60 Fun Facts About Sundials

What is the shadow-casting part of a sundial called?

The word comes from Ancient Greek for 'one that knows or examines'; only its time-telling edge is called the style.

How many degrees does the Sun appear to move around Earth's axis each hour?

That is why the hour lines on a dial whose plate faces the celestial pole are evenly spaced.

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

That alignment is what makes the shadow sweep round at a constant rate whatever the season.

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

A dial built for London therefore does not keep good time in Cairo unless it is tilted.

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

The path traced by a nodus over a day is a conic section such as a hyperbola.

From roughly what date do the earliest known Egyptian and Babylonian shadow clocks survive?

Egyptian obelisks were long assumed to be sundials too but are now thought to have been purely memorials.

Which Greek thinker is credited with bringing the sundial to Greece around 560 BC?

He was a pupil of Thales in Miletus and also drew one of the first maps of the known world.

Which Roman author catalogued the known sundial types in Book IX of De Architectura?

He wrote around 25 BC, and every dial on his list was a shadow-tip type that differed mainly in the shape of its receiving surface.

Which Sun-centred-model astronomer is credited with the bowl-shaped scaphe dial?

None of his original works survive; the dial was a hemispherical bowl with a vertical pin whose tip was level with the rim.

According to Pliny, in what year was a sundial first recorded in Rome?

A character in a Plautus comedy later complained that his day was 'chopped into pieces' by the ubiquitous dials.

What did the Greeks call the axe-shaped pattern of hyperbolae traced by a shadow tip?

The curves are narrow near the noon line and flare out toward the early morning and evening hours.

Which emperor set up an Egyptian obelisk as a shadow-caster in the Campus Martius in 10 BC?

The obelisk had originally been erected under the pharaoh Psamtik II and now stands in Piazza di Montecitorio.

On which date did the Solarium Augusti's shadow fall across the centre of the Ara Pacis?

That was the emperor's own birthday, linking his birth to the arrival of peace.

Which pope re-erected the Montecitorio obelisk in Rome in 1789?

The obelisk had been thrown down after the 8th century and lay buried until it was rediscovered in 1512.

Which octagonal marble monument in Athens carried eight sundials and a water clock?

It is the only surviving horologium from classical antiquity and was completed by about 50 BC.

What bronze figure originally topped Athens' octagonal clock tower as a weather vane?

The figure held a rod that pointed out the wind direction; it has completely disappeared.

Which Anglo-Saxon cross in Cumbria carries the earliest surviving English sundial?

The dial is a canonical one divided into the 'tides' that governed the medieval working day.

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

His shadow table assumed a monk's height was six times the length of his foot.

Which Damascus timekeeper built a sundial for the Umayyad Mosque minaret in 1371?

His dial used hours of equal length all year, unlike the seasonal hours of the old Greek dials.

Which Italian astronomer's 1570 treatise explained how to lay out wall and horizontal dials?

Sundials have been in common use in Europe since that century.

The villages around which French Alpine town were famed for painted sundials in the 1800s?

The Hautes-Alpes department has at least 400 painted dials, a hundred of them by Giovanni Francesco Zarbula.

What is the up-to-16-minute correction between a sundial and an ordinary clock called?

It arises from Earth's elliptical orbit and tilted axis, and it is the same everywhere on Earth.

Around which date is a sundial furthest ahead of clock time, by 16 min 33 s?

The dial runs slowest, roughly a quarter of an hour behind, around 11 February.

How many times a year does a sundial agree exactly with mean clock time?

The zeros fall near 15 April, 13 June, 1 September and 25 December.

What shape does the analemma, the Sun's yearly path at a fixed clock time, resemble?

The long axis spans about 47 degrees, twice the tilt of Earth's axis.

Where on a terrestrial globe is the analemma usually printed?

It is the only large tropical region with very little land to obscure.

How many minutes of time does one degree of longitude represent when correcting a sundial?

A dial 5 degrees west of its zone's reference meridian therefore reads 20 minutes slow all year.

Which way do the hour numbers run on a horizontal sundial in the Southern Hemisphere?

A dial made for one hemisphere has to be reversed to work at the same latitude in the other.

Which sundial type has its plate perpendicular to the style and evenly spaced hour lines?

It is also called an equinoctial dial because the plate is parallel to Earth's equator.

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

Some are built so that a person standing on the date scale becomes the shadow-caster.

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

It is also called a pillar dial or chilindre and gets a mention in Henry VI, Part 3.

Which mathematician likely invented the universal equinoctial ring dial around 1600?

Edward Wright later mounted one over a compass to make the 'sea ring' for mariners.

Which physicist devised a cheap sundial using a windowsill mirror and the ceiling?

The mirror stands in for a shadow tip, and a ceiling gives a dial large enough to be very accurate.

What material were the faces of pocket diptych sundials typically made of?

A string stretched between the two hinged faces formed both a vertical and a horizontal dial.

Which 1922 design by Hugo Michnik tells time from the crossing of two thread shadows?

The two threads do not intersect and are usually set at right angles above the dial plate.

Where was Walter Gordon Benoy, inventor of the sheet-of-light Benoy dial, from?

Examples stand at Jodrell Bank, Longleat and the Science Museum in London.

What does a sundial cannon do at solar noon?

A lens focuses the Sun onto the firing pan; the lens position has to be adjusted with the season.

In which country is the only sundial cannon still in regular operation, at Adelsnäs Manor?

Installed in 1853, its 6-pounder fires daily from May to August, lit by hand on cloudy days.

Which American published a joke recipe for a 'striking sundial' audible for ten miles?

It appeared in Poor Richard's Almanack, noting a great saving of powder on cloudy days.

The Vrihat Samrat Yantra at Jaipur is roughly how tall?

Its face is angled at 27 degrees, Jaipur's latitude, and its shadow visibly moves about 1 mm per second.

Which Rajput ruler built the Jantar Mantar observatory at Jaipur, completed in 1734?

He founded Jaipur itself and built five observatories to produce more accurate astronomical tables.

Which architect designed the Sundial Bridge in Redding, California, completed in 2004?

Its 217-foot mast points due north and casts the shadow, though it is only accurate on one day a year.

The Sundial Bridge spans which body of water?

The bridge crosses without touching the water to protect salmon spawning grounds beneath it.

Which skyscraper acts as the style of a giant sundial laid out in its adjoining park?

The tower was the world's tallest when it opened in 2004, and the park's clock runs on wind shear from the building.

Which national flag features an armillary sphere, a ringed model of the heavens?

Manuel I adopted it as a personal symbol during the Age of Discovery.

What is the simplest sundial, which marks only the moment of midday, called?

In colonial American houses one might be carved into a floor or windowsill to reset the household clock.

How can a correctly built horizontal sundial be used as a compass?

Its style then points true north or south, which beats a magnetic compass where declination is large.

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 around 240 BC using an obelisk's shadow and a well?

Centuries later Ptolemy charted the latitudes of cities from the angle of the Sun.

What is a precision sundial accurate to the minute, first devised by Philipp Hahn c. 1763, called?

Abbé Guyoux improved the design around 1827; it reads minutes of Universal Time.

In which California city does the world's largest axial-gnomon dial, a Calatrava mast, stand?

The mast of the Turtle Bay span took the title from the Jaipur gnomon, which is raised 26°55' to match its latitude.

How fast does the shadow move across Jaipur's 27-metre Samrat Yantra?

That is roughly a hand's breadth every minute on the 'Supreme Instrument' built between 1727 and 1733.

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

For a west-facing dial the centre time is 6 p.m.; the hour-line spacing diverges to infinity six hours away from it.

A portable altitude dial shaped like a little ship is known as what?

The 'little ship of Venice' sits alongside card dials and ring dials among altitude-based instruments.

By what angle is the Foster-Lambert dial's gnomon tilted north, where Φ is the site's parallel?

At 40° latitude that means a gnomon leaning 25° from vertical, and it must also be moved north each season.

Which Shakespeare play evokes the shepherd's dial?

The portable cylinder dial can be unrolled into a flat plate, and hollow versions fold the gnomon inside.

Where is a digital sundial, which forms the time in numerals from sunlight, installed in Germany?

Another stands in the Sundial Park in Genk, Belgium, and the design is patented.

A meridian line, such as Cassini's in San Petronio, Bologna, indicates what rather than the time?

Francesco Bianchini laid a similar line in Santa Maria degli Angeli in Rome.

Which astronomer created the Wilanów Palace sundial in about 1684?

The Sciothericum telescopicum of the same century used a telescopic sight to fix noon to 15 seconds.

How long is the shadow of a 170 cm person at 27° latitude on the summer solstice?

Chaucer's Canterbury Tales describe telling time from the length of a human shadow.