given in Appendix I.

second (of angle):

See arc-second.

second

. Fluctuations in the Earth’s rotational rate since about 1969 have been such that the day is between 1 and 3 ms longer than this number of seconds. These variations are totally insignificant to even the best sundial. The word ~ derives from the Latin “secunda minuta” or second minute.

semidiameter

. As the distance from the Earth to the Sun varies during its orbit, the semi-diameter varies from 15.76 arc-minutes in July to 16.29 arc-minutes in January. In dialling, it is usual to take the sun’s full diameter as ½º
.

septentrional

A term now rarely used for “of the north” and sometimes applied to north-facing dials.

shadow sharpener

Any of the various devices for sharpening the edge of a shadow, allowing more accurate time readings to be made. Usually a physical addition to the gnomon or nodus, it casts a secondary shadow, with its own penumbra, in which the primary shadow can be located more accurately (although it may have less contrast). The term is sometimes also used to refer to a movable lens which produces an image of the shadow edge.

shadow square

A square (or rectangular) scale often found on quadrants and astrolabes which allows the tangent or cotangent of the altitude of a celestial body to be found.

shortest day: a term in common parlance, defined as the day of the year with the least (astronomical)

sunrise to sunset period. It is normally used synonymously with the winter solstice although, strictly, it
can vary by a day depending on the exact time of the solstice and the relationship between the rate of
change of the EoT and that of the local sunset/sunrise.

sidereal time, sidereal day

See time (types of).

signs of the zodiac

See Zodiac.

Sirius

The brightest star in the night sky, used by the Egyptians as a means of determining the beginning of the Nile floods. See heliacal rising.

skylight

Light which reaches the observer from the general (blue) sky. It is sunlight which has undergone multiple scattering events with the molecules of the Earth’s atmosphere (i.e. Rayleigh scattering) or with clouds or other aerosols in the atmosphere. High levels of skylight reduce the contrast of a shadow. It also tends to be polarised.

slant

See obliquity.

small circle

A circle on the surface of a sphere whose centre does not coincide with that of the sphere (and hence it must always have a smaller diameter).

solarium

Latin for sundial. Beware, it can also be interpreted as “sunning place”.

solar compass

An instrument for direction finding which uses dialling principles. The most common are modified versions on an analemmatic dial with a vertical gnomon. Sometimes called an astro-compass, although these latter more properly use sightings of the fixed stars.

solar longitude

The ecliptic longitude of the Sun, it varies from 0º (at the vernal equinox) to 360º during the year. By Kepler’s Second Law, the rate of change of the solar longitude is such that the Earth sweeps out equal areas on the ecliptic plane in equal times.

solar parallax: the difference between the Sun’s altitude as observed from the Earth’s surface and its true

astronomical value from the centre of the Earth./

solar time

See time(types of).

solstices

The same as the longest and shortest days, respectively. Astronomically, they are the occasions when the Sun’s ecliptic longitude is 90º or 270º, respectively, and correspond to the extreme values of declination. See Figure 1.

South

One of the cardinal points of the compass, it is the direction opposite north, in the direction of the south celestial pole. It is also the direction of the Sun at local noon (in the northern hemisphere).

southing

Another term for a southern transit.

South Pole

The location on the Earth’s surface where it intersects the axis, and opposite the North Pole. It has a latitude of -90º.

spherical angle

The angle whose vertex is at the intersection of two great circles of the celestial sphere. Spherical trigonometry deals with spherical angles and triangles.

spherical triangle

The figure formed on the surface of a sphere by three intersecting great circles. The fundamental (or nautical) triangle is the special case of a spherical triangle on the celestial sphere with vertices at the zenith, North celestial pole and the Sun.

split noon

See noon gap.

standard time zone

[TZ] a geographical region which uses the same civil time. These are approximately regions between two lines of longitude, set 15º
apart, and hence with 1 hour time difference between adjacent zones. The standard time for each zone is the mean solar time at the central or standard meridian for the zone. For the UK, which is in Zone 0, the standard meridian is the Prime Meridian at Greenwich, and the zone nominally extends from 7½º W to 7½º E. For political reasons, other time zones have their boundaries adjusted to follow country borders or other features. The zones were defined at the same international conference in 1884 that set Greenwich as the Prime meridian.

steradian

Unit of solid angle. It is the central solid angle of a sphere subtended by a surface area equal to the square of its radius. The whole sphere supports an angle of 4p steradians around its centre.

stereography

. Its use was advocated by 17th century diallists.

string gnomon

A gnomon in the form of a flexible cord which is pulled tight when the dial, typically in diptych form, is opened.

style

{stile}: the line in space which generates the shadow edge used to indicate the time on the dial plate. Note that a gnomon with finite thickness will have two styles (one along each of the upper edges) which will each be operational for parts of every day. If the gnomon is in the form of a long rod, the style will be the virtual line running along the centre of the rod and the dial is read by estimating the centre of the shadow. Note: this modern distinction between gnomon and style is not the one found in earlier literature where (from 1577) the word style was used to indicate a polar-pointing gnomon (a polos) or, more rarely, a nodus. Hence stylar: pertaining to the style or gnomon of a dial (first used 1688). See Figure 1.

style height

[SH] of a polar style is the angle that the style makes with the sub-style line. Note that this is an unusual use of the word “height”, and style angle could be regarded as a better term. For a style which is perpendicular to the dial plane, style height is simply the distance from its top to the foot. See Figure 1.

sub-nodus (point)

The point on the dial plane that lies perpendicularly below (or behind for a vertical dial) a nodus. The distance from this point to the nodus is sometimes called the ortho-style distance.

sub-style angle

[SD] the angle that the sub-style makes with the noon line, measured positively clockwise (towards the p.m. hours for a south-facing vertical dial).

sub-style (line)

The line lying in the dial plane which is perpendicularly below (or behind for a vertical dial) the style. See Figure 1.

sub-style triangle

The right angled triangle formed with the polar style as the hypotenuse, with the other sides lying along the sub-style and the ortho-style distance.

summer solstice

; see solstices.

Sun

The star at the centre of our solar system. The mean distance to the Earth (designated the Astronomical Unit or AU) is 149.6 x 10⁶ km. It has a surface temperature of about 5800 º
K. The solar spectral irradiance reaching the Earth’s surface (at AM1 – air mass 1 – i.e. looking through a standard atmosphere with the Sun at the zenith) ranges from about 250 nm to 2000 nm, with the main peak at 490 nm. See semi-diameter for the apparent size of the Sun.

sun clock

See Dial types.

sun compass

See solar compass

sundial

An instrument for telling the time and/or date from the position of the Sun. More generally, it can give any function of the Sun’s co-ordinates. See dial for the origins of the term, and Dial (types of) for types.

sunlight

Light reaching the observer directly from the Sun. Contrast with skylight. Note that the Sun’s rays reaching the Earth are always taken as parallel, but coming from an extended source (see semi-diameter).

sunrise, sunset

The first (last) appearance of the Sun above the horizon each day. This occurs when the sun’s altitude reaches -0º 50′. Note that astronomers define the rising of an object as an altitude of 0º. The difference is due to the combined effects of the Sun’s mean semi-diameter (16 arcmin) and atmospheric refraction (34 arcmin). See Equations for expressions to calculate sunrise and sunset.

sunshine recorder

A meteorological instrument for recording the hours in which the Sun shines. The most interesting type is the Campbell-Stokes ~, which uses a spherical lens to focus bright sunlight onto a paper chart, burning a track along it.

superior

Refers to an event on the celestial sphere above the horizon. Opposite of inferior.

synodic

Pertaining to the successive conjunctions of a planet (or moon) with the Sun.