55 Fun Facts About Sundials

In which US state can a sundial's upright pointer cast no shadow at noon on two days a year?

Hawaii is the only state south of the Tropic of Cancer, so the noon Sun can stand straight overhead there twice a year, a moment named Lahaina Noon.

By what angle does the Sun appear to swing around the sky each hour?

15°, near enough: a full 360° circuit takes 24 hours, which is also why each ideal time zone spans 15 degrees of longitude.

For a sundial to be accurate all year, its style must be parallel to what?

Earth's rotational axis: set that way, the style's shadow turns at the same rate in every season, so the dial can mark equal hours all year.

What must a horizontal sundial's style angle above the horizontal equal?

The local latitude, so a dial built for London keeps poor time in Cairo until it is tilted by the difference between the two latitudes.

What is a sundial's single time-telling point, such as a style's tip, called?

Nodus: its shadow falls on a different curve in each season, so a dial with one can show the date as well as the hour.

Which ancient civilisation left the oldest known shadow clocks, from about 1500 BC?

Ancient Egypt: Egyptians were apparently among the first to divide the day into parts much like our hours, and shadow clocks measured them.

Which Greek thinker from Miletus is credited with bringing the sundial to Greece?

Anaximander is said to have set up a gnomon at Sparta to mark the solstices and equinoxes, though Herodotus says the Greeks learned the device from Babylon.

Which Roman author listed the sundial types known in his day, naming their Greek inventors?

Vitruvius wrote about dials because he counted the making of timepieces as one of architecture's three departments, beside building and machinery.

Which Greek astronomer is credited with inventing the scaphe, a bowl-shaped sundial?

Aristarchus of Samos also put the Sun at the centre of the cosmos, but the Greeks did not accept the idea and Earth-centred models prevailed.

Pliny says Rome's first public sundial, set up in the First Punic War, was taken from which island?

Sicily: the dial was laid out for Catania, about 4 degrees further south, so it never quite matched Rome's hours, yet it kept the city's time for about a century.

The word 'dial', once meaning a sundial, traces back to the Latin word for what?

Day: it apparently came through Medieval Latin 'dialis', meaning 'daily', and the same word now names clock faces and telephone dials.

Which emperor set up an obelisk as a shadow-caster in Rome's Campus Martius?

Augustus's obelisk cast its shadow on lines of brass, but by Pliny's day the readings had stopped agreeing, and he wondered whether the Sun or the Earth had shifted.

Which TV science educator helped design the sundials carried by NASA's Spirit and Opportunity rovers?

Bill Nye helped create the MarsDials, inscribed 'Two Worlds, One Sun', which doubled as targets for calibrating the rovers' cameras.

The obelisk that served as a giant shadow pointer in ancient Rome now stands before which building?

The Chamber of Deputies meets in Palazzo Montecitorio, where the obelisk, fallen and buried for centuries, was raised again by Pope Pius VI.

Which octagonal marble monument in Athens bore sundials outside and housed a water clock?

The Tower of the Winds is named for its frieze of wind gods, and a bronze Triton on top is said to have turned to point at whichever wind was blowing.

In the Bible, which king was given the sign of a shadow moving backwards on the dial of Ahaz?

Hezekiah, ill and promised fifteen more years, asked for the harder sign, and the shadow went back ten degrees on his father Ahaz's dial.

Which motto shares the sundial side of the 1787 Fugio cent, an early US coin?

Mind Your Business: set beside the sundial and the word 'Fugio', 'I fly', it read as a rebus: time flies, so mind your business.

Which English monk promoted canonical sundials for fixing the times of prayer?

The Venerable Bede also helped popularise counting years from the birth of Christ, using AD dates in his Ecclesiastical History of the English People.

Which Damascus timekeeper built a sundial for a minaret of the Umayyad Mosque?

Ibn al-Shatir's dial was later taken down, and a replica now stands in its place on the minaret.

Which Holbein painting shows a celestial globe, a polyhedral sundial and a torquetum on a shelf?

The Ambassadors also hides a stretched skull between the two men that only snaps into shape when you view the painting from the side.

German astronomer Nicolaus Kratzer made sundials as royal astronomer to which king?

Henry VIII's astronomer Kratzer also teamed up with Hans Holbein to devise a painted ceiling of planets for a banqueting house at Greenwich Palace.

What is the year-varying gap of up to 16 minutes between a sundial and a clock called?

The equation of time was once used the other way round: early clocks were corrected with it to match sundial time.

Before sundials reached Rome, how did the city learn that it was midday?

An official announced it: the consuls' attendant called out midday when, from the Senate house, he saw the Sun between the Rostra and the Graecostasis.

Which Shakespeare king wishes he could sit on a hill carving out dials?

Henry VI says it sitting on a molehill at Towton, then watches a son who has killed his father and a father who has killed his son.

If you photographed the Sun at the same clock time all year, what shape would it trace?

A figure eight: Earth's tilt makes the two loops, while its slightly oval orbit makes one loop larger than the other.

Varro judged which Roman playwright wrote the comedy in which a hungry parasite curses whoever set up a sundial?

Plautus, Varro was sure: the parasite of The Boeotian Woman grumbles that the sundial cuts and hacks his days into small portions.

In which country do sundials in the far west read noon when clocks there show about three in the afternoon?

China: Beijing time covers the whole country, so in Xinjiang many people also keep a local Xinjiang Time that runs two hours behind.

In which city do the hour numbers on a horizontal sundial run anticlockwise?

Sydney: in the Southern Hemisphere a dial's gnomon points south and its shadow turns the other way, while clock hands copied northern sundials.

Which sundial type has a plate at right angles to the style, with evenly spaced hour lines?

Equatorial: the Sun lights its plate from above in summer and from below in winter, so an opaque dial needs hour lines on both faces.

Which sundial type has a vertical rod moved daily and hour markers set out in an ellipse?

Analemmatic dial: some are laid out on the ground so that a person standing on the date scale becomes the shadow-casting gnomon.

A portable cylinder dial hung from a string is named after which trade?

Shepherds: the dial is simply twisted on its string until its gnomon faces the Sun, so unlike a fixed dial it needs no north-south alignment.

Which mathematician, who introduced the × sign for multiplication, also wrote on laying out sundials?

William Oughtred also built an early slide rule, but Richard Delamain published a circular one first, and a bitter priority row followed.

Which future scientist, as a Lincolnshire schoolboy, drew a sundial which local people often checked for the hour?

Isaac Newton marked its hours with fixed pins, and he also carved two dials into the wall of Woolsthorpe Manor, his birthplace.

Jesuit astronomer Christopher Clavius, who wrote a treatise on sundials, helped which pope reform the calendar?

Gregory XIII: his reform made 15 October 1582 follow straight after 4 October, and Clavius spent years writing in its defence.

Which novelist set a balustrade from the demolished old Rochester Bridge on his lawn and topped it with a sundial?

Charles Dickens: the bridge's contractor gave him the stone, and he wrote to his daughter that the dial would be 'a very good object indeed'.

How does a sundial cannon set itself off at noon?

A lens focusing sunlight lights the fuse, and its frame had to be moved with the seasons because the winter Sun rides lower than the summer Sun.

Which invention by Christiaan Huygens in the 1650s finally let clocks outdo sundials as timekeepers?

The pendulum clock was also Huygens's bid to find longitude at sea, so he built several and sent them on sea trials.

On the Moon, how long does a sundial's shadow take to circle once?

A month, near enough: the Moon keeps one face toward Earth, so it turns once per orbit, and its Sun comes round about every 29.5 Earth days.

Which Rajput king built the Jantar Mantar observatories, including the one in Delhi?

Sawai Jai Singh founded Jaipur and built five observatories to produce more accurate astronomical tables than the ones in use.

Which architect designed the Sundial Bridge, whose leaning mast is the pointer of a giant sundial?

Santiago Calatrava's bridge crosses the river with no supports in the water, sparing the salmon spawning grounds beneath it.

Which river does the Sundial Bridge in northern California span?

Sacramento River: the tip of the mast's shadow moves about a foot a minute, so you can watch the time pass.

Which skyscraper, once the world's tallest, stands beside a park laid out as a giant sundial?

Taipei 101 steadies itself against typhoon winds with a 660-tonne tuned mass damper hung high up inside the tower.

Which country's flag today features an armillary sphere, a ringed model of the heavens?

Portugal's armillary sphere began as King Manuel I's personal badge during the Age of Discovery and later became a national symbol.

Complete the classic sundial motto: 'I count only the ___ hours.'

Other dials carry sombre memento mori lines such as 'Do not kill time, for it will surely kill thee.'

Which Greek estimated Earth's circumference from noon shadows at Alexandria and a sunlit well at Syene?

Eratosthenes found Alexandria's noon shadow tilted by about a fiftieth of a circle, so he multiplied the distance to Syene by 50.

In London, at what point in the year does a sundial cast its shortest noon shadow?

The summer solstice: its name means the Sun 'stands still', as the noon Sun climbs no higher and then turns back.

What pushed towns across Victorian Britain to give up their own local time, set by the Sun?

The railways: timetables needed exact times, but local noon falls four minutes later for every degree of longitude west.

At what time does the shadow fall straight down the centre of a direct east-facing polar dial?

6 a.m.: an east-facing wall catches the Sun only in the morning, and its hour lines spread ever wider apart as noon approaches.

Why did an hour on an ancient Greek sundial run longer in summer?

Daylight was split into 12 parts, so long summer days made long hours, until mechanical clocks, whose hours never varied, brought in equal hours.

Why do many well-made sundials seem to have two XII o'clock lines?

The gnomon plate is thick: each edge casts the shadow in turn, so the dial is really two half dials meeting at noon.

Which Shakespeare comedy has a fool draw 'a dial from his poke' and muse on how the world wags?

As You Like It: Jaques, who tells of the meeting, is so taken that he cries 'O that I were a fool!' and wants a motley coat of his own.

Where in the sky is the Sun when a sundial in London reads exactly noon?

Due south: that is why a dial's XII line and the raised end of its gnomon are set along the north-south line.

Cassini's meridian line in Bologna's San Petronio basilica let him test whose idea that Earth's orbit is an ellipse?

Johannes Kepler: the Sun's image on the church floor grew and shrank through the year, and Cassini found those changes fitted an elliptical orbit.

Which astronomer designed the astronomical side of the ornate sundial on Warsaw's Wilanów Palace?

Johannes Hevelius also honoured the palace's royal owner, John III Sobieski, naming a constellation Scutum Sobiescianum, 'Sobieski's Shield'.

Which medieval English poet described telling the hour from the length of one's own shadow?

Geoffrey Chaucer also wrote his astrolabe guide in English for 'little Lewis my son', a ten-year-old who knew as yet little Latin.