The ancient Egyptians of around 4500 BC were familiar with the fact that when the star Peret Sepdet, which we call Sirius, rose just before the Sun in June, the River Nile was about to flood and cover the land with a thick coat of nutritious silt. But by 150 BC, no heliacal rising of Sirius was taking place to herald the flood. Why?
Scientific astronomy hinges on three things: accurate observations, making a permanent record of these observations, and subjecting the observations to precise mathematical analysis in order to extract celestial laws.
By the time of the Greek astronomer Hipparchus, there were lots of astronomical records around, especially in the Babylonian world of the Tigris and Euphrates valley, where they were inscribed as cuneiform characters into baked clay tablets. To a lesser extent, records were also available in Egypt where they were written on papyrus 'paper' scrolls.
We would like to know more about the life and travels of Hipparchus, who lived on the Greek island of Rhodes, where he died around 127 BC. it is clear that he was familiar with Babylonian and other astronomical data that were already ancient in his day, for one of the several things this assiduous observer and calculator came to realise was that the sky was slowly changing and the seasons were slipping back.
Knowing as he did that the suns annual orbit through the zodiac constellations or ecliptic, is included to the celestial equator at an angle of around 23.5°, he came to realise that the midsummer and midwinter solstices and the two equinoxes had not always fallen on the same dates as they did in his own time.
By comparing his own observations of the positions of the star Spica with earlier Greek records, he concluded that in a single century the equinox points and solstices would slip back by one degree, eventually moving through a full circle in 36,000 years.
And while we now know that Hipparchus's value for what today we call precession was wrong - in fact, it takes 25,800 years to complete the circle - he was the first astronomer to fully appreciate that the heavens undergo gradual, long-term changes. Equally, he realised that historical records can be goldmines of data for astronomers who have learnt how to interpret them correctly. And it was because of precession that Sirius no longer rose with the Sun when the Nile was due to flood.