The lower wheel should be rotated until the sun aligns to the current zodiacal sign. The upper wheel should be rotated until the moon aligns to the point on the ecliptic where it rises. The width of the resulting black band represents the level of the tide during the day.

On the left you see a day in which the sun, moon and Earth are in alignment: on such a day the solar tide has an additive effect on the lunar tide, creating extra-high tides, and very low tides.

On the right you see the situation one week later, when the sun and moon are at right angles to each other, and the solar tide partially cancels out the lunar tide, producing moderate tides known as neap tides.

When the two volvelles are in the correct position, this works as a clock, predicting the level of the water at every hour of the day.