on first arriving in France - driving
France is not England
Cathedrals in France
Futuroscope
Vulcania
viaduct de Millau
the French umbrella &
Aurillac
the forest
as seen by francois mauriac, and today
places and
playtime
roundabout art of
Les Landes
50 years old:
Citroën DS
the Citroën 2CV:
a French motoring icon
Grand Palais, Paris
Marianne - a French national symbol, with French definitive stamps
the calendar of the French
Revolution
Motorway Aires
le pique-nique
Hermès scarves
bastide towns
mardi gras! carnival in Basque country
what a hair cut! m & french pop/rock
country life in France: the poultry
fair
the greatest show on Earth - the Tour
de France
short biography of Pierre
(Peter) Abelard
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Motorway aires are designed to provide a suitable environment for relaxing,
refreshing and recovering during the long, hard journeys. As well as
facilities of often dubious nature, picnic tables and seats, a telephone
kiosk, there are often optional extras such as a play area or a display
related to some local interest or event.
Tavel
nord aire, A9 and its sundials
From the north, showing the scale of the monumental
Tavel sundial.
Note the circular illustration of the zodiac on
the ground. [Photo composite]
From the south, showing different parts of
this multiple sundial structure
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:
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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. |
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:
-
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.
- 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).
- 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. 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:
- The earth spins at an irregular rate around its axis of rotation.
- 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]:
- 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.
- 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.)
- 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
- During Summer/Daylight Saving Time, add one hour to the sundial time.
-
- 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.
Table of corrections for
the Nef Solaire sundials at Tavel, A9
[based on the Equation of Time] |
January |
May |
October |
1st - 2nd |
45 mins |
1st - 28th |
38 min |
1st - 2nd |
31 min |
3rd - 4th |
46 min |
29th - 31st |
39 min |
3rd - 5th |
30 min |
5th - 7th |
47 min |
June |
6th - 9th |
29 min |
8th - 9th |
48 min |
1st - 4th |
39 min |
10th - 12th |
28 min |
10th -12th |
49 min |
5th - 9th |
40 min |
13th - 17th |
27 min |
13th - 14th |
50 min |
10th - 14th |
41 min |
18th - 23rd |
26 min |
15th - 17th |
51 min |
15th - 19th |
42 min |
24th - 31st |
25 min |
18th - 21st |
52 min |
20th - 23rd |
43 min |
November |
22nd - 25th |
53 min |
24th - 28th |
44 min |
1st - 14th |
25 min |
26th - 30th |
54 min |
29th - 30th |
45 min |
15th - 18th |
26 min |
31st |
55 min |
July |
19th - 22nd |
27 min |
February |
1st - 3rd |
45 min |
23th - 26th |
28 min |
1st - 23rd |
55 min |
4th - 9th |
46 min |
27th - 29th |
29 min |
23rd - 28th |
54 min |
10th - 19th |
47 min |
30th |
30 min |
March |
20th - 31st |
48 min |
December |
1st |
54 min |
August |
1st |
30 min |
2nd - 6th |
53 min |
1st |
48 min |
2nd - 4th |
31 min |
7th - 10th |
52 min |
2nd - 10th |
47 min |
5th - 6th |
32 min |
11th -13th |
51 min |
11th - 15th |
46 min |
7th - 8th |
33 min |
14th - 17th |
50 min |
16th - 20th |
45 min |
9th - 11th |
34 min |
18th - 20th |
49 min |
21st - 24th |
44 min |
12th- 13th |
35 min |
21st - 24th |
48 min |
25th - 27th |
43 min |
14th - 15th |
36 min |
25th - 27th |
47 min |
28th - 31st |
42 min |
16th - 17th |
37 min |
28th - 30th |
46 min |
September |
18th - 19th |
38 min |
31st |
45 min |
1st - 3rd |
41 min |
20th - 21st |
39 min |
April |
4th - 6th |
40 min |
21st - 23rd |
40 min |
1st - 3rd |
43 min |
7th - 9th |
39 min |
24th - 25th |
41 min |
4th - 6th |
44 min |
10th - 11th |
38 min |
26th - 27th |
42 min |
7th - 10th |
43 min |
12th - 14th |
37 min |
28th - 29th |
43 min |
11th - 14th |
42 min |
15th - 17th |
36 min |
30th - 31st |
44 min |
15th - 18th |
41 min |
18th - 20th |
35 min |
|
|
19th - 23rd |
40 min |
21st - 23rd |
34 min |
|
|
24th - 29th |
39 min |
24th - 26th |
33 min |
|
|
30th |
38 min |
27th - 29th |
32 min |
|
|
|
|
30th |
31 min |
|
|
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
- 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.
- Gnomonist or dialist:
one who constructs dials to show the hour of the day by the shadow of
a gnomon.
- 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.
- 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.
- The term day lengths can
refer to two different things : the approximately twenty-four hour day,
or the time of daylight.
- “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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