As today marks the Lunar New Year, for thousands of years, it was the accurate and reliable calendar of Earth.
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The lunar calendar is based on the moon's movement around the Earth, and the visible monthly cycles its appearance goes through.
A lunar day, the time it takes the moon to rotate on its axis, which on Earth is 24 hours, is the same as a lunar year, the time it takes to go around the Earth, much like it takes 365.2564 days for the Earth to go around the sun.
This is all because the moon is tidally locked to the Earth. Over millions of years, the gravity between the Earth and moon has slowed the rotation of the moon to reach the point where its rotation (day) and orbit (year) are happening at the same rate.
Relative to the stars and rest of the sky, the moon does one orbit around the Earth every 27.29 days. However, the lunar months are based on the phases - the time between each new moon or each full moon. It takes the moon about 29.53 days to go through the phases - two days longer than how long it takes the moon to go around Earth.
Since the Earth is still going around the sun while the moon is going around the Earth, the moon has to catch up a bit from the time it takes to go around the Earth relatively to stars, giving the extra two days.
Twelve of these cycles, or 12 lunar months, gives us only 354.56 days - far off 365 days in our current calendar. However, lots of cultures measured that the solar year, Earth's orbit around the sun, was closer to 365, and specifically, 365.25 days.
Julius Caesar recruited Greek mathematicians and others to reform the calendar system, eventually called the Julian calendar, from one based on the moon phases to one based on the sun.
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They devised that the calendar year would change between 355 days, 377 days, 355 days, 378 days, and then repeat. While this makes the average year 366.25 days, they had a solution. Over eight years every 24 years, there would only be three periods of 377 days which then averages out to 365.25 days. How do you determine the eight years? You let the politicians decide. Of course, the results were less than stellar and the calendar system ended up a bit loony.
Over time, the Julian calendar ended up with too many days, and the calendar ended up being off from seasonal equinoxes and solstices, and was impacting Christian holidays and ceremonies. In 1582, the Gregorian calendar, after Pope Gregory, was brought in. By adding an extra day in February every four years, this makes the average year 365.25 days long.
This became the basis of our modern-day calendar system. When it was eventually brought in in England nearly 200 years later, there was a difference of about 11 days between the Gregorian calendar and the Julian calendar.
When it swapped over, 11 days were lost. As we celebrate the Lunar New Year today, it has a beautiful history based on the physics of our moon, and hasn't had any politicians stealing days away from you.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
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