Editing Solar time (section)

==Introduction==
[[File:EarthsOrbit_en.png |thumb|The Earth's orbit around the Sun, showing its eccentricity]]

A tall pole vertically fixed in the ground casts a shadow on any sunny day. At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead). That instant is called [[solar noon|''local apparent noon'']], or 12:00 local apparent time. About 24 hours later the shadow will again point north–south, the Sun seeming to have covered a 360-degree arc around Earth's axis. When the Sun has covered exactly 15 degrees (1/24 of a circle, both angles being measured in a plane perpendicular to Earth's axis), local apparent time is 13:00 exactly; after 15 more degrees it will be 14:00 exactly.

The problem is that in September the Sun takes less time (as measured by an accurate clock) to make an apparent revolution than it does in December; 24 "hours" of solar time can be 21 seconds less or 29 seconds more than 24 hours of clock time. This change is quantified by the [[equation of time]], and is due to the [[Orbital eccentricity|eccentricity]] of Earth's orbit (as in, Earth's orbit is not perfectly circular, meaning that the Earth{{endash}}Sun distance varies throughout the year), and the fact that Earth's axis is not perpendicular to the plane of its orbit (the so-called [[obliquity of the ecliptic]]).

The effect of this is that a clock running at a constant rate{{Snd}}e.g. completing the same number of pendulum swings in each hour{{Snd}}cannot follow the actual Sun; instead it follows an imaginary "'''mean Sun'''" that moves along the celestial equator at a constant rate that matches the real Sun's average rate over the year.<ref>{{cite web|url=https://aa.usno.navy.mil/faq/asa_glossary#solar-time,-mean|work=Glossary, Astronomical Almanac Online|date=2021|publisher=[[Her Majesty's Nautical Almanac Office]] and the [[United States Naval Observatory]]|title=solar time, mean}}</ref> This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes. {{As of|2008}}, a mean solar day is about 86,400.002 [[International System of Units|SI]] seconds, i.e., about 24.0000006 hours.<ref>{{cite web|url=http://tycho.usno.navy.mil/leapsec.html|title=Leap Seconds|date=1999|website=Time Service Department, United States Naval Observatory|archive-url=https://web.archive.org/web/20150312003149/http://tycho.usno.navy.mil/leapsec.html|archive-date=March 12, 2015}}</ref>