# Why the solar term gaps differ up to 1 day?

Solar term is any of twenty-four periods in traditional Chinese lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic.

The dates vary, according to this calendar

• Year 2020 Dong Zhi is December 21st, 17:55.
• Year 2021 Xiao Han is January 5th, 11:18.

-- so in between there is 14day, 17hours, and 24 minutes.

• Year 2021 Xia Zhi is June 21st, 11:18.
• Year 2021 Xiao Shu is July 7th, 04:49.

-- in between 15days, 17 hours, 31 minutes.

So the gap is almost 1 day.

However, at summer time the earth is at the aphelion, at winter at Perihelion, the difference is about 3.287%

At perihelion, the Earth's center is about 0.98329 astronomical units (AU) or 147,098,070 km (91,402,500 mi) from the Sun's center. The aphelion distance between the Earth's and Sun's centers is currently about 1.01671 AU or 152,097,700 km (94,509,100 mi).

Then the timing shall be about

$$365.25 \cdot \frac{15}{360} \cdot (1-\frac{147098070}{152097700}) = 0.500$$

days. Almost only half of the 1 day timing gap above.

Pls enlighten me, where did I get it wrong?

The ratio of the speeds at perihelion and aphelion is about $$1.034$$ and the distance the earth needs to move to cover 15 degrees is also reduced by the same factor.
The ratio of overall time to cover a given small angle, at aphelion compared to perihelion, is about $$1.034^2 = 1.069$$ and will approximately double the 0.5 days in your calculation.