Many people believe that the hot summer is due to the Earth “getting closer” to the Sun, while the cold winter is because the Earth is “moving away” from the Sun. However, this is an unscientific explanation.
Here are the analyses by Mr. Đặng Vũ Tuấn Sơn, President of the Vietnam Astronomical and Cosmological Society, regarding the scientific basis for the division of seasons on Earth.
Seasonal Changes Due to Earth’s Tilt
The reality is that there are seasons in the year due to the influence of Earth’s axial tilt of 23.5 degrees in relation to its orbital plane around the Sun, and not because the Earth is closer or farther from the Sun. This means that summer in the Northern Hemisphere occurs when the North Pole of the Earth is tilted toward the Sun. At the same time, the South Pole is tilted away from the Sun, leading to the onset of winter in the Southern Hemisphere.
Seasons on Earth based on the Earth’s movement cycle with the solar system.
It’s important to note the distance between the perihelion and aphelion (the closest and farthest points from the Earth to the Sun) during the Earth’s orbit. The Earth reaches perihelion around January 2-5, when the distance from the Earth to the Sun is about 147.1 million kilometers, while it reaches aphelion around July 4-7, with a distance of 152.1 million kilometers.
This shows that the difference between these two distances is 5 million kilometers, which translates to a percentage difference of only about 3%. This 3% is a very small number and cannot create the seasons on Earth; the difference in distance only results in a variation in the amount of sunlight each hemisphere receives during the same season. Specifically, summer in the Southern Hemisphere receives more sunlight than summer in the Northern Hemisphere.
Summer and winter are centered around the days we refer to as the solstices, marked by the Sun’s highest and lowest points in the sky. In the Northern Hemisphere, the Sun is typically at its highest position on June 21, reaching over 40 degrees higher than its lowest position on December 21. For this reason, summer begins around June 21 in the Northern Hemisphere, while winter also starts in the Southern Hemisphere on the same day.
Weather-Based Calendar
Solar terms and intercalary periods, commonly referred to collectively as solar terms or simply terms, are different phases throughout the year, each lasting about 15 to 16 days, characterized by distinct weather patterns.
These terms are derived from ancient observations of the weather over many years and are named to reflect the corresponding weather conditions. For example, the first term of the year is called “Lập Xuân,” which marks the beginning of spring, while the last term of the year is called “Đại Hàn,” indicating that this period often experiences severe cold. From an astronomical perspective, these terms correspond to different positions of the Earth in its orbit around the Sun.
There are a total of 24 alternating solar terms and intercalary periods that make up a year, corresponding to one weather cycle. It is noteworthy that these terms do not have fixed dates in the Lunar calendar but are nearly fixed in the Solar calendar. This is because the Solar calendar is based on the Earth’s orbit around the Sun, which does not include leap months like the Lunar calendar, thereby preventing any date discrepancies concerning the weather. For instance, the term “Lập Xuân” always falls on February 4 or 5 in the Solar calendar but can be any day from the end of the Lunar December to mid-January in the Lunar calendar, depending on the year.
Today, with our knowledge of astronomy, we understand that a year is a cycle of the Earth’s orbit around the Sun, which corresponds to the Solar calendar year. This year aligns with the cycle of weather changes.
Historically, Eastern cultures did not rely on the position of the Sun but rather on the Moon’s position, so when estimating weather cycles, they found this cycle to correspond to about 12 lunar months, thereby defining a year as the length of 12 lunar months.
However, it was later realized that this 12 lunar month cycle is shorter than the actual weather cycle by about 10 days. Thus, if a year remained at 12 months, every three calendar years would lag behind the weather cycle by about one month, and the deviation would increase over the years. Consequently, ancient Eastern cultures introduced intercalary months.
An intercalary month is added approximately every three years. The calculation rule takes the winter solstice each year as a reference point. In any year where there are 13 solar terms between two winter solstices, that year will have an additional month. This year with an intercalary month is referred to as a lunar leap year, and the intercalary month is chosen as the first month of the year that does not contain any solar terms, named after the preceding month.
With improvements made to ensure the Lunar calendar “keeps pace” with the Solar calendar and aligns with weather cycles, the type of Lunar calendar we use today is sometimes referred to as “Lunar-Solar Calendar”. However, fundamentally, it still retains the original basis of the initial Lunar calendar, hence, for brevity, we can still refer to it as “Lunar calendar” in common parlance.
