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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Solstice | 4/4 | https://en.wikipedia.org/wiki/Solstice | reference | science, encyclopedia | 2026-05-05T11:13:30.644057+00:00 | kb-cron |
=== Solstice determination === Unlike the equinox, the solstice time is not easy to determine. The changes in solar declination become smaller as the Sun gets closer to its maximum/minimum declination. The days before and after the solstice, the declination speed is less than 30 arcseconds per day which is less than 1⁄60 of the angular size of the Sun, or the equivalent to just 2 seconds of right ascension. This difference is hardly detectable with indirect viewing based devices like sextant equipped with a vernier, and impossible with more traditional tools like a gnomon or an astrolabe. It is also hard to detect the changes in sunrise/sunset azimuth due to the atmospheric refraction changes. Those accuracy issues render it impossible to determine the solstice day based on observations made within the 3 (or even 5) days surrounding the solstice without the use of more complex tools. Accounts do not survive but Greek astronomers must have used an approximation method based on interpolation, which is still used by some amateurs. This method consists of recording the declination angle at noon during some days before and after the solstice, trying to find two separate days with the same declination. When those two days are found, the halfway time between both noons is estimated solstice time. An interval of 45 days has been postulated as the best one to achieve up to a quarter-day precision, in the solstice determination. In 2012, the journal DIO found that accuracy of one or two hours with balanced errors can be attained by observing the Sun's equal altitudes about S = twenty degrees (or d = about 20 days) before and after the summer solstice because the average of the two times will be early by q arc minutes where q is (πe cosA)/3 times the square of S in degrees (e = earth orbit eccentricity, A = earth's perihelion or Sun's apogee), and the noise in the result will be about 41 hours divided by d if the eye's sharpness is taken as one arc minute. Guo Shoujing, a Chinese astronomer, found that the taller the gnomon, the more acurately the journey of the sun could be measured. He designed the 12.6 metres (41 ft) gnomon constructed at Gaocheng Astronomical Observatory in 1276. The measurements from Gaocheng determined the length of the year to within one minute of the current measurement, a value in accord with the value of the Gregorian Calendar, but obtained 300 years earlier.. Astronomical almanacs define the solstices as the moments when the Sun passes through the solstitial colure, i.e. the times when the apparent geocentric celestial longitude of the Sun is equal to 90° (June solstice) or 270° (December solstice). The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. Because the earth's orbit takes slightly longer than a calendar year of 365 days, the solstices occur slightly later each calendar year, until a leap day re-aligns the calendar with the orbit. Thus the solstices always occur between June 20 and 22 and between December 20 and 23 in a four-year-long cycle with the 21st and 22nd being the most common dates, as can be seen in the schedule at the start of the article. Currently, government organizations like USNO and IMCCE publish the date and time of the solstice.
== In the constellations == Using the current official IAU constellation boundaries—and taking into account the variable precession speed and the rotation of the ecliptic—the solstices shift through the constellations as follows (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):
The northern solstice passed from Leo into Cancer in year −1458, passed into Gemini in year −10, passed into Taurus in December 1989, and is expected to pass into Aries in year 4609. The southern solstice passed from Capricornus into Sagittarius in year −130, is expected to pass into Ophiuchus in year 2269, and is expected to pass into Scorpius in year 3597.
== See also ==
Analemma Geocentric view of the seasons Iranian calendars Perihelion and aphelion Wheel of the Year Zoroastrian calendar
== References ==
== External links ==
Equinoxes and Solstices Calculator (1600 to 2400) "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion (2000–2025)". United States Naval Observatory, Astronomical Applications Department. Retrieved December 9, 2015. Weisstein, Eric (1996–2007). "Summer Solstice". Eric Weisstein's World of Astronomy. Retrieved October 24, 2008. The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years.