Created on thick stock paper using Alberti’s Horizon instrument. 380mm diameter.
Alberti’s Horizon is an instrument that allows any person to recreate the map of Rome using a radial co-ordinate system invented by Alberti. The city walls, gates, river and significant buildings were all measured and mapped from a central point in the city, each being given a radial distance (on a horizontal plane) and degrees from N at the central point. These are laid out in tables so that they can be transcribed using the instrument.
The horizon instrument is made up of a ring of a user defined diameter divided into 48 degrees and 4minutes per degree. A rule divided into 50 sections with each section containing 4 minutes sits so that it pivots from the centre allowing for radial measurements.
Users therefore read off the co-ordinates from the tables and find them using the rule. For example a co-ordinate of 28’3″, 43’2″ would be found by rotating the rule to an angle of 28 degrees and 3 minutes, then reading off the rule the point of 43’2″ and mark this on the page. This is continued through all the tables and in the case of the walls of the city require you to join the dots to form the map.
Instrument: 14th Century European Astrolabe
Plates: 40 – 45 degrees N
Plate replicated: 43 degrees N
To determine the time of day or night:
Begin by measuring the angle of a specific star (or the sun) using the alidade on the back. This piece has two small holes to look through as you hold the astrolabe from a ring attached to the top (if aiming at the sun do not look through the alidade but instead align it as such so that a beam of light passes through both holes onto a surface near you). Once our view is aligned read off the angle of the sun or star on the outer edge.
Flip over the astrolabe and position your star to the correct altitude by rotating the rete. (If measuring the suns angle you have to know the date according to the zodiac and use the eclipse on the rete.) Double check with a compass to determine which side of the projection your star should be. Once positioned you can use the rule to read off the time of day. On this particular astrolabe there are no times displayed but it is marked at 15 degree intervals that correspond to an hour of the day. E and W mark 6am/pm respectively.
For the sun you need to position the correct date on the eclipse with the angle of the sun determined and again use the rule to measure off the time of day.
To determine sunrise and sunset times:
Select your correct date according to the zodiac and line up the corresponding mark on the eclipse with the corresponding side of the horizon line. Left side being East therefore sunrise and the right side being West therefore sunset.