Shubun (Autumnal equinox) is one of the 24 seasons in the solar year.
It falls around September 23
Or, it is the period until the Cold Dew (Kanro) season in the same solar calendar. Mid August.
In astronomy the Shubun is defined as being the moment the sun transits the autumn equinoctial point, namely, the very moment that the sun is considered to be positioned at 180 degree celestial longitude.
Similar to Shunbun (the spring equinox), Shubun is where day and night are of similar length.
In historical almanacs it is explained as being 'Mid point of cosmic forces.'
However, in actual fact, daytime is longer than the night. In the vicinity of Japan there are variations from year to year but, on average daytime is approximately 14 minutes longer than night time.
This is because of the following reasons:
Due to refraction in the atmosphere, the sun can look higher than it actually is.
The portion of the sun that is visible due to the higher aspect results in an earlier dawn and later sunset. Refraction makes the sun look larger the closer it is to the horizon. At the National Astronomical Observatory of Japan, when the sun is close to the horizon, the angle is estimated to be 35 minutes 8 seconds. From this it can be calculated that the time difference between dawn and sunset is 2 minutes 20 seconds.
Because dawn or sunset is defined as the very moment when the top edge of the sun is aligned with the horizon
Due to this, the radius of the sun can cause dawn to be early or sunset late. The variance in timing between dawn and sunset can be extrapolated from this to be 1 minute 5 seconds.
Putting both these together, at dawn the centre of the sun rises from the horizon 3 minutes 25 seconds faster and, at sunset sinks 3 minutes 25 seconds slower from the centre of the sun to the horizon. Accordingly, the length of daytime at Shubun is approximately 12 hours 7 minutes and, the night approximately 11 hours 53 minutes long. Also, actually the day when the variance between day and night is substantially smaller occurs approximately 4 days after the autumnal equinox.
During those days forming the autumn equinox, the sun rises from due East and sets due West. If a person makes observations from above the equator, the sun passes through the zenith at mid day. If a person makes observations at the North or South Pole, the Autumnal Equinox sun will appear to be moving right along the horizon; not rising or sinking.
Shubun No Hi (Autumnal Equinox Day) is a public holiday in Japan. The Japanese Cabinet decides upon Shubun No Hi based on the astronomical Autumnal Equinox calculated by the National Astronomical Observatory of Japan and, an announcement of the decision is officially gazetted in February the previous year. It is unusual elsewhere in the world to have a public holiday decided upon each year based on astronomy. Also, there is the equinoctial day of Higan (equinoctial week).
The 72 'climates' of the year (based on the 24 solar seasons further divided into three).
The 72 'climates' occurring during the Shubun are as follows:
Thunder sounds: Thunder sounds and reverberates (Japan/China)
Hibernating insects: Insects enter and seal the holes they have burrowed (Japan/China)
Water starts to dry up: Water in paddies starts to dry (Japan/China)
Approximate order of solar calendar seasons
Shirazume (Glistening Dew) => Shubun => Kanro (Late Autumn)