Astrolabe

A period drawing of an astrolabe

The astrolabe is an ancient astronomical calculator for determining time and the positions of the Sun and stars.

There are several types of astrolabe including the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator.

Astrolabes were typicaly made from brass with a diameter of around 15cm.

Period construction

The following instructions for constructing an Astrolabe are from "The arte of nauigation" by Cortes, Martin (1532-1589), published in 1572.

The.vii.Chapter, of the making and vse of the Astrolabie, with the whiche the Marryners take the altitude of the Sunne.

Take a plate of copper, or laton, (which for this purpose is better then any other metal) of the bignesse that you desire to make the Astrolabie, & is commonly of the bignesse of a spanne the Diameter, and let it be of the thickenesse of half a fynger at ye least: for the the waightier that it shal be, so much shal it be more steadie to take the altitude. This plate must be made rounde by a circle, leauing comming foorth of the circle a corner, in the whiche you shall put a ringe or handle with a hole, wherby you may hang the Astrolabie, by a threede or lyne to take the altitude. After it is thus made, with the ring or handle annered therto, make it bright, and smoothe, pullished on both sydes, & all of one equal thicknesse, that one side be not heauier then an other, which you shall trye in this manner: Hang the plate by the ryng or hole that you haue made, and from the same hole hang a ploinet of leade, fastened to a heere, or fine threede of silke. The Astrolabie thus hangyng, free, and at libertie with the plomet, yf then the threede fall uppon the center of the Astrolabie, it is well: but if the threede doo leane of swarue to the one side, or the other, from the center, then is that side thicker and heauier then the other, and must therefore be made thynner, untyl the threede fal iustly uppon the center. This donne, make a circle upon the sayde center, a litle within the circumference of the Astrolabie. Then drawe a Diameter from the center of the hole, in the whiche the ring or handle is, unto the center of ye Astrolabie, trauersing or ouerthwartyng the whole Circle. And this shalbe cut with an other Diameter upon the center makyng the ryght angles with it. And this Diameter shalbe called the Horizontal lyne. These two Diameters shall divide the Circle into foure partes. After this, you shall make an other Circle, so much more within the second, that betweene the circumferences of both the circle, may be conteyned the numbers of the degrees. Then (the Astrolabie hangyng before you) you shall divide the one parte (beyng the superior and least part) fyrst, into three equal partes, and euerye parte shall conteyne 30 degrees. Then shall you divide euery parte of these into oother three equall partes, and they shall conteyne 10 degrees : and euerye of these divided into two partes, and they shall contayne 5 degres. This donne, put a ruler uppon the center of the Astrolabie, applyng it to every of the poyntes that divide the sayde partes, and drawe certayne lynes that passe from the circumference of the firste Circle, unto the lesse circumference: and in the spaces of the lesse Circle, wryte the numbers of hte degrees, begynnyng in the Horizontall lyne: and in that space put fyue, and in the seconde tenne, and so foorth of the other, untyll the 90 degrees ende in the lyne of the Zenith: then shall you divide the spaces are betweene the fyrste Circle and the seconde, euerye space into fyue, whiche shall make the 90 degrees. The Astrolabie thus made, you shall make the Alhidada or Labrill. for the whiche, you shall take a plate of laton, of the breadth of scarsclye two fingers, and as thicke as the Astrolabie: also, as long as the Diameter of the Astrolabie, and make a line in the myddest thereof, by the longitude: in the myddest of this line make a Circle so great, that it may touche in the sydes of the plate: then cut this plate on the one syde, that whiche it hath from the lyne to the ryght hande: and on the other side that it hath from the lyne to the left hande, leauyng the Circle whole. This lyne that shall passe y the center of the Circle, is called Linea fiduci槧, (that is) the lyne of confidence: whiche is that, that sheweth in the degrees, the altitude that is taken. Then shall you take away the endes or corners of the Alhidada that are without the lyne, so that you touche not the lyne. This donne, you shall make two lytle rysyng or raysed tablettes or plates of the same metalll whereof the Astrolabie is made, and of the selfe same thickenesse that is the Alhidada, or litle lesse: and of the breadth of the Diameter of the circle of the Alhidada. And let them be a thumbe in height or bredth. In the myddest of these two plates, by the heyght, you shall make a lyne. When these are made equal, and al theyr angles ryght, in euerye lyne of these that you haue made, you shal also make two holes, equally distant from the sydes or edges of the sayde plates or tablettes. And of the two holes of euerye of these lytle plates, the one hole muste be as bygge as maye conteyne a great pynne: and these shal serue to take the altitude of the Starres. The other muste be so subtile and smale as a fyne sowyng needle: and these serue to take the altitude of the Sunne. They must be made in suche manner, that the outwarde parte of them be bygger, and lesse within, of the quantitie that I have sayde. These tablettes or erected plates beyng thus made, they muste be sothered in the Alhidada, betweene the center and extremities or endes of the same, makyng in it certaine notches where they maye be made faste, and sothered: or leauing first in the plates, certayne sharpe poyntes or corners, that may ebter into certayne holes made in the Alhidada, whereby they may be made fast underneath with pynnes: And they must be so sette, that the lyne where the holes of the plates are, maye fal uppon the line of confidence of the Alhidada: So that the one halfe of the plate be set uppon the Alhidada, and the other halfe without it, or at large. In lyke manner shal you take good aduertisement, that the greate hole of the one plate, stande directly agaynst the great hole of the other plate, and be nothing at al a wrye. This donne, you shal bore the Astrolabie through by the Center, makyng a very rounde hole, that may haue midst of it the Center of the Astrolabie. This hole shal be of the byggnesse of a Goose quyl. And the lyke shal you make in the Center of the Circle of the Alhidada. Then shal you make a pynne or nayle of the same laton, the whiche on the upper parte of the Alhidada maye haue a playne and Rounde head. This pynne also shalbe very rounde and smooth, that it maye enter iuste and close into the hoale of the Alhidada and Astrolabie. And at the ende or poynte thereof, commyng foorth on the other syde of the Astrolabie, must be a hoale made syde way through the pynne. close to the plate of the Astrolabie, of the bygnesse of a little nayle or pynne, that may be put therein, to make fast togeather the Alhidada with the astrolabie, so that the Alhidada may therby be turned rounde about the Alstrolie, as appeareth in this fygure folowyng.Also the asrtolabe was used in science class and other resoures expeciallly in other parts of the world such islam and in the middle ages in Europe.