Unequal hours

Before the introduction of clocks, the time between sunrise and sunset was commonly divided into 12 equal hour divisions. Similarly, the night between sunset and sunrise was also divided into 12 equal hour divisions. The result was that the length of an hour during the daytime was not equal to the length of an hour during the night (except at the equinoxes) and the lengths of the hours were not equal from day to day (except at the solstices). We commonly refer to this system of timekeeping as the unequal hour system.

The traditional astrolabe had a scale of equal hours around the outside edge in a 12 + 12 hour (clock) arrangement. This equal hour time system was used by medieval astronomers and we use the same system to govern our lives today. Once the rete had been correctly positioned, arcs on the plates of most astrolabes also allowed the time to be determined according to the unequal hours system.

The upper part of the front of the astrolabe is used to determine the positions of the Sun and stars in the sky. The lower part of this area is marked by the horizon arc. The unequal hours arcs are drawn in the area beneath the horizon arc and are drawn from the circle of the Tropic of Cancer, through the Equator, to the circle of the Tropic of Capricorn, which is the circumference of the plate.

The horizon marks the separating point between night and day on the left and day and night on the right. The vertical line down the centre of the lower half of the astrolabe indicates the time of 6 o'clock by the unequal hours system. Five arcs between the horizon arc on the right and this 6 o'clock line indicate the unequal hour divisions before 6 o'clock, and five more arcs between the 6 o'clock line and the horizon on the left indicate the divisions between 6 o'clock and 12 o'clock.

To determine the time, the position of the Sun on the ecliptic circle must first be ascertained, preferably using the Zodiac and calendar scales on the back of the astrolabe. The rete should then be correctly positioned according to the Sun's position in the sky during the day or by the positions of the stars during the night. If the Sun is below the horizon, its position among the unequal hour arcs determines the time of night. If the Sun is above the horizon, the pointer must be used. When placed over the position of the Sun on the ecliptic circle, the pointer also rests on the opposite side of the ecliptic circle. The position of this point amidst the unequal hours arcs indicates the time of day.

It was common to have an unequal hours diagram in the upper two quadrants on the back of the astrolabe. This diagram showed a circle plus arcs which were tangential to the centre of the astrolabe and engraved to the left and right of it. Occasionally, these arcs were crossed by other arcs whose centres were at the centre of the astrolabe.

This diagram allowed the time to be found according to the unequal hours system, using the alidade.

First, it was necessary to find the elevation of the midday Sun on that day. A diagram showing sigma curves was ideal for this purpose, but it was also possible to find this elevation from the front of the astrolabe. Knowing the position of the Sun on the ecliptic, this point on the rete was rotated until it was in the midday position. The Sun's elevation was then found using the almucantars on the plate.

The set of arcs concentric with the centre were then examined, and one was chosen which was marked with the midday elevation of the Sun. The astrolabe was hung from its suspension system, and the alidade was pointed at the Sun. The point on the selected arc where the alidade crossed it was then examined to find its position amidst the arcs around the circle. These arcs indicated the time in unequal hours, with the start and end of the day being on the horizontal line and the arcs and the circle being counted as hours from the left to the right, the circle representing the sixth unequal hour.

However, on most medieval astrolabes, the arcs which indicated the midday altitude of the Sun were not included. In this case, the alidade should be marked with an appropriate scale.

Sometimes, two quadrants were used to display a conversion diagram between equal and unequal hours. (Very occasionally, only one quadrant was used.) Assuming the diagram extended over two quadrants, it consisted of a set of concentric semicircles crossed by curves.

To use this conversion diagram, it was first necessary to find the time of sunrise (or sunset) in equal hours, which was most conveniently found using the front of the astrolabe. It was also necessary to have found the time by the unequal or the equal hours system.

The semicircles are numbered along their ends according to the time in equal hours of sunrise in the quadrant on the left and sunset in the quadrant on the right, and the appropriate semicircle is selected. The curves which this semicircle crosses are marked with the time according to the equal hours system. The time by the unequal hours system is marked around the outer part of the diagram. Thus, if the Sun rises at 6 o'clock in the morning in equal hours, and the time indicated by the astrolabe is 10 o'clock in the morning (equal hours), the pointer is rotated until it is over the crossing point of the 6 o'clock sunrise semicircle and the 10 o'clock curve. The pointer will then be pointing to the 4 o'clock unequal hour.

To convert between times after sunset, the titles of sunrise and sunset are mentally transposed. That is, the numbers along the bottom of the quadrant on the left are considered to be sunset times, and the numbers on the right as sunrise times. Thus, if the Sun sets at 6 o'clock in the evening in equal hours, and the time indicated by the astrolabe is 10 o'clock in the evening (equal hours), the pointer is rotated until it is over the crossing point of the semicircle marked 6 o'clock on the left and the 10 o'clock curve. The pointer will then be pointing to the 4 o'clock unequal hour, this being the beginning of the fourth hour of the night.

Some medieval astrolabes omitted the sunrise/sunset semicircles from the conversion diagram. In this case, the pointer (alidade) was calibrated appropriately.

*1 North, J.D. (1988). 'Chaucer's Universe', Clarendon Press, Oxford. p77.