Equation of Time
Compute the equation-of-time correction and approximate solar noon for any date.
Overview
The Equation of Time tool reports the gap between solar noon (when the Sun crosses your local meridian) and clock noon for any date in the year. The result is in minutes and seconds, positive when the Sun is "ahead" of mean time and negative when it lags — the same correction printed on sundials and used by astronomers calibrating telescope mounts.
Useful for sundial makers calibrating gnomons, solar-panel installers tuning tracker alignment, photographers planning the exact moment of solar noon for harsh-shadow shots, and amateur astronomers timing transit observations.
How it works
Mean solar time progresses at a constant 24 hours per day, but apparent solar time speeds up and slows down through the year because Earth's orbit is elliptical (Kepler's second law) and its axis is tilted (obliquity of the ecliptic). The Equation of Time is the difference between the two, sitting inside roughly ±16 minutes and crossing zero four times per year.
The calculation here uses the standard NOAA approximation valid for years 1800–2100: solve for solar declination and right ascension at the given Julian Date and subtract mean solar position. Apparent solar noon is then mean noon plus the equation, adjusted for the longitude difference from the centre of your time zone.
Examples
Feb 11 → EoT ≈ −14m 13s (Sun lags, latest sunrise around now)
May 14 → EoT ≈ +3m 41s
Nov 3 → EoT ≈ +16m 27s (annual maximum, Sun is "ahead")
Apr 15 → EoT ≈ 0 (one of four zero crossings)
FAQ
Why isn't solar noon at 12:00 on my clock?
Three reasons: the equation-of-time correction, your longitude offset from the centre of your time zone (up to ±30 minutes), and any daylight-saving shift (+60 minutes). All three combine to push solar noon as far as two hours from 12:00.
When does the Sun rise earliest?
Not on the summer solstice. The earliest sunrise and latest sunset are offset by a few days from the solstice because the equation of time is also changing — a classic non-obvious consequence of this curve.
Why is the curve shaped like a figure-eight?
Plotting daily solar position at the same clock time across a year traces an "analemma" — a figure-eight where the vertical axis is declination (axial tilt) and the horizontal axis is the equation of time.
Does it depend on latitude?
No — the equation of time is the same everywhere on Earth. Latitude only affects the length of daylight and the elevation of the Sun at noon.
Is it accurate enough for navigation?
The NOAA approximation is accurate to within a few seconds across the 1800–2100 window, more than enough for amateur celestial navigation but not for spacecraft tracking.