Equation of Time & Sundial Correction Calculator

Two reasons combine. Eccentricity: Earth's orbit isn't circular — it's at perihelion (closest to the Sun) around January 3 and aphelion around July 4. By Kepler's second law it moves fastest at perihelion, so the Sun appears to drift east against the stars faster in January, slower in July. Obliquity: Earth's spin axis is tilted 23.44° relative to its orbit, so the Sun's motion along the ecliptic doesn't project uniformly onto the celestial equator. The two effects each contribute about ±9 minutes; they sometimes add (peak ±16 min) and sometimes partially cancel (zero crossings).

Time zones are 1-hour bands centered roughly every 15° of longitude. Seoul is at 127° E but Korea Standard Time uses the 135° E meridian — so Seoul's mean solar noon is 32 minutes after KST noon (longitude correction = 4 min/° × (135 − 127) = +32 min). The EoT is the same everywhere on Earth on a given date; the longitude correction depends on where you stand relative to the timezone's reference meridian.

The 9.87/7.53/1.5 formula is accurate to about ±15 seconds for any year in the current epoch. Modern astronomical libraries use a longer Fourier series that's accurate to fractions of a second. For sundial design or rough astronomy this fit is more than enough; for a high-precision ephemeris use the JPL DE files or Skyfield/NOVAS implementations.

Because of the longitude correction. EoT = 0 means the apparent Sun crosses your local meridian at exactly 12:00 mean local time. If you're not on your timezone's reference meridian, that 12:00 mean local time is not 12:00 wristwatch time. Add the longitude correction (this tool's 'longitude correction' field) to get the wristwatch time of true solar noon. Add then EoT for any other date.