24 Solar Terms Calendar

Overview

Solar terms are not even calendar divisions but markers where the sun reaches each fifteen degrees of apparent longitude, which is why neighbours sit anywhere from 14.7 to 15.7 days apart. This tool truncates the VSOP87 series for the earth longitude and adds nutation, aberration and a ΔT correction to give every term from 1900 to 2100 in UTC+8 with a minute-level time, plus the 72 hou phenology, solar longitude, declination and day length for a chosen latitude. The ΔT correction matters most for years far from the present, where the earth rotation estimate has a wider spread; near 2026 it stays a small term, which is why the one-minute agreement holds. The sequence never inverts across a new year boundary. The reason the interval varies is orbital rather than calendrical: the earth moves faster near perihelion in early January, so terms cluster a little closer there and spread out near aphelion in July. That is also why a fixed table copied from an old almanac drifts, and why the calculation here is rebuilt for every year instead of being interpolated between a few anchors. The phenology lines are kept alongside because the classical calendar pairs each term with three five-day periods, and readers often want the field signs rather than the instant. A one-minute difference matters mainly when a printed almanac is compared with an authoritative table.

Features

• All 24 terms in one annual table: name, date, time and days from today
• Minute-level times: against the 2026 authority table 22 terms match exactly, the worst gap is one minute
• 72 hou phenology, three per term, such as Lichun with thawing wind, waking insects and rising fish
• Date lookup returns the term in force, time to the next term, solar longitude and declination
• Day length in hours and minutes for a given latitude, useful for farming and photography
• TXT export plus a full-year ICS calendar that imports into phone calendars

How to use

1. Type a year and the 24-row table appears at once
2. Press This year to jump back to the current one for comparison
3. For a specific day, pick a date and read the term in force with days to the next
4. Enter a latitude to add day length to the lookup result
5. Use the ICS export to put the whole year into a calendar app

Notes

• Terms divide the ecliptic every 15 degrees, two per month, and Xiaohan starts at 285, yet the spacing is not a fixed 15 days but moves between 14.719 and 15.736.
• Values come from a truncated VSOP87 series with nutation, aberration and a ΔT correction, given in UTC+8; against the 2026 authority the worst gap is one minute.
• Each of the 72 hou lasts five days with three per term, following the classical phenology book, so these are field signs rather than a weather forecast.
• Day length is geometric from latitude and declination without refraction or altitude, so sunrise and sunset shift by minutes; the range is 1900 to 2100.
• ICS entries are all-day reminders without a timed alarm; lunar conversion and sexagenary work are out of scope and out-of-range years are refused rather than interpolated.