Editing Solstice (section)

===Solstice determination===
Unlike the equinox, the solstice time is not easy to determine. The changes in [[Position of the Sun|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 {{frac|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 scale|vernier]], and impossible with more traditional tools like a [[gnomon]]<ref>{{cite web
|title=Solstice Determination based on Observations
|last=Mollerup
|first=Asger
|date=January 12, 2008
|url=http://sundial.thai-isan-lao.com/solstice-determination.html
|access-date=September 27, 2010
|archive-url=https://web.archive.org/web/20090211044959/http://sundial.thai-isan-lao.com/solstice-determination.html
|archive-date=February 11, 2009}}</ref> or an [[astrolabe]]. It is also hard to detect the changes in sunrise/sunset azimuth due to the [[atmospheric refraction]]<ref>{{cite journal
|last = Exton
|first = Harold
|title = A Fresh Analysis of Some Recent Data on Atmospheric Refraction Near the Horizon with Implications in Archaeoastronomy
|journal = Journal of History of Astronomy, Archaeoastronomy Supplement
|volume = 23
|page = S57
|date = 1992
|issue = 17
|bibcode = 1992JHAS...23...57E
|doi = 10.1177/002182869202301707|s2cid = 118484271
}}</ref> 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.<ref>{{cite journal
|last = Hugh
|first = Thurston
|title = Early Greek Solstices and Equinoxes
|journal = Journal for the History of Astronomy
|volume = 32, Part 2
|issue = 107
|pages = 154–156
|date = 2001
|doi = 10.1177/002182860103200208
|bibcode = 2001JHA....32..154T
|s2cid = 118464897
|issn = 0021-8286|doi-access = 
}}</ref>
In 2012, the journal DIO [http://www.dioi.org/cot.htm#ngmw 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.

Astronomical almanacs define the solstices as the moments when the Sun passes through the [[colure|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).<ref>{{cite book
|last=Meeus
|first=Jean
|author-link1=Jean Meeus
|date=1998
|title=Astronomical Algorithms
|edition=Second English
|url=https://archive.org/details/astronomicalalgo00meeu
|url-access=limited
|location=Richmond
|publisher=Willmann-Bell, Inc.
|pages=[https://archive.org/details/astronomicalalgo00meeu/page/n184 177]–182
|isbn=0-943396-61-1}}</ref> 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 <ref>{{cite web|title=December solstice|url=https://www.timeanddate.com/calendar/december-solstice.html|access-date=March 21, 2018}}</ref><ref>{{cite web|title=June solstice|url=https://www.timeanddate.com/calendar/june-solstice.html|access-date=March 21, 2018}}</ref> 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.