If a, b, c and d each have 24 solar terms.

(1) As can be seen from the schematic diagram of the earth's revolution, when the earth is in position A, the sun is in the tropic of cancer, which is the summer solstice in the northern hemisphere, and the date is around June 22nd. When the earth is in position B, the latitude of direct sunlight is the equator, which is the autumn equinox in the northern hemisphere, and the date is around September 23; When the earth is in the third position, the latitude of direct sunlight is the tropic of Capricorn, which is the winter solstice in the northern hemisphere, and the date is about 65438+February 22; When the earth is in position D, the latitude of direct sunlight is the equator, that is, the vernal equinox in the northern hemisphere, and the date is around March 2 1.

(2) As can be seen from the schematic diagram of the earth's revolution, the period from the vernal equinox to the solstice in the northern hemisphere is represented by the letter A in the diagram.

(3) When the earth is in orbit A, the northern hemisphere is in summer.

(4) Qingshen is located in the southwest of Chengdu Plain, Sichuan Province, China, belonging to the northern hemisphere; Every year around September 23, it is the autumn equinox in the northern hemisphere, and the earth revolves around B. At this time, the length of day and night in the world is: day and night are equal; The national day (65438+ 10/) lies between the autumnal equinox (around September 23rd) and the winter solstice (around February 22nd, 65438+around February 22nd), and the earth's running time is C. 。

So the answer is: (1) June 22nd; September 23; 65438+February 22nd; March 21; (2) A; (3) summer; (4)b； The length of day and night is equal; C.