The figure-8 shaped curve is called an analemma. It’s a photo of the sun, taken every day at the same time, in this case 7:30 a.m. local time in Hong Kong. Matthew Chin created this analemma in 2014. He also add the Chinese solar terms, which comprise a calendar of 24 periods and climate, set up to govern agriculture in ancient China and referred to in China even now. Matthew wrote:

Due to cloudy or rainy days, the image may be taken before / after the solar term day and are marked +/- nd(ays).

That year’s two equinoxes on March 21 (0° longitude) and September 23 (180° longitude) correspond to the mid-points but not the cross-over point, on the curve.

While summer solstice on June 21 (90° longitude) and winter solstice on December 22 (270° longitude) are located at the top-left and bottom-right on the curve respectively.

The solar term days are listed as below (based on Hong Kong Time) for year 2014 by Hong Kong Observatory.