: a demonstration model, similar to an orrery, but showing the Earth-Sun system, or the Earth-Sun-Moon system. This latter is sometimes referred to as a lunarium. Note: the tellurian is sometimes called a tellurium but this term is best avoided as it is the name of the 52nd element in the periodic table.
The edge of the shadow cast by a self-shadowing object, such as the edge of the illuminated part of the Moon.
From the Latin for “little globe”. See globe dials.
The divisions of a day used in the Anglo-Saxon period. The time from sunrise to sunset was divided into four tides or time periods. See Appendix II for the names of the tides. Lines showing the tides are found on Anglo-Saxon and some mass dials. Some dials also have lines denoting the half-tide. Note that this use of the word has no connection with the marine tides.
Time, (types of):
the measure of time based on the diurnal motion of the true sun.
: [BST] civil time in the UK during the “summer”, one hour ahead of GMT. Invented by William Willett and first introduced in 1916. A sundial showing BST in Petts Wood, near Chislehurst, Kent, is his memorial. BST usually begins on the last Sunday in March, and ends on the last Sunday in October. These dates are now co-ordinated with Summer Time in the rest of the EU.
The legally-accepted time scale in a particular country or region. It is based on the standard time for that standard time zone, but may have fixed differences (eg BST). Measured in modern hours from the most recent midnight, with either a 24 hour or 2 x 12 hour format.
Simply the times shown by a clock, usually civil time. Hence the appendage “o’clock” to some times.
[or Terrestrial Dynamical Time, TDT] “scientific time” – it superseded ephemeris time in 1984, and is based on a uniform scale of time derived from atomic clocks (i.e. not subject to fluctuations in the Earth’s rate of rotation). Now usually called international atomic time (TAI).
ephemeris ~: [ET] “scientific time” – used between 1960 and 1983, this uniform timescale was based on the ephemeris second, itself derived from the period of rotation of the Earth at a particular date. It was succeeded by dynamical time when the second was redefined in 1984.
French revolution ~
A decimal timescale (10 equal hours or decidays per day) devised in 1790 by the French Academy after the French Revolution. Each hour was divided into 100 millidays (of 86.4 seconds) and each milliday into 1000 microdays (0.0864 seconds each). The decimal timescale, which had been used previously in ancient Egypt and China, was never fully implemented and was quickly dropped, with the result that sundials so calibrated are extremely rare.
: [L.A.T. – the use of the full-stops is encouraged to avoid confusion with the common contraction of “latitude”] this is solar time, as derived from the real Sun at any particular location. It is the hour angle of the Sun + 12 hours. Some authors (non-UK) may refer to it a Local True Time.
: [LMT] this is solar time which has been corrected for the EoT but not for longitude, so it is still location specific. English towns used this form of time prior to the coming of national railways and the telegraph, e.g. Oxford time.
Mean Solar Time
The authoritative (by the National Physical Laboratory) definition is: a measure of time based conceptually on the diurnal motion of the fictitious mean Sun, under the assumption that the Earth’s rate of rotation is constant.
A colloquial term used for Greenwich time as it began to replace local time with the introduction, in the mid-1800s, of railways and the resulting need for unified timetables.
: [t, SDT] “astronomical or star time”. This is timekeeping based on the sidereal day, and hence it runs ahead significantly with respect to solar-based time. Local sidereal time is equal to the hour angle of the first point of Aries and is, to a first approximation, sidereal time with a longitude correction.
~: the same as Local Apparent Time.
~ :[UT or UTC] this is the basis for terrestrial and civil timekeeping, and was adopted in January 1972. It is tied to the rotation of the Earth, and hence has to be periodically adjusted by the addition of leap seconds to account for the gradual slowing of the Earth, and the vagaries of its rotation. UT is by definition measured from the superior transit of the fictitious mean sun (i.e. mean noon at Greenwich), and hence is 12 hours behind GMT (although this difference only tends to be recognised by astronomical calculations). UT measurement is based on standard seconds. The version referred to as UTC (Universal Time, Co-ordinated) simply means the value averaged over a number of atomic clocks world-wide. In aviation, it is referred to as Z or zulu.
zonal solar time
– End of Time (types of) –
In dialling, a mechanical aid to drawing lines of declination on dial plates. It consists of an instrument which is fitted to, and can swivel around, the nodus on a polar-pointing gnomon, and can be set at an angle equal to the sun’s declination angle to the gnomon. Often used with an associated auxiliary dial. Trigon is also an archaic term for a triangle. From the Greek “trigonon” or three-cornered.
Literally “three leaves”, it refers to a set or compendium of three instruments, including at least one dial. Other instruments often include a compass and perpetual calendar.
Geographical bands of the Earth’s surface, extending from the equator to latitude 23º
26′ N (tropic of Cancer) or to 23º
26′ S (tropic of Capricorn). The terms are also used to refer to these specific latitudes. Note that they represent the extremes of the region where the Sun can reach the zenith when the sun’s declination is at its extreme values.
The interval after sunset or before sunrise when the Sun illuminates the upper atmosphere and hence it is not completely dark. It is determined by the sun’s altitude falling within a given range, as follows:
- civil twilight: -0º 50′ and -6º
- nautical twilight: -6º and -12º
- astronomical twilight: -12º and -18º
These values reflect the need for decreasing light levels for various activities.