Solar time

On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1-2 is a complete rotation of the Earth, but because the revolution around the Sun affects the angle at which the Sun is seen from the Earth, 1-3 is how long it takes noon to return.

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time ( sundial time) and mean solar time (clock time).


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 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 the 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 the 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. As explained in the equation of time article, this is due to the eccentricity of the Earth's orbit (i.e. the Earth's orbit is not perfectly circular, meaning that the Earth-Sun distance varies throughout the year), and the fact that the 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 – e.g. completing the same number of pendulum swings in each hour – 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. [1] This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes. Currently a mean solar day is about 86,400.002 SI seconds. [2]

The two kinds of solar time ( apparent solar time and mean solar time) are among the three kinds of time reckoning that were employed by astronomers until the 1950s. (The third kind of traditional time reckoning is sidereal time, which is based on the apparent motions of stars other than the Sun.) [3] By the 1950s it had become clear that the Earth's rotation rate was not constant, so astronomers developed ephemeris time, a time scale based on the positions of solar system bodies in their orbits.

Other Languages
العربية: توقيت شمسي
भोजपुरी: सौर समय
català: Temps solar
čeština: Sluneční čas
dansk: Soltid
Deutsch: Sonnenzeit
español: Tiempo solar
Esperanto: Suntempo
français: Temps solaire
Gaeilge: Grian-am
한국어: 태양시
हिन्दी: सौर समय
hrvatski: Sinodički dan
Bahasa Indonesia: Waktu Matahari
interlingua: Tempore solar
íslenska: Sólartími
Latina: Dies solaris
Lëtzebuergesch: Sonnenzäit
lietuvių: Saulės laikas
македонски: Сончево време
Bahasa Melayu: Waktu suria
Nederlands: Zonnetijd
日本語: 太陽時
norsk: Soltid
norsk nynorsk: Soltid
occitan: Temps solar
română: Timp solar
Scots: Solar time
Simple English: Solar time
slovenčina: Slnečný čas
slovenščina: Sončev čas
svenska: Soltid
Türkçe: Güneş zamanı
українська: Сонячний час
Tiếng Việt: Thời gian Mặt Trời
粵語: 太陽時間
中文: 太阳日