Nychthemeron

It has not yet been found by Observation, whether Mercury turns upon its own Axis, ... that is, whether it has the vicissitude of Day and Night, much less what is the space of its Nychthemeron or Natural Day; but one may safely enough conjecture that it does revolve about its Axis, since the other Planets do.

Ideler passes over the Indians, and with good reason ; for they had not the week, and could not have had it, since they divided the nychthemeron into thirty hours, which the following out the same method would give rise to an entirely different series of planetary regents.

Now, the Arabs assumed as the beginning of their Nychthemeron the point where the setting sun intersects the circle of the horizon. Therefore their Nychthemeron extends from the moment when the sun disappears from the horizon till his disappearance on the following day.

For the division of the nychthemeron into equal parts, we are indebted to the Sumerians. Following the fiction of the ideal year of 12 months of 30 days to each, they divided the nychthemeron into 12 danna (46), each of 30 giš (47).

Geminus says in his Introduction to the Phenomena (VI, 1-4) that the nychthemeron (a day and night together) is not of constant length.

The Hindus consider the day as the first, the night as the second, part of the nychthemeron. There-fore they call the former Sâvana, i.e. a day depending on the rising of the sun. Besides, they call it Manu-shyâhorâtra, i.e. a human day, because, in fact, the great mass of their people do not know of any other kind of day but this.

The first sphere moves in each nychthemeron 0° 3′ in opposite sense to the zodiac. The second sphere moves 13° 11′ in each nychthemeron in the sequence of the signs of the zodiac.

Another difference at this age is the distribution of sleep and wake across the nychthemeron.