Ancient astronomers used the passage of the Sun across the sky to determine time. The Sun is a large object, which makes it difficult to observe small divisions of time, such as minutes. The stars at night, because of their small size, provide more precision but appear to rotate in the heavens 366 times a year instead of the Sun’s 365.
Later ancient astronomers found that the planet Venus provided a means of correction. The orbit of Venus is closer to the Sun than that of Earth, and therefore rotates faster around the Sun. When Venus is opposite the Sun from Earth, it appears to travel across the night sky slightly faster than the stars.
Using Venus instead of the stars as a clock to time the rotation of the Earth, their year thus became 365.33 days long.
This more accurate time allowed the calculation of a more perfect Nautical Mile of 6000 Attic Feet or 6250 Roman Feet, using a simple length correction to the Sumerian One-Second Pendulum.
Figure 1 The geometry of the orbits of Earth and Venus and the equations of motion
Table 1 Dividing the day using a line of sight to Venus using the Earths rotation angle of one degree (Fraction-360)
and using an angle of 360/366 degrees (Fraction-366)