The abacus or tide computer is used to figure the time of the tides. The tides, with their constant change in the water level, greatly affect navigation both in harbours and for fishing due to the energy generated by the immense quantity of water in motion and the effect this has on submarine currents.

In this instrument the outer circle is divided into 360º in 1º intervals. The middle into twice 12 hours; the inner into the 32 wind directions. There follows the solar volvelle with a pointer, the rim divided to 30 days, approximately the age of the Moon (actually 29 ½ days). Finally comes the lunar volvelle with aspectarium, and a pointer marked by a crescent. The face is completed with the aspects: opposition, trine, quartile, sextile.

According to Waters (1958), the earliest known English tide computer is that on Cole´s compendium of 1569, and he considers that this one can be dated to the period shortly after 1570 because the Moon´s age is given 30 days and not 29 ½ days. However, 30 is more practical for seamen when working out the date of the New Moon.

The tides are the periodic change of the water level in the oceans and they are caused by the attraction that the Moon and Sun exercise over them, especially the Moon because it is close to the Earth. The change of the water level is produced every 6 hours by the rotation of the Earth, so that in one day the water level will rise twice and fall twice. The highest level of the tide is called the High Tide and the lowest level is called the Low Tide.

Although astronomically, the high and low tides are related to the position of the Moon, in fact there is a difference between the time that the Moon passes over a given place on the Earth and the rise or fall of the water level; this difference in time is called the "establishment of the port" and is defined as the interval between the passage of the Moon and the high tide over the local meridian.

There are distinct variations or ranges in the water level between high and low tide; at some times of the year, the range is greater than at other times. This distinct variation in the levels of the tides occurs because at certain times, the attraction of the Moon and Sun work together, and at other times, they work against each other. This causes the range between the syzygy or spring tides and the quadrature or neap tides. The Moon moves slowly around the Earth, each revolution taking almost a month; thus during this time two spring and two neap tides occur.

The spring tides have the greatest range because the Moon and Sun exercise their attraction along the same line. The high and low tides vary greatly in their water levels. They coincide with the Full Moon and New Moon.

In the neap tides the Moon and Sun form a right angle with the Earth; the attraction of the Moon is counteracted by that of the Sun. The difference between high and low tide is at its least range. They occur in the First Quarter and Last Quarter of the Moon.

Method of Use

Place the indicator of the disk marked with the days of the lunar cycle at 12, which corresponds with North (fleur-de-lis), then turn the upper disk until the long indicator is located on the number that equals the days since the last New Moon (the age of the moon). If this is not known, the moon can be observed and the indicator can be moved until the approximate position of the moon is found in the orifice. The indicator will point to a number on the scale of hours. To know the time of the high tide we must know the establishment of the port of the place where we take the reading.

Courtesy of Leonard Honey, a knowledgable chap.