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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?

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The earth moves in an ellipse around the sun. Kepler's 2nd law says that the earth sweeps out equal areas in equal times. If the earth is close to the sun, it'll have to travel further each second to complete the same area.

In December/January the earth is closest to the sun and moving faster than in June/July, so it takes less time to sweep out the 15 degree angle.

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.

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  • $\begingroup$ thank you, now i get it.. the distance is also longer $\endgroup$
    – athos
    Jun 21 '21 at 14:11
  • $\begingroup$ Yes, the distance the earth has to travel to do 15 degrees, when far from the sun is longer, the speed is lower, so the combination is 1.034 squared, glad it helped. $\endgroup$ Jun 21 '21 at 14:55

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