Editing Sundial (section)

==Adjustments to calculate clock time from a sundial reading==
The most common reason for a sundial to differ greatly from clock time is that the sundial has not been oriented correctly or its hour lines have not been drawn correctly. For example, most commercial sundials are designed as ''horizontal sundials'' as described above. To be accurate, such a sundial must have been designed for the local geographical latitude and its style must be parallel to the Earth's rotational axis; the style must be aligned with [[true north]] and its ''height'' (its angle with the horizontal) must equal the local latitude. To adjust the style height, the sundial can often be tilted slightly "up" or "down" while maintaining the style's north-south alignment.<ref>{{harvp|Waugh|1973| pp= 48–50}}</ref>

===Summer (daylight saving) time correction===
Some areas of the world practice [[daylight saving time]], which changes the official time, usually by one hour. This shift must be added to the sundial's time to make it agree with the official time.

===Time-zone (longitude) correction===
A standard [[time zone]] covers roughly 15° of longitude, so any point within that zone which is not on the reference longitude (generally a multiple of 15°) will experience a difference from standard time that is equal to 4 minutes of time per degree. For illustration, sunsets and sunrises are at a much later "official" time at the western edge of a time-zone, compared to sunrise and sunset times at the eastern edge. If a sundial is located at, say, a longitude 5° west of the reference longitude, then its time will read 20&nbsp;minutes slow, since the Sun appears to revolve around the Earth at 15° per hour. This is a constant correction throughout the year. For equiangular dials such as equatorial, spherical or Lambert dials, this correction can be made by rotating the dial surface by an angle equaling the difference in longitude, without changing the gnomon position or orientation. However, this method does not work for other dials, such as a horizontal dial; the correction must be applied by the viewer.

However, for political and practical reasons, time-zone boundaries have been skewed. At their most extreme, time zones can cause official noon, including daylight savings, to occur up to three hours early (in which case the Sun is actually on the [[Meridian (astronomy)|meridian]] at official clock time of 3&nbsp;{{sc|pm}}). This occurs in the far west of [[Alaska]], [[China]], and [[Spain]]. For more details and examples, see [[time zones]].

=== Equation of time correction ===
[[File:Equation of time.svg|thumb|The [[Equation of Time]] – above the axis the equation of time is positive, and a sundial will appear ''fast'' relative to a clock showing local mean time. The opposites are true below the axis.]]
{{main|Equation of time}}
[[File:Derby Sundial C 5810.JPG|thumb|170px|The [[Whitehurst & Son sundial]] made in 1812, with a circular scale showing the equation of time correction. This is now on display in the [[Derby Museum and Art Gallery|Derby Museum.]] ]]

Although the Sun appears to rotate uniformly about the Earth, in reality this motion is not perfectly uniform. This is due to the [[Eccentricity (mathematics)|eccentricity]] of the Earth's orbit (the fact that the [[Earth's orbit]] about the Sun is not perfectly circular, but slightly [[ellipse|elliptical]]) and the tilt (obliquity) of the Earth's rotational axis relative to the plane of its orbit. Therefore, sundial time varies from [[Local mean time|standard clock time]]. On four days of the year, the correction is effectively zero. However, on others, it can be as much as a quarter-hour early or late. The amount of correction is described by the [[equation of time]]. This correction is equal worldwide: it does not depend on the local [[latitude]] or [[longitude]] of the observer's position. It does, however, change over long periods of time, (centuries or more,<ref>
{{cite web
 | first= Kevin  | last= Karney
 | title= Variation in the equation of time
 | url= http://www.precisedirections.co.uk/Sundials/E-o-T_Variability.pdf
 | access-date= 2014-07-25  | url-status= live
 | archive-url= https://web.archive.org/web/20160610094901/http://www.precisedirections.co.uk/Sundials/E-o-T_Variability.pdf
 | archive-date= 2016-06-10
}}
</ref>)
because of slow variations in the Earth's orbital and rotational motions. Therefore, tables and graphs of the equation of time that were made centuries ago are now significantly incorrect. The reading of an old sundial should be corrected by applying the present-day equation of time, not one from the period when the dial was made.

In some sundials, the equation of time correction is provided as an informational plaque affixed to the sundial, for the observer to calculate. In more sophisticated sundials the equation can be incorporated automatically. For example, some equatorial bow sundials are supplied with a small wheel that sets the time of year; this wheel in turn rotates the equatorial bow, offsetting its time measurement. In other cases, the hour lines may be curved, or the equatorial bow may be shaped like a vase, which exploits the changing altitude of the sun over the year to effect the proper offset in time.<ref>
{{cite web
 |title       = The Claremont, CA Bowstring Equatorial Photo Info
 |url         = http://www.wsanford.com/~wsanford/exo/sundials/ca/claremont/info.html
 |access-date = 2008-01-19 |url-status     = dead
 |archive-url = https://web.archive.org/web/20080422090829/http://www.wsanford.com/~wsanford/exo/sundials/ca/claremont/info.html
 |archive-date = 2008-04-22
}}
</ref>

A ''heliochronometer'' is a precision sundial first devised in about 1763 by [[Philipp Matthäus Hahn|Philipp Hahn]] and improved by Abbé Guyoux in about 1827.<ref name=Daniel2008>{{cite book
 | last= Daniel
 | first= Christopher St.&nbsp;J.H.
 | title= Sundials
 | year= 2004
 | publisher= Osprey Publishing
 | isbn= 978-0-7478-0558-8
 | pages= 47&nbsp;ff
 | url= https://books.google.com/books?id=x7-cO24xCMcC&pg=PA47
 | access-date= 25 March 2013
 }}{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
It corrects [[solar time|apparent solar time]] to [[mean solar time]] or another [[standard time]]. Heliochronometers usually indicate the minutes to within 1 minute of [[Universal Time]].

[[File:ROE-2012-05-07-0010.jpg|thumb|Sunquest sundial, designed by Richard L. Schmoyer, at the [[Mount Cuba Observatory]] in [[Greenville, Delaware]].]]

The [[Sunquest sundial]], designed by Richard L. Schmoyer in the 1950s, uses an analemmic-inspired gnomon to cast a shaft of light onto an equatorial time-scale crescent. Sunquest is adjustable for latitude and longitude, automatically correcting for the equation of time, rendering it "as accurate as most pocket watches".<ref>
{{cite web
 |last=Schmoyer |first=Richard L.
 |year=1983
 |title=Designed for accuracy
 |website=Sunquest Sundial
 |url=http://sunquestsundial.org/index.php/designed-for-accuracy/
 |access-date=17 December 2017 |url-status=live
 |archive-url=https://web.archive.org/web/20180319213757/http://sunquestsundial.org/index.php/designed-for-accuracy/
 |archive-date=19 March 2018
}}
</ref><ref>
{{harvp|Waugh|1973|p=34}}
</ref><ref>
{{cite book
 |last=Cousins |first=Frank W.
 |year=1973
 |title=Sundials: The art and science of gnomonics
 |publisher=Pica Press
 |place=New York, NY
 |pages=189–195
}}
</ref><ref>
{{cite magazine
 |last1=Stong |first1=C.L.
 |year=1959
 |title=The Amateur Scientist
 |magazine=Scientific American
 |volume=200  |issue=5  |pages=190–198
 |doi=10.1038/scientificamerican0459-171 |bibcode=1959SciAm.200d.171S
 |url=http://sunquestsundial.org/wp-content/uploads/2015/10/Scientific-American-1959.pdf
 |url-status=live  |access-date=2017-12-17
 |archive-url=https://web.archive.org/web/20190303123221/http://sunquestsundial.org/wp-content/uploads/2015/10/Scientific-American-1959.pdf
 |archive-date=2019-03-03
}}
</ref>
Similarly, in place of the shadow of a gnomon the sundial at Miguel Hernández University uses the solar projection of a graph of the equation of time intersecting a time scale to display clock time directly. [[File:Sundial with Equation of Time correction.jpg|thumb|Sundial on the Orihuela Campus of [[Miguel Hernández University]], Spain, which uses a projected graph of the [[equation of time]] within the shadow to indicate clock time.]]

An analemma may be added to many types of sundials to correct apparent solar time to [[mean solar time]] or another [[standard time]]. These usually have hour lines shaped like "figure eights" ([[analemma]]s) according to the [[equation of time]]. This compensates for the slight eccentricity in the Earth's orbit and the tilt of the Earth's axis that causes up to a 15&nbsp;minute variation from mean solar time. This is a type of dial furniture seen on more complicated horizontal and vertical dials.

Prior to the invention of accurate clocks, in the mid 17th&nbsp;century, sundials were the only timepieces in common use, and were considered to tell the "right" time. The equation of time was not used. After the invention of good clocks, sundials were still considered to be correct, and clocks usually incorrect. The equation of time was used in the opposite direction from today, to apply a correction to the time shown by a clock to make it agree with sundial time. Some elaborate "[[equation clock]]s", such as one made by Joseph Williamson in 1720, incorporated mechanisms to do this correction automatically. (Williamson's clock may have been the first-ever device to use a [[Differential (mechanical device)|differential]] gear.) Only after about 1800 was uncorrected clock time considered to be "right", and sundial time usually "wrong", so the equation of time became used as it is today.<ref>
{{cite book
 |last=Landes |first=David S.
 |year=2000
 |title=Revolution in Time : Clocks and the making of the modern world
 |place=London, UK
 |publisher=Viking
 |isbn=0-670-88967-9 |oclc=43341298
 |url=https://www.worldcat.org/oclc/43341298
 |access-date=2022-02-13 |url-status=live
 |archive-url=https://web.archive.org/web/20230421080818/https://www.worldcat.org/title/43341298
 |archive-date=2023-04-21}}</ref>