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Early Egyptian astronomers discovered that the start of the annual Nile floods coincided closely with the first visibility of the star Sothis, now known as Sirius, in the morning sky (known as the heliacal rising).

It is likely that the Egyptian calendar year started on the date of this rising.

Eight points on these lines deserve further attention; Klaudios Ptolemaios called them Phaseis.

Thus the astronomical refraction is defined as the arc between apparent and true altitude of the star.Sirius rises earlier in the evening every day and its apparent acronychal rising, which is his last observable rising at dusk, occurs on December 29 The arc of vision or arcus visionis of a star is defined as the difference in altitude between the star and the Sun at the moment when the star is observed at the horizon, calculated without the effect of refraction.The light of the star has to pass through the Earth's atmosphere before reaching the observer, and during its passage a ray of light suffers a change in direction owing to refraction.Egyptians realised early on that the first appearance of the star Sirius in the morning sky - its so called heliacal rising - coincides with the beginning of the inundation of the Nile.The Egyptian solar or civil year consisted of 12 months with 30 days and 5 additional, so called epagomenal days, thus it totalled 365 days. This means that the astronomical and the civil year drifted apart by approximately one day in four years.This means, in ancient times Sirius should have been visible already at a smaller arcus visionis values than today.

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