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The astrolabe The astrolabe exists since Antiquity (200 before J.C). It was designed by Hipparque. Ptolemee studied it later in the IIIe century. The astrolabe is a plane representation of the armillaire sphere. It is buit for a given latitude. It makes it possible to locate the position of the stars. It is made up : the fixed "tympan" on which are engraved the circles wich define the astronomical positions of the "araignée". the "araignée" : a projection of the sky chart. the "limbe" : a ring graduated external. It shows the apparent revolution of the celestial sphere around the Earth and makes it possible to establish the position relating of the stars to one unspecified moment. It makes it possible to measure the height of the sun compared to the horizon and thus to calculate the hour.

Arsenius's astrolabe (XVIe century)

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The armillary sphere The armillary sphere, invented by Hipparque consist of five circles of metal. These varouis circles represent : the ecliptic the meridian line the equator the positions of stars the fifth mobile circle carries two sights at the end of an diameter to carry aimings. In its center, a small sphere represents the Earth. The observer can thus represent the apparent movement of the sky according to all latitudes. Ptolemee used larger spheres to make measurements.

The armillary sphere

The Octant This apparatus belogs to the same family as the sextant. Its name comes from its size : the angular sector is 45° (a eightth of the circle). It is used for the measurements of the celestial bodies by employement of mobile mirrors. It thus makes it possible to determine the latitude of the observer, one of the corodinates giving the osition of the boat. It seemed instruments of navigation about the middle of the XVIIIe century.

Octant

Sextant Instrument est utilisé pour déterminer la position géographique de l'observateur. Il était particulièrement utilisé pour la navigation. Il a été inventé vers 1730, en même temps en Amérique et en Angleterre, par le mathématicien anglais John Hadley (1682-1744) et par l'inventeur américain Thomas Godfrey (1704-1749). Il comporte : une lunette fixée horizontalement sur le cadre plusieurs miroirs mobiles Sa construction repose sur les lois de la réflexion de la lumière. Le sextant a la forme d'un secteur angulaire de 60°.

Sextant

The gregorian telescope In the telescope of gregory a real , reversed image is formed in the principal focus of the large mirror placed at the bottom of the tube. In opposite direction of the large mirror and on the same axis, is laid out a small concave mirror. The real image is an object for this second mirror. It is thus formed a second reversed real image so what this one is a right image of the object observed which is increased by eyepiece.

The gregorian telescope

The compass A piece of magnetized iron is aligned spontaneoulsly in the North-South axis. This discovery was alloted with Chinese, the Arabs and the Greeks. It however does not seem that the compass was used by Europeans before 1400. After its appearance, at the beginning of the 15th century, the compass was not change during more than 300 years. The manufacturers of instruments of navy were unable to solve the three problems raised by the compass : The magnetic north is not the true north The difficult conditions disturb the compass The iron and the steel of the hull of the ships causes disturbances It is only about 1800 that we solve these problems.

The compass

The roman balance   The roman balance is one of the instruments of weighing in the past know. Its extremely simple construction, rests on the principle of mechanics according to : "the equilibre of the balance depends of the weights of two heavy bodies acting at the ends of two arms and the lengths of the arms" The weight, advanced or moved back along the divided stem, fact balances with the heavy bodies suspended on the hook. After its appearance, at the beginning of the 15th century, the compass was not change during more than 300 years.

La balance romaine

The hour glass Approximately 3000 years before J.-C, apppeared in Egypt the "clepsydres", or clock with water. Later, at the XIVe century, the hour glasses are usually used. These apparatuses measure time by the flow of a fluid, water or sand. They do not make it possible to determine the hour but are used for its conservation during a more or less long time. They are thus in fact the first chronometers. The precision of such instruments is always very weak, but sufficient if the measured time is short. Now, the hour glasses are used than on a decorative basis.

the hour glass

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The compass The compass is an instrument of layouy or measurement made up of two articulated branches. It is used in navy to calculate end defer distances on cards.

The compass

 

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The Roberval's balance Gilles Personne de Roberval (1602 - 1675), professor of mathematics to the "College de France" in Paris, invented a balance which bears its name. The two plates of this balance rest, on two knives whose edges are turned to the top and are fixed at two mobile stems, connected between them at their lower ends by a mobile lever. This position which does not change the equilibrium conditions, makes very convenient the use of this balance.

The Roberval's balance

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The barometer The simplest barometer is the Torricelli's tube who filled with dry mercury in order to not contain neither gas, nor vapor above mercury. The pressure measured by the barometer is expressed in millimeter of mercury. It makes it possible to meausure altitudes indirectly. The mercury contained in the tube rises of 1 mm by 10 m rise. It is very useful for wheather forecasting.

The barometer

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