Weather vane

A weather vane, wind vane, or weathercock is an instrument used for showing the direction of the wind. It is typically used as an architectural ornament to the highest point of a building. The word "vane" comes from the Old English word "fana", meaning 'flag'.
Although partly functional, weather vanes are generally decorative, often featuring the traditional cockerel design with letters indicating the points of the compass. Other common motifs include ships, arrows and horses. Not all weather vanes have pointers. When the wind is sufficiently strong, the head of the arrow or cockerel (or equivalent depending on the chosen design) will indicate the direction from which the wind is blowing.
The weather vane was independently invented in ancient China and Greece around the same time during the 2nd century BCE. The earliest written reference to a weather vane appears in the "Huainanzi", and a weather vane was fitted on top of the Tower of the Winds in Athens.
History.
The oldest textual reference to a weather vane comes from the ancient Chinese text "Huainanzi" dating from around 139 BC, which describes a "wind-observing fan" ("hou feng shan", ). The Tower of the Winds on the ancient Greek "agora" in Athens once bore on its roof a weather vane in the form of a bronze Triton holding a rod in his outstretched hand, rotating as the wind changed direction. Below this was a frieze adorned with the eight Greek wind deities. The eight-metre-high structure also featured sundials, and a water clock inside. It dates from around 50 BC.
Military documents from the Three Kingdoms period of China (220–280) refer to the weather vane as "five ounces" ("wu liang", ), named after the weight of its materials. By the 3rd century, Chinese weather vanes were shaped like birds and took the name of "wind-indicating bird" ("xiang feng wu", ). The "Sanfu huangtu" (), a 3rd-century book written by Miao Changyan about the palaces at Chang'an, describes a bird-shaped weather vane situated on a tower roof, which was possibly also an anemometer:
The Han 'Ling Tai' (Observatory Platform) was eight "li" north-west of Chang'an. It was called 'Ling Tai' because it was originally intended for observations of the Yin and the Yang and the changes occurring in the celestial bodies, but in the Han it began to be called Qing Tai. Guo Yuansheng, in his "Shu Zheng Ji" (Records of Military Expeditions), says that south of the palaces there was a Ling Tai, fifteen "ren" (120 feet) high,
upon the top of which was the armillary sphere made by Zhang Heng. Also there was a wind-indicating bronze bird ("xiang feng tong wu"), which was moved by the wind; and it was said that the bird moved when a 1000-"li" wind was
blowing. There was also a bronze gnomon 8 feet high, with a 13 feet long and 1 foot 2 inches broad. According to an inscription, this was set up in the 4th year of the Taichu reign-period (101 BCE). 
The oldest surviving weather vane with the shape of a rooster is the "Gallo di Ramperto", made in 820 CE and now preserved in the Museo di Santa Giulia in Brescia, Lombardy.
Pope Leo IV had a cock placed on the Old St. Peter's Basilica or old Constantinian basilica.
Pope Gregory I said that the cock (rooster) "was the most suitable emblem of Christianity", being "the emblem of St Peter", a reference to in which Jesus predicts that Peter will deny him three times before the rooster crows.
As a result of this, the cock gradually began to be used as a weather vane on church steeples, and in the 9th century Pope Nicholas I ordered the figure to be placed on every church steeple.
The Bayeux Tapestry of the 1070s depicts a man installing a cock on Westminster Abbey.
One alternative theory about the origin of weathercocks on church steeples is that it was an emblem of the vigilance of the clergy calling the people to prayer.
Another theory says that the cock was not a Christian symbol but an emblem of the sun derived from the Goths.
A few churches used weather vanes in the shape of the emblems of their patron saints. The City of London has two surviving examples. The weather vane of St Peter upon Cornhill is not in the shape of a rooster, but a key; while St Lawrence Jewry's weather vane is in the form of a gridiron. 
Early weather vanes had very ornamental pointers, but modern weather vanes are usually simple arrows that dispense with the directionals because the instrument is connected to a remote reading station. An early example of this was installed in the Royal Navy's Admiralty building in London – the vane on the roof was mechanically linked to a large dial in the boardroom so senior officers were always aware of the wind direction when they met.
Modern "aerovanes" combine the directional vane with an anemometer (a device for measuring wind speed). Co-locating both instruments allows them to use the same axis (a vertical rod) and provides a co-ordinated readout .
World's largest weather vane.
According to the Guinness World Records, the world's largest weather vane is a Tío Pepe sherry advertisement located in Jerez, Spain. The city of Montague, Michigan also claims to have the largest standard-design weather vane, being a ship and arrow which measures 48 feet tall, with an arrow 26 feet long.
A challenger for the title of world's largest weather vane is located in Whitehorse, Yukon. The weather vane is a retired Douglas DC-3 CF-CPY atop a swiveling support. Located at the Yukon Transportation Museum beside Whitehorse International Airport, the weather vane is used by pilots to determine wind direction, used as a landmark by tourists and enjoyed by locals. The weather vane only requires a 5 knot wind to rotate.
A challenger for the world's tallest weather vane is located in Westlock, Alberta. The classic weather vane that reaches to 50 feet is topped by a 1942 Case Model D Tractor. This landmark is located at the Canadian Tractor Museum.
Slang term.
The term "weathervane" is also a slang word for a politician who has frequent changes of opinion. The National Assembly of Quebec has banned the use of this slang term as a slur after its use by members of the legislature.