Predicting the passage of time preoccupied our ancestors from the earliest recorded history. The journey of the Sun, Moon and stars across the great expanse of the sky provides clues for numerous methods of marking time. The most obvious method for human beings is the passage of a day (light/dark) and that of a month (based on phases of the Moon).
In ancient times measuring the exact length of a year was very difficult. Our ancient ancestors used natural parameters such as when a certain tree would bloom and this was sufficient proof to know the beginning of a new year.
In ancient times Egyptians knew that to calculate an accurate measurement of a year, important thing is to take note of where the stars are in the sky at any given time. The priests of Egypt used stars like Sirius, the Dog Star to predict the flooding of the Nile annually, and they were able to foretell this event. Guessing time by studying Sirius also enabled the Egyptians to become the first civilization to switch from a lunar to a solar calendar.
Lunar Calendar
Lunar calendar was used by the ancient Babylonians. The Muslim and Jewish calendars, even today, are lunar-based. But using a lunar calendar poses a major problem. Basically a lunar month is 29.5 days which means 12 lunar months add up to 354 lunar days, that is about 11 days short of a solar year. That is why some lunar calendars add an extra month every now and then to make up for lost time.
The Egyptian priests’ study of Sirius allowed them to count the exact number of days in a solar year. The lunar months were arranged into 12 month intervals making each of them 30 days in length with five added days at the end of the year. But there was a problem that every four years Sirius shows up a day late; main reason for this is that the solar year is really closer to 365 days and six hours which the Egyptians never took into account though they were aware of the issue. As a result of this the calendar start taking a backward slide as a lunar one would do.
Induction of Leap Year in Solar Calender
With the help of Sosigenes who was a renowned astronomer from Alexandria; Julius Caesar started a new calendar on January 1, 45 B.C. This calendar came closer to the solar year than any of its predecessors and became known as the “Julian Calendar”. Sosigenes informed Caesar that the actual length of the solar year is 365 days and six hours as the Egyptian priests had known. The logical solution was to simply add a day to February, which was the shortest of the Roman months, every fourth year. With this clever idea the leap year was born.
Leap year comes after every four years and in this year the month of February has 29 days instead of 28.