Solar terms

Twelve boundaries, thirty degrees apart

A solar term is a position of the sun rather than a date. The calculation this site uses marks twelve of those positions, thirty degrees of ecliptic longitude apart, and each one opens the period that the month column is read from.

The twelve, each with the longitude that opens it: 立春 315, 驚蟄 345, 清明 15, 立夏 45, 芒種 75, 小暑 105, 立秋 135, 白露 165, 寒露 195, 立冬 225, 大雪 255, 小寒 285. Positions that fall between these twelve are not used as month boundaries here.

The branch of the month column follows the same order, starting from the period that 立春 opens. So the month column is not a reading of the calendar month at all; it is a reading of which of the twelve periods a birth moment landed in.

Why the dates are not the same every year

Each boundary is one instant, not a day. In the calculation this site uses, the instant that opened 立春 in 2026 fell on February 3 at 20:01 UTC, and the next one fell on February 4, 2027 at 01:46 UTC: about five and three quarter hours later on the clock, and a day later on the calendar.

The periods are not the same length either. Across the twelve that followed 立春 in 2026, the shortest ran just under 29.5 days and the longest just over 31.4 days. They are set an equal number of degrees apart rather than an equal number of days apart, and the two do not come out the same.

That is why a birth moment near the end of one period cannot be placed by a date that is fixed from year to year. It has to be read against the instant, which is the reason the screen prints the instant rather than the date.

The two rows the result screen prints

Under a chart the calculator prints two rows for this: the solar term period the birth moment falls in, and the start of that period in UTC. The first prints as the characters listed above, so the list is what converts that row; the second prints as a full timestamp down to the second.

The end of the period is not printed. The next boundary is used inside the calculation, but it does not appear as a row, so where the current period closes is read from the next of the twelve rather than from the screen.

Which clock those boundaries are read on is a row of its own, and it is one of three settings the screen lets a reader change. Changing one does not redraw a chart already on screen; the choice is applied the next time the "Calculate the chart" button is pressed, and the screen then records that the choice came from the reader rather than from the site default.

The two minutes on either side of a boundary

Within two minutes of one of these boundaries the year and month columns are not produced at all. Independent calculations of the same instant differ by around that much, so an input inside that band is left uncalculated rather than pushed to one side of it.

The day and hour columns are still produced there, and the two rows above still print. What the screen shows in that case is a chart with two columns named, two left empty, and the boundary that emptied them printed next to it.