Shunbun (Vernal Equinox) is one of the 24 seasons in the solar year. It falls around March 21. Or, it is the period from that day until Clear and Bright (Seimei) season in the same solar calendar.
In astronomy Shunbun is defined as the very moment that the sun transits through the vernal equinox. That is, just when the sun is visible at the celestial longitude of 0 degrees.
In historical almanacs the vernal equinox is explained as being 'Mid point between the sun and the heavens when the division between day and night is identical' and, holds true that at the vernal equinox, day and night are roughly the same length.
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.
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 the vernal equinox 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 vernal equinox.
During those days forming the vernal 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 vernal equinox sun will appear to be moving right along the horizon; not rising or sinking.
In Japan, Shunbun No Hi (Vernal Equinox Day) is a public holiday. The Japanese Cabinet decides upon Shunbun 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, Vernal Equinox Day occurs during the 7 days of spring Higan (equinoctial) week. Prior to WWII the day was a national holiday in recognition of the imperial ceremony of ancestor worship.
In Europe etc, the vernal equinox marks the opening of spring. The day is a public holiday in a number of countries.
In Christianity, the vernal equinox is used as a reference point when calculating the date of Easter. The Sunday following the first full-moon after the vernal equinox is set as Easter. This calculation method is called "Computus." That said, while the calendar 'vernal equinox' date is fixed as March 21, the dates of astronomical vernal equinox days do not always coincide. While the 'full moon' dates can also be obtained from simplified calculations, the astronomical full moon dates may not always coincide. This method of calculation was decided upon at the First Council of Nicaea.
The 72 'climates' of the year (based on the 24 solar seasons further divided into three).
The 72 'climates' occurring during the Vernal Equinox are as follows:
Sparrow's First Nest: Sparrows start to stand in the nest (Japan). Swallows Arrive: Swallows arrive from the South (China).
Sakura First Blossom: Sakura begins to bloom. (Japan)
Thunder Begins to Sound: In the distance the sound of thunder begins (China).
Thunder Begins to Sound: In the distance the sound of thunder begins (Japan). First Thunder: First flashes of lightning (China).
Approximate order of solar calendar seasons.
Keichitsu (Insects Awaken) => Shunbun (Vernal Equinox) => Seimei (Clear and Bright)