What it is
The equation of time (EoT) is the difference between apparent solar time — what a sundial shows — and mean solar time, the steady time a clock keeps. It is positive when the sundial is ahead of the clock.
Over a year it ranges from about −14 minutes in mid-February to about +16 minutes in early November, crossing zero around 15 April, 13 June, 1 September and 25 December.
Why the sun keeps imperfect time
- The tilted axis. The sun moves along the ecliptic, tilted 23.44° to the equator. Its eastward progress projected onto the equator is faster near the solstices and slower near the equinoxes.
- The elliptical orbit. The Earth moves fastest at perihelion in early January and slowest in early July, so the sun's apparent motion changes speed too.
The two effects add up to the familiar double-humped curve; together with declination they draw the figure-eight analemma — switch the sun map to Year mode to see it traced over your location.
| Date | Declination | Equation of time | Noon elevation at 40° N | at 50° N | at 35° S |
|---|---|---|---|---|---|
| 1 Jan | -22.98° | −3.6 min | 27.0° | 17.0° | 78.0° |
| 15 Jan | -21.07° | −9.4 min | 28.9° | 18.9° | 76.1° |
| 1 Feb | -17.02° | −13.5 min | 33.0° | 23.0° | 72.0° |
| 15 Feb | -12.58° | −14.1 min | 37.4° | 27.4° | 67.6° |
| 1 Mar | -7.48° | −12.3 min | 42.5° | 32.5° | 62.5° |
| 15 Mar | -2.02° | −8.9 min | 48.0° | 38.0° | 57.0° |
| 1 Apr | 4.65° | −3.8 min | 54.7° | 44.7° | 50.3° |
| 15 Apr | 9.87° | −0.0 min | 59.9° | 49.9° | 45.1° |
| 1 May | 15.17° | +2.9 min | 65.2° | 55.2° | 39.8° |
| 15 May | 18.94° | +3.7 min | 68.9° | 58.9° | 36.1° |
| 1 Jun | 22.09° | +2.2 min | 72.1° | 62.1° | 32.9° |
| 15 Jun | 23.32° | −0.5 min | 73.3° | 63.3° | 31.7° |
| 1 Jul | 23.08° | −3.9 min | 73.1° | 63.1° | 31.9° |
| 15 Jul | 21.47° | −6.0 min | 71.5° | 61.5° | 33.5° |
| 1 Aug | 17.94° | −6.4 min | 67.9° | 57.9° | 37.1° |
| 15 Aug | 13.95° | −4.5 min | 64.0° | 54.0° | 41.0° |
| 1 Sep | 8.17° | −0.0 min | 58.2° | 48.2° | 46.8° |
| 15 Sep | 2.91° | +4.8 min | 52.9° | 42.9° | 52.1° |
| 1 Oct | -3.30° | +10.3 min | 46.7° | 36.7° | 58.3° |
| 15 Oct | -8.63° | +14.2 min | 41.4° | 31.4° | 63.6° |
| 1 Nov | -14.52° | +16.4 min | 35.5° | 25.5° | 69.5° |
| 15 Nov | -18.56° | +15.4 min | 31.4° | 21.4° | 73.6° |
| 1 Dec | -21.84° | +11.0 min | 28.2° | 18.2° | 76.8° |
| 15 Dec | -23.28° | +4.9 min | 26.7° | 16.7° | 78.3° |
Finding solar noon on the clock
solar noon = 12:00 − 4 min × (λ − λzone) − EoT (+ 1 h in summer time)
λ is your longitude and λzone the central meridian of your time zone (15° × the UTC offset). Kraków lies at 19.94° E in a zone centred on 15° E: the sun reaches noon 19.8 minutes earlier than 12:00, and on 3 November another 16.4 minutes earlier still — at about 11:24.
Every place page lists solar noon for each month, computed exactly rather than with this approximation.