How was the length of the year measured?

Fireworks light up the sky, glasses meet and warm hugs celebrate the arrival of a new year. Since time immemorial, the turn of the year has been a milestone in many cultures, a moment of reflection, renewal and hope for the future. It's as if the beginning of a new revolution of the Earth around the Sun has the power to revolutionize our lives too, making the next 365 days a better time.

Scientifically, there is nothing that guarantees this. In fact, the date of the New Year itself has no scientific basis. It's just a convention, adopted in Rome more than two millennia ago and which has established itself as a global standard. However, the true length of the year was established by Science. Its period has been precisely measured over centuries, through Astronomy!

Thousands of years ago, long before atomic clocks and digital calendars, our ancestors already perceived astronomical cycles as a way of marking time. The first astronomers, with their rudimentary observatories and their incredible capacity for observation, realized that the Sun, in its apparent movement across the sky, could indicate not only the days and nights, but also the seasons. We can see this in Stonehenge, in England, and in other megalithic structures around the world. Gigantic stones positioned in perfect alignment to mark the exact directions of sunrise and sunset during the solstices and equinoxes. These were probably the oldest ways of measuring the length of the year and, although imprecise, were essential for agriculture, religious festivities and the organization of life in the first human societies.

Over time, these observations gave rise to more organized attempts to keep time. It was in this way that the Romans created the calendar that is a precursor to the one we use today in much of the world. The Roman calendar was originally based on the solar cycle, composed of 10 months of 30 or 31 days starting on the spring equinox in the Northern Hemisphere, which currently begins on March 21st. But if we add up the days of these 10 months, we arrive at 304 days. The 61 days left to complete the year coincided with the winter period and were completely ignored by the calendar, as it was a very cold and unproductive period.

If this seems confusing to you, relax because it gets worse. Around 713 BC, King Numa Pompilius reduced the months from 30 days to 29 days and added the months of Januarius, with 29 days, and Februarius, with 28, at the end of the calendar, transforming it into a luni-solar calendar, with the beginnings of the months coinciding with New Moons. The problem is that at the end of these 12 months, only 355 days would be counted. And to resolve this difference, caused by the asynchrony between the solar and lunar periods, every two years an extra month of 22 or 23 days was added to the calendar. In addition to the confusion involved in managing this calendar, the Roman year in this period had, on average, 366 days and 6 hours, which created a lag in relation to the solar year.

Those who didn't have this problem were the Egyptians, who developed a refined technique to measure the length of the year precisely. For them, marking the year made it possible to anticipate the period of flooding of the River Nile, a vital event for agriculture and the survival of what was one of the oldest civilizations. His technique was based on the cycle of the star Sirius, the brightest in the night sky. The year began on the day of the first appearance of Sirius immediately before sunrise. The evolution of astronomy in Egypt and this systematic observation made over the years, allowed the Egyptians to calculate, around two centuries before the Romans, the duration of the year in 365 days and 6 hours.

In fact, the Romans didn't even calculate this. Egyptian measurement was so precise that, around the year 46 BC, when the emperor of Rome, Julius Caesar, decided to reform his calendar, he consulted the astronomer Sosigenes of Alexandria and adopted the same model used in Egypt, with 12 months of 30 or 31 days, with the exception of February, which would have 29 and every 4 years there would be 1 extra day. The Julian calendar assumed a year of 365 days and 6 hours, as measured by the Egyptians. It was a huge advance, but there was still a small inaccuracy that made a big difference a few centuries later.

It turns out that the seasons and many religious dates are related to the equinoxes and solstices. Easter, for example, is celebrated on the first Sunday after the Full Moon that occurs after the March equinox. And as astronomers observed the occurrence of the equinoxes over the years, they realized that it was occurring earlier each year, and this was messing up religious celebrations and causing the Julian calendar to decouple from the solar year.

In 1582, Pope Gregory XIII, concerned mainly with the mismatch of religious dates, promoted a reform in the Julian calendar. The new calendar was defined by a commission of scientists and eliminated the extra day in the month of February for years ending in 00, unless they are multiples of 400. In the end, this represents a year with an average length of 365 days, 5 hours, 49 minutes and 12 seconds. The Gregorian calendar, still used today in almost all countries in the world, corrected the error accumulated over the centuries, but this was not easy. The month of October in 1582 had 10 days removed to correct this difference. That year, people went to sleep on October 4th and woke up on the 15th. The days of October 5th to 14th, 1582 simply never existed.

With the advancement of astronomy and the development of instruments such as the telescope, scientists were able to measure the length of the year with even greater precision. Today we know that it is just 26 seconds shorter than the period measured more than 400 years ago for adopting the Gregorian calendar. And, contrary to what many people imagine, a year is not the period in which the Earth takes to go around the Sun.

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The year we adopt in our calendar is the so-called “tropical year”, based on the cycle of seasons, and which is measured based on the passage of the equinoxes, in the same way as our ancestors did millennia ago. The tropical year is about 20 minutes and 24 seconds shorter than the “sidereal year,” based on the Earth's complete orbit around the Sun. This small difference exists due to a slow wobbling motion of the Earth's axis of rotation that makes with the equinoxes and solstices occurring at slightly earlier times in each orbit.

Humanity's journey in measuring time is a fascinating story that reflects our desire to understand the cosmos and our place in it. From our ancestors' observation of the Sun and stars to the precision of atomic clocks, the quest to measure time has led us not only to define what a year is, but also to unlock some of the secrets of the universe. So, as we celebrate the arrival of another year, we can thank astronomy for giving us the tools to mark time, appreciate the cycles of life, and marvel at the cosmic dance that connects us to the stars.—