The first quantity that people could measure

The first quantity that people could measure with any degree of accuracy, and on which all people could agree, was time, although only fairly large amounts of time. Large amounts of time can be easily measured because the universe itself supplies “clockwork” in the daily and annual motion of Earth and the moon. Even so, the measurement of time was not easy to work out. A day is one revolution of Earth; a Moon is from one new Moon to the next; but it is not so easy to measure a year. Even the day is not as easy to measure as it seems. It took a while to learn to measure the day from one noon to the next (noon is when the Sun reaches its highest point in the sky).

The ancient Egyptians were the first to establish a good length for the year, possibly because the Nile floods around the same time each year. This flooding generally coincides with the heliacal
rising of Sirius: that is, when Sirius rises at about the same time as the Sun. Although there are 365 days between such risings, the year is actually 365 days, 5 hours, 48 minutes, and 46 seconds, or about a quarter of a day longer than 365 days. Since the year is not exactly 365 days, the Egyptian calendar gradually went into and out of alignment with the seasons with a period of about 1460 years.

When did the Egyptians began using a 365- day calendar

No one knows for sure when the Egyptians began using a 365- day calendar. The Egyptian calendar accurately matched the seasons with dates in 139 CE, since according to Roman historian Censorinus, the heliacal rising occurred on the Egyptian New Year in that year. Knowing this one date, astronomers have speculated that the year of 365 days was instituted in Egypt around 1322 BCE, 2782 BCE, or even 4242 BCE, going backward in leaps of 1460 years.

Hellenic astronomers, Julius Caesar and Pope Gregory

Hellenic astronomers added the missing quarter day to the Egyptian calendar by adding an extra (leap) day every four years, but most people ignored it. The calendar with a leap day was finally adopted by the Romans under Julius Caesar in 46 BCE with the first leap year added in 45 BCE. But errors in applying the rule in the following years (priests thought Caesar meant a leap year every third year!) resulted in restarting the system in 8 CE, with leap years every four years thereafter. Since then, the calendar has had one major modification, when Pope Gregory, in 1582 CE, on the advice of astronomers, dropped the leap day in years that end in two zeros unless the year is also divisible by 400 (for example, 2000 was a leap year but 2100 will not be).

Astronomy and making astronomical observations

A large –– 22 m (72 ft) high, weighing more than 250 tons  standing stone, or menhir, known as the Grand Menhir of Locmariaquer, is erected in Brittany, probably for use in making astronomical observations. Stone structures in Europe and in Egypt are aligned so that the Sun or a particular
star shines through a chosen opening at special times of the year, such as at the summer solstice. Thutmose III erects in Heliopolis the so-called Cleopatra’s Needle. Its shadow is used to calculate time, season, and solstices.