A sundial is a device that measures time by
using a light spot or shadow cast by the position
of the Sun on a reference scale.
As the Earth turns on its polar axis, the
sun appears to cross the sky from east to
west, rising at sun-rise from beneath the
horizon to a zenith at mid-day and falling
again behind the horizon at sunset.
Both the azimuth (direction) and the altitude
(height) can be used to create time measuring
devices.
Sundials have been invented independently
in all major cultures and become more accurate
and sophisticated as the culture developed.
== Introduction ==
A sundial uses local solar time.
Before the coming of the railways in the 1830s
and 1840s, local time was displayed on a sundial
and was used by the government and commerce.
Before the invention of the clock the sundial
was the only source of time, after the invention,
the sundial became more important as the clock
needed to be reset regularly from a sundial-
as its accuracy was poor.
A clock and a dial were used together to measure
longitude.
Dials were laid out using straight edges and
compasses.
In the late nineteenth century sundials became
objects of academic interest.
The use of logarithms allowed algebraic methods
of laying out dials to be employed and studied.
No longer utilitarian, sundials remained as
popular ornaments, and several popular books
promoted that interest- and gave constructional
details.
Affordable scientific calculators made the
algebraic methods as accessible as the geometric
constructions- and the use of computers made
dial plate design trivial.
The heritage of sundials was recognised and
sundial societies were set up worldwide, and
certain legislations made studying sundials
part of their national school curriculums.
== Ancient sundials ==
The earliest household clocks known, from
the archaeological finds, are the shadow clocks
(1500 BCE) in ancient Babylonian astronomy.
Ancient analemmatic sundials of the same era
(about 1500 BCE) and their prototype have
been discovered on the territory of modern
Russia.
Much earlier obelisks, once thought to have
been used also as sundials, placed at temples
built in honor of a pharaoh, are now thought
to serve only as a memorial.
Presumably, humans were telling time from
shadow-lengths at an even earlier date, but
this is hard to verify.
In roughly 700 BCE, the Old Testament describes
a sundial — the "dial of Ahaz" mentioned
in Isaiah 38:8 and 2 Kings 20:9 (possibly
the earliest account of a sundial that is
anywhere to be found in history) — which
was likely of Egyptian or Babylonian design.
Sundials are believed to have existed in China
since ancient times, but very little is known
of their history.
There is an early reference to sundials from
104 BCE in an assembly of calendar experts.The
ancient Greeks developed many of the principles
and forms of the sundial.
Sundials are believed to have been introduced
into Greece by Anaximander of Miletus, c.
560 BCE.
According to Herodotus, Greek sundials were
initially derived from their Babylonian counterparts.
The Greeks were well-positioned to develop
the science of sundials, having founded the
science of geometry, and in particular discovering
the conic sections that are traced by a sundial
nodus.
The mathematician and astronomer Theodosius
of Bithynia (ca. 160 BCE-ca. 100 BCE ) is
said to have invented a universal sundial
that could be used anywhere on Earth.
The Romans adopted the Greek sundials, and
the first record of a sundial in Rome is 293
BC according to Pliny.
Plautus complained in one of his plays about
his day being "chopped into pieces" by the
ubiquitous sundials.
Writing in ca. 25 BC, the Roman author Vitruvius
listed all the known types of dials in Book
IX of his De Architectura, together with their
Greek inventors.
All of these are believed to be nodus-type
sundials, differing mainly in the surface
that receives the shadow of the nodus.
the hemicyclium of Berosus the Chaldean: a
truncated, concave, hemispherical surface
the hemispherium or scaphe of Aristarchus
of Samos: a full, concave, hemispherical surface
the discus (a disc on a plane surface) of
Aristarchus of Samos: a fully circular equatorial
dial with nodus
the arachne (spiderweb) of Eudoxus of Cnidus
or Apollonius of Perga: half a circular equatorial
dial with nodus
the plinthium or lacunar of Scopinas of Syracuse:
an example in the Circus Flaminius)
the pros ta historoumena (universal dial)
of Parmenio
the pros pan klima of Theodosius of Bithynia
and Andreas
the pelekinon of Patrocles: the classic double-bladed
axe design of hyperbolae on a planar surface
the cone of Dionysodorus: a concave, conical
surface
the quiver of Apollonius of Perga
the conarachne
the conical plinthium
the antiboreum: a hemispherium that faces
North, with the sunlight entering through
a small hole.The Romans built a very large
sundial in 10 BC, the Solarium Augusti, which
is a classic nodus-based obelisk casting a
shadow on a planar pelekinon.
The Globe of Matelica is felt to have been
part of an Ancient Roman sundial from the
first or second century.
The custom of measuring time by one's shadow
has persisted since ancient times.
In Aristophanes' play, Assembly of Women,
Praxagora asks her husband to return when
his shadow reaches 10 feet (3.0 m).
The Venerable Bede is reported to have instructed
his followers in the art of telling time by
interpreting their shadow lengths.
However, Bede's important association with
sundials is that he encouraged the use of
canonical sundials to fix the times of prayers.
== Modern dialing ==
The Greek dials were inherited and developed
further by the Islamic Caliphate cultures
and the post-Renaissance Europeans.
Since the Greek dials were nodus-based with
straight hour-lines, they indicated unequal
hours — also called temporary hours — that
varied with the seasons, since every day was
divided into twelve equal segments; thus,
hours were shorter in winter and longer in
summer.
The idea of using hours of equal time length
throughout the year was the innovation of
Abu'l-Hasan Ibn al-Shatir in 1371, based on
earlier developments in trigonometry by Muhammad
ibn Jābir al-Harrānī al-Battānī (Albategni).
Ibn al-Shatir was aware that "using a gnomon
that is parallel to the Earth's axis will
produce sundials whose hour lines indicate
equal hours on any day of the year."
His sundial is the oldest polar-axis sundial
still in existence.
The concept later appeared in Western sundials
from at least 1446.The oldest sundial in England
is a tide dial incorporated into the Bewcastle
Cross, Cumbria, and dates from the 7th or
early 8th century.
== Renaissance sundials ==
The onset of the Renaissance saw an explosion
of new designs.
Italian astronomer Giovanni Padovani published
a treatise on the sundial in 1570, in which
he included instructions for the manufacture
and laying out of mural (vertical) and horizontal
sundials.
Giuseppe Biancani's Constructio instrumenti
ad horologia solaria (ca. 1620) discusses
how to make a perfect sundial, with accompanying
illustrations.
== The dials of Giovanni Francesco Zarbula
==
Painted vertical declining dials in villages
around Briançon, Hautes-Alpes, France.
There are 400 painted dials in this one French
department dating form the 18th and 19th centuries.
The most famous sundial maker was Giovanni
Francesco Zarbula (fr) who created a hundred
of them between 1833 and 1881.
== Twentieth and twenty-first century dialing
==
Designers of the Taipei 101, the first record-setting
skyscraper of the 21st century, brought the
ancient tradition forward.
The tower, tallest in the world when it opened
in Taiwan in 2004, stands over half a kilometer
in height.
The design of an adjoining park uses the tower
as the style for a huge horizontal sundial.
== Gallery ==
Modern
== See also ==
Foucault pendulum
Francesco Bianchini
Horology
Scottish sundial — the ancient renaissance
sundials of Scotland.
Tide dial — early sundials which show the
canonical hours ("tides") of the day
Wilanów Palace Sundial, created by Johannes
Hevelius in about 1684
