The altitude of
the Sun is the vertical angle it makes above the horizon (from -90° to
+90°). The Sun's altitude and azimuth define its position at any given
moment. At sunrise the altitude of the Sun is 0°.
angle measured from the meridian. It is positive towards the East and
negative towards the West. The Sun's azimuth at a particular time is
its bearing at that moment. The azimuth is 0° when it crosses the
meridian of a place.
complementary angle of the latitude of a place, equal to 90°-latitude.
of a wall: the
angle which a vertical wall makes relative to the meridian. When one
designs a vertical sundial, one must take account of the declination
of the surface on which it will be placed. In Europe, the declination
of a south-facing wall is 0° of an east-facing wall is -90° of a wall
facing west +90° and one facing north is 180°.
of the Sun: the
position of the Sun north or south of the celestial equator.
surface of a horizontal dial. The lines and numbers of a dial are laid
out on the dial plate.
An artisan who
designs, constructs and installs sundials.
containing the orbit of the Earth around the Sun, which by extension
projects a circle on the celestial sphere.
The value of the
difference between Apparent Solar Time and Mean Solar Time. The
difference arises from the movement of the Earth in its elliptical
orbit as well as the fact that its axis of rotation is inclined to the
ecliptic. The difference can vary by as much as +/- 16 minutes. The
Equation of Time is sometimes represented in the form of a figure of
eight placed on the noon line of a sundial.
The line of
declination on a sundial corresponding to the equinox. This line makes
a right angle with the gnomon in a plane sundial.
The day of the
year when the Sun crosses the celestial equator in its apparent
movement. At this moment, the Sun rises exactly in the East and sets
exactly in the West. It is the period of the year when night and day
are of equal length. The equinox occurs on 20 March and 23 September.
The name given
to a style placed vertically to the plane of a sundial. The shadow
cast by the tip of the gnomon can indicate both the time and the date.
The science of
Hours: an ancient system which divides the day into 24 hours,
commencing at sunrise. They thus measure the number of hours elapsed
an ancient system which divides the day into 24 hours, commencing at
sunset. They thus measure the time left until sunset.
the time shown by clocks, which will be the same for all inhabitants
of a particular time zone. It is Mean Solar Time corrected for the
difference in longitude between a place and the reference longitude of
the time zone in which it is located.
Mean Solar Time:
Obtained by adding the Equation of Time to the Apparent Solar Time.
The Mean Solar Time gives a constant hour duration over the year. It
varies with the location.
Time: the time naturally shown by a sundial. It is defined as 1/24th
of the interval between two successive passages of the Sun across the
meridian. This period of time varies throughout the year.
an ancient system dividing the interval between sunrise and sunset
into 12 hours. The length of an hour therefore varied throughout the
year! (between 40-80 minutes). These hours are sometimes referred to
as Biblical Hours.
the angle formed
between the vertical of a place and the plane of the equator. Northern
latitudes are positive; southern latitudes are negative.
lines drawn on the dial plate of a sundial which permit us to tell the
time from the shadow cast by the style. Hour lines are drawn for hours
and sometimes for half-hours and quarter-hours but much more rarely
for 5 minute intervals.
Lines: lines drawn on the dial plate of a sundial which permit us to
determine the date from the shadow cast by the style. It is
conventional to show the dates on which the Sun enters certain signs
of the Zodiac, for example, for the Sun declination 0° (Aries and
Libra), +/- 11° 29' (Pisces and Virgo), +/- 20° 20' (Gemini and
Sagittarius). These lines are also called diurnal arcs.
of a place: the
angle formed by the meridian of a place with the Prime Meridian in
Greenwich. Longitude is positive for locations to the west of
Greenwich and negative for those to the east.
correction: The time difference between the location's meridian and
the time zone's meridian (Greenwich or local time zone). There is a
difference of 4 minutes for each degree of longitude.
of a place: the
geographical meridian which passes through a place.
great circle passing though the poles and the zenith of a place.
The moment in
the year when the Sun reaches in maximum declination in the north or
south of the celestial equator. The Summer Solstice (21 June) in
Europe is the longest day of the year and the Sun is in its highest
position in the sky. The Winter Solstice (21 December) is the opposite
— the shortest day and the lowest point the Sun reaches in the sky.
The name given
to the shaft or triangle which casts its shadow on a dial. A polar
style is oriented parallel to the Earth's axis. A vertical style or
gnomon is placed vertical to the dial plate.
corresponding to the projection of the style on the dial plate of a
sundial. The angle which the sub-style makes with the noon hour line
indicates the declination of the dial