- The moon determines its dates – such as Chinese New Year – but it also has ‘solar terms’ – for example winter solstice – based on the sun’s movements
- Young Post has partnered with Hong Kong Science Museum and Hong Kong Space Museum to answer your questions about the fantastic world around us
Lunar New Year is one of Hong Kong’s biggest celebrations with a flurry of cultural festivities.
This year’s holiday begins on January 22 and marks the start of the year of the rabbit. But it began on February 1 last year and February 12 in 2021. Have you ever wondered why the festival falls on a different date every year?
The reason for the changing date is that Chinese New Year’s timing is based on the lunar year used for the Chinese calendar, which is sometimes called the Chinese Agricultural Calendar. This is different from the calendar most of us use today, called the Gregorian calendar, based on the solar year.
A lunar year contains 12 lunar months, which are based on the time it takes the moon to go around the Earth. Twelve lunar months are equal to about 354 days. But the solar year follows the time it takes for the Earth to go around the sun, so it has about 365 days.
This difference explains why Lunar New Year never falls on the same day on the Gregorian calendar.
What is the Chinese Agricultural Calendar?
While most people are familiar with lunar months, not many are aware that the Chinese Agricultural Calendar integrates the lunar and solar calendars because it tracks the movement of the moon and the sun.
The Chinese Agricultural Calendar uses lunar months to determine its dates, but it also has “solar terms” which are based on the solar year. These solar terms helped the ancient Chinese people plant and harvest their crops at the right time – this was crucial knowledge for their survival.
How did solar terms guide the ancient Chinese?
Solar terms are related to the Earth’s positions with respect to the sun, and hence they are also related to the seasons (see graphic above). This guided the ancient Chinese on when to perform various agricultural activities.
But how did people back then determine the dates for these 24 solar terms? Well, they did so by observing how the sun’s path in the sky would change during different seasons.
By observing changes in the sun’s shadow over extended periods of time, the ancient Chinese were able to determine the length of a solar year. They then divided the solar year into 24 parts (see graphic above) and came up with 24 solar terms which were essential for their schedule of planting crops.
The diagram below represents a location at a latitude of 40 degrees north. The three tilted arcs illustrate the different paths that the sun takes during four of the solar terms – winter solstice, summer solstice, spring equinox and autumn equinox.
Other solar terms describe the weather, for example, grain rain, white dew, frost’s descent, bright and clear, minor and major heat, and minor and major snow. There are also solar terms such as grain buds, grain in ear and awakening of insects that are related to crop growth and animal activities.
By referring to the chart below, the ancient Chinese predicted changes in seasons and figured out the best times to sow and gather their crops every year.
Now that you know more about the Chinese Agricultural Calendar, do you think there is a relationship between the solar terms and the Gregorian calendar? Can you identify which Chinese festivals always appear around the same Gregorian calendar dates, and why?
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Young Post is pleased to partner with Hong Kong Science Museum and Hong Kong Space Museum on our STEM Lab series. Our aim is to encourage you in your pursuit of science. Every month, the two museums will answer your burning questions about the fantastic world around us, the cosmos and beyond.