motorway aires: 11

motorway aires[1]
Tavel nord aire, A9 auto route:
giant sundials

Tavel nord aire, A9 and its sundials How to reach Tavel nord aire - sketch map Time, the sun and clocks Varying day lengths Clock time - mean time Summer time Telling the time using a sundial To convert solar time to clock [civil/legal] time Why a sundial is more accurate than a clock or watch Some facts and figures Bibliography and useful links

Tavel nord aire, A9 and its sundials

The busy A9 autoroute between Orange and Nimes leads to the western stretch of the French Mediterranean coast. The southbound Tavel aire is dominated, not by the service station, shop and café clustered at the base of a small hill, but by the structure visible from the motorway : the asymmetric, futuristic yet mathematical and educational monumental sculpture - an enormous set of sundials, the Nef Solaire : the Solar Ship or Nave. This edifice is like huge sailing ship, or a cavernous cathedral. It is built of four white concrete sail-like structures, butted one against another with the highest point 17 metres high. In all there are five sundials : three vertical ones on three of the four ‘sails’, and two horizontal ones (on the ground) for morning and afternoon.

Morning horizontal sundial
[taken in the afternoon, so the shadow does not mark the actual solar time]

As well as the five dials, there are two gnomons [that’s the stick bit, also known as a style] which throw shadows on the various sundials. At any time of day, at least two of the dials are active - one horizontal and one vertical.

Built in 1993, the sundials are surrounded by Mediterranean garrigue planting - olives, pines and poplars, together with strong-smelling rosemary and thyme. The main originators were sculptor Odile Mir, gnomonist Denis Savoie, and engineer Robert Queudot.

The afternoon horizontal sundial.
This detail shows the shadow from the afternoon gnomon
[on the left] marking just after 16.00 solar time.
Note the shadow time is marked on both the ground and the sail.
This time would need to be converted to match the equivalent clock time.

The enormous sundials are set to Universal Time [UT], which is the same as Greenwich Mean Time [GMT]. However, the Tavel sundial is not on the same longitude as Greenwich. The solar time at Tavel is GMT plus 18 minutes 45 seconds.
[Click for detailed information on converting the solar time as indicated by these sundials, into clock time as shown on a watch.]

Mounted on knee-high blocks around the sundial are information plaques, some of which give instructions on how to correct the time provided by the sundials into clock (civil) time. Unfortunately, the plaques are only in French.

There are also other panels that describe the history of different sundials since early times. Here are translations of the French text for those panels:

	Gnomon This primitive sundial functions with an obelisque or stick planted vertically in the ground, the shadow of which marks the time of day. The gnomon was used from earliest antiquity by practically all ancient peoples. Despite its imprecision, it served [...] to make astronomic measurements.
	Scaphé - sphère creuse - hémisphère This type of sundial dates from the 3rd century BC, it was made by the Greeks and Romans. The scaphé was most often in a hemispherical or conical form. The time between sunrise and sunset was divided into 12 equal parts, whatever the season. This resulted in a variable length or an hour, for France a hour would be 40 minutes in winter and 80 minutes in summer.
	Tall sundial - pole sundial During the Renaissance there appeared portable sundials of several types - the shepherd’s watch, the astronomer’s ring. The orientation of the pole sundial depends on the the position of the Sun through the day.
	Analemmatic sundial First described by Francis Vauzelard in 1640, the Analemmatic sundial has an elliptic shape. It functions with a vertical style, which is moved according to the month. Later, it was simplified and extended to work in with different longitudes (different orientations) and latitudes (different inclinations).
	Polar style sundial Towards the 18th century, there appeared a flat sundial equipped with an inclined style that pointed towards the Pole star. Already known to the Arabs for at least two centuries, this arrangement brought and important improvement - the indication of the hours remained the same all year round, unlike for previous sundials. This type of sundial had several variations and could include curves to indicate the solstices and equinoxes.
	Canonical dial This sundial appeared on church facades during the 18th century. It was in the form of a circle (or more often a semi-circle) divided into six, eight or twelve equal sections. There were no numbers. At the centre of the dial, a horizontal stick threw a shadow which showed the prayer hours. [Sundials bridged the gap between art science, and varied considerably. A dial may be labelled, for example, with Prime,Tierce, Sext, None, Vespers.]
	Two-wire sundial Invented in 1922 by a German mathematician, Hugo Michnik, the two-wire sundial is rather special. The hour is indicated by the intersection of the shadows of two wires, set at different heights and which are perpendicular to one another. This makes possible having a constant interval of 15° between each hour line.

There are examples of other types of sundial included in the outdoor displays at the Cité de l’espace - Space City - at Toulouse.

As well as being a sundial, the structure at Tavel aire includes a ring of illustrations showing how the signs of the zodiac relate to constellations.

The Earth revolves around the Sun in just over 365 days. From here on Earth, it appears that that the Sun moves relative to the stars. The constellations amongst which the Sun appears to traverse successively are the constellations of the zodiac. There are twelve constellations, decided because the related stars, when joined up like dots, apparently make outlines that have some relation to constellation’s name. Thus, at the Spring Equinox (20 March), the Sun appears to traverse before the constellation of Pisces; at the Summer Solstice (21 June) before the constellation of Gemini, and so on.

Zodiac constellation - Gemini

How to reach the Tavel nord motorway aire

The Tavel North aire reviewed on this page is accessible from both directions of the A9 autoroute, from the northbound direction by a connecting road tunnel between the two sides. The aire on the southbound side of the autoroute also has fuel, shop, a café. [abelard.org has not yet visited Tavel Sud].

The Tavel aires are in Département 84 - Vaucluse.

Time, the sun and clocks

Sun time is different from clock time.
For a sundial at any place on the Earth, noon [12 midday, 12 p.m.] is when the sun passes the local meridian line [line of longitude]. This is when the sun is highest in the sky for that place and that day of the year. It is also when shadows are at their shortest. But the amount of clock time between that noon and the next noon will probably differ.

A clock or a watch marks the same amount of time, 24 hours, passing between one noon and the next, whatever the time of year. Clocks and watches show a mean or average time that ignores the real slight differences in day length through the year, and also ignores the fact that a day is not exactly 24 hours long, nor a year exactly 365 days long. (A year is, in fact, 365 ¼ days long.) Thus, solar time can be faster or slower than clock time.

Small differences between the length of day for sundial [solar] time and clock/mean time gradually compound to become larger differences between the two types of time at some periods of the year.

Varying day lengths

Day lengths vary for three main reasons:

1. Obliquity: the Earth’s axis of rotation is tilted [inclined] as it circles the Sun, so the Equator is not parallel to the orbit of the Earth around the Sun. The angle of this tilt is called the angle of obliquity. This angle measures 23°26'. This means that the plane of the Earth’s equator is inclined relative to the plane of the Earth’s orbit around the Sun.
   
   Because of the inclination, the Sun is at its highest point (overhead) in the sky [at the time known as noon] at different latitudes, at different times of the year. The higher the Sun reaches, the longer that day lasts. All this also means that the length of daylight varies according to the latitude of the observer.

The inclination of the Earth results in the seasons. In the Northern hemisphere, the Sun is high at noon in June (summer) and low on the horizon in December (winter). This is reversed in the Southern hemisphere, where the Sun is high at noon in December (summer)and low on the horizon in June (winter). At the Equator, the length of day hardly varies throughout the year. Because of this, the length of daylight time differs during the year, being shorter in winter when the sun is low and so above the horizon for a smaller part of a day, and longer in summer when the sun is above the horizon for more of a day.

2. Eccentricity: the Earth’s orbit around the Sun not a circle, but an ellipse. Thus the apparent motion of the Sun varies throughout the year, with the Sun appearing to move fastest when the Earth is closest to the Sun in winter, and slower in summer when it is further away. [The sun is closest to Earth on 3rd January - 91.3 million miles/147.5 million km, this is the perihelion; the sun is furthest away on 4th July - 94.4 million miles/152 million km, this is the aphelion.] The Sun’s gravity pulls on the Earth harder when the Earth is closer to it. This pull increases the Earth’s speed by about 20 miles per hour. To put this in context, The Earth orbits the Sun at an average speed of 18 ½ miles per second, or 67,000 miles per hour. [Detailed explication: 2-page .pdf.]
   
   The longest solar day is 19th December, it is 24 hours, 28 seconds long. The shortest day is 14th September and is 23 hours, 59 minutes and 38 seconds long.

These two differences between solar time and clock time are corrected by using the Equation of Time, or EOT. [The EOT is usually presented as a table, or as a graph, of calculated differences between solar and clock time.]

The following diagram is a graphical presentation of how solar time differs from clock (mean) time during a year, with the components caused by eccentricity (the Earth’s eliptic orbit) and obliquity (the tilt of the Earth).

3. Differences because of your longitude
   A further difference in time results from the Sun passing at its highest point at different moments consecutively as it goes round the Earth. The Sun moves from East to West in the Northern Hemisphere, thus noon for someone in London, England actually occurs before someone in Bristol, although both cities are in the same country and the same time zone. Bristol is 2 ° 35' west of Greenwich, and so noon at Bristol is a little over 10 minutes later than noon at Greenwich [15° longitude = 1 hour; 1° longitude = 4 minutes time].
   
   Longitudes are imaginary half-circles running around the world, starting at the North Pole and ending at the South Pole. Thus London and Bristol are at different longitudes, although they are at a roughly similar latitude (distance from the Equator). The Sun passes over longitudes consecutively as the Earth completes its daily rotation on its axis.
   
   In the days before clocks were able run accurately, public sundials, like those to be seen on church facades, were used to reset the clocks.
   
   Before the Industrial Revolution in Britain, there was no particular need for clocks in different towns to show exactly the same time. Each town ran on its own time, based on the town’s longitude - this determines when noon [the Sun’s highest point in the sky during a day] occurs at any location.
   
   The introduction of railway transport stretching from one end of Britain to the other, and related communications brought by the telegraph, changed that. It became necessary for all railway stations, and so all towns, to run on the same time, which was first known as Railway Time, first introduced in 1845. Thus towns gradually came to use a standardised time - Greenwich Mean Time [GMT] , based on the longitude that passes through Greenwich, south London. GMT is now called Universal Time [UT].

Other, lesser reasons for variation in the length of a day are:

1. The earth spins at an irregular rate around its axis of rotation.
   
   
2. The earth ‘wobbles’ on its axis.[6]



Clock time - Mean time

A clock measures a day assumed to be the same length - 24 hours - every day of the year, despite day lengths varying through out the year [see varying days lengths]. This is Mean Time, where all days are taken to last exactly 24 hours.



Summer time

When Summer Time - Daylight Saving Time - adds an hour to the standard time, there are three reasons why a watch or clock does not show the same time as that shown by almost any sundial [these differences are described in more detail in other sections of this web page]:

1. By assuming the days are all the same length. Because of the effect of the Earth’s inclination (tilt) and its elliptical orbit, a day’s length can vary by up to about 15 minutes. The Equation of Time corrects for these differences.
   
   
2. By not being on the local standard meridian. The Greenwich Meridian running through South London at Greenwich is probably the most famous, but each time zone has its meridian on which the local time is based. The sundials at the Tavel Aire are marked according to Universal Time, based on the Standard Meridian [or longitude] , which is the Greenwich Meridian at 0°.
   
   However, France uses Central European Time, based of the meridian of 15°E that passes through Prague. Therefore for these sundials, an hour has to be added to convert to Central European Time.
   (There is a 4 minute variation for every 1° you are east or west of your designated standard meridian. Because Clock Time is an average or mean time, no allowance is made for this smaller variation from the actual time at the standard meridian.)
   
   
3. Because of the hour added as required for Summer Time/Daylight Saving, there can be a further 60 minutes difference.

Telling the time using a sundial

On a sundial, a stick is called a gnomon casts its shadow on a marked dial. Where the shadow falls on the dial gives the solar time for that particular latitude and longitude.

The length of the shadow the gnomon varies throughout the year, being long in winter and short in summer. A sundial marked with curves, called diurnal arcs, can indicate the seasons, as well as the time (shown by the position of the shadow).

To convert sundial time to clock [civil/legal] time

1. During Summer/Daylight Saving Time, add one hour to the sundial time.
   
   
2. Add a correction for the variation in the length of the solar day. This correction is called the Equation of Time [EOT]. This is included in the correction shown by the Tableau de corrections (table of corrections) plaque at the Tavel aire.
   
   

3. Add a correction for not being on the same longitude as the Standard Meridian for the local time zone. In the case of the sundials at the Tavel Aire, you add 18 minutes 48 seconds. The standard meridian for France being the Greenwich Meridian - 0° longitude.

Examples:
On the 10th of July, you have seen the shadow at 14 hours on one of the sundials. Being during the Summer Time period, you must add one hour. Thus, the clock time is 15 hr 47 (3.47 pm).
On the 27 February, you read 11 hr on one of the sundials. The table indicates a correction of 54 minutes. Thus the clock time is 11 hr 54 (11.54 am).

Why a sundial is more accurate than a clock or watch

A sundial is a scientific instrument calibrated for a specific latitude. It uses the natural motions of the earth around the sun to find local solar time at each instant. Clocks keep mean time, which is the average solar time for a broad 15° longitudinal section of the earth known as a Time Zone. While convenient to running modern businesses, clocks give us only a coarse approximation of local solar time.

Some facts and figures

The Nef Solaire:
• Construction date: 1992 - 1993
• Height: 17 metres/56 feet
• Weight: total 240 tonnes, each ‘sail’: 60 tonnes
• Precision: within 30 seconds
• Latitude: x = 44° 00' 30" North
• Longitude: y = 4° 42' 0 "East
  
  Sundials in the world:
• Largest: Currently, this is the Samrat Yantra (Supreme Instrument) at Jaipur, in India.
  Built in 1724, 27m./89ft high, 45 m./148 ft wide, 0.4 ha/1 acre ground area
• Pajala, Sweden, circular 38 m./42 yards diameter [link to webcam]
• Carefree, Arizona: 27 m./90 ft diameter, gnomon 19 m./62 ft high, 1959
• Disney World in Orlando, Florida, 36 m./119ft diameter
• Singleton, NSW, Australia: largest monolithic sundial
• Lloydminster, Canada: dial 60m/197ft diameter
  [Note that there are many “world’s largest sundial”. Like American presidents, title-holding sundials keep their title, even when they have been superceded by bigger sundials.

Bibliography and useful links

• Le nef solaire de Tavel - by the Montpellier Education Authority. The pages are in French and mostly provide the text and images of the plaques at the Tavel Nord aire.
• Detailed explanation on the Equation of Time and why sundial time differs from clock time depending on the time of year. Also how the Equation of time is calculated.
  Daily sun data: a detailed table for the current Equation of Time.
• Comprehensive site on the Analemma, or why and how the sun does not appear at the same height in the sky each noon. Introduction and eight further pages, with diagrams and animations.

end notes

1. aire: in this context, an area —
   aire de loisirs: recreation area;
   aire de pique-nique: picnic area;
   aire de repos: rest area;
   aire de services: services , motorway (GB) or freeway (US) service station.
   
   
2. Gnomonist or dialist: one who constructs dials to show the hour of the day by the shadow of a gnomon.
   
   
3. Each time zone in the world is based on a local standard meridian, a particular longitude. For Universal Time [UT], also known as Greenwich Mean Time, the meridian is 0°. For Central European Time, which is one hour in advance of UT, the local standard meridian is 15°E, which runs through Prague.
   
   
4. Angles between two lines, or angles describing the size of the segment of a circle, are measured in degrees, minutes and seconds - °, ', ".
   Time is measured in hours, minutes and seconds - hr, '," or hr, min, sec.
   Minutes and seconds of arc [as measurements of an angle are often labelled] are not the same as the minutes and seconds as used to measure and describe time.
   The Earth rotates on its axis approximately once every 24 hours. That is, the Earth rotates 360 degrees [360°] once in 24 hours. So in one hour, the Earth rotates (360 / 24) degrees or 15°. In one minute of time, the Earth rotates (15 / 60) degrees or ¼°. This is more usually wrtten as 15 minutes, or 15', of arc. This is not the same as the amount of time in one quarter of an hour, which is also labelled as 15 minutes.
   
   
5. The term day lengths can refer to two different things : the approximately twenty-four hour day, or the time of daylight.
   
   
6. “The moon, moreover, has a wobble in its orbit. This wobble doesn’t affect relative sea level, but it does drive a cycle in tidal ranges. It pushes tides higher and higher for nine years and then increasingly lower for nine years. Every 18.6 years, the “lunar-nodal cycle” adds five centimeters (two inches) to high tides along the Atlantic coast.” [Quoted from scseagrant.org]

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