The equation of time equals which difference of times?

• A. Local Apparent Time - Local Mean Time + 12 hours
• B. Local Apparent Time - Local Mean Time
• C. Local Mean Time - Local Apparent Time
• D. Local Apparent Time - Local Mean Time - 12 Hours

Answer: (B)

The difference between apparent solar time and mean solar time is what the equation of time measures. Local Apparent Time is the true sun time based on the Sun’s actual position in the sky, while Local Mean Time is the averaged, regular clock time that treats every hour as equal. The equation of time is LAT minus LMT, the amount by which the sundial time (apparent) leads or lags behind the clock time (mean). This difference is usually a few minutes and changes over the year because the Sun’s apparent motion is affected by the Earth's elliptical orbit and axial tilt. The other formulations with a 12-hour offset or reversing the order don’t describe this small seasonal difference.