The equation of time vanishes twice during one year.
The equation of time is the difference between apparent solar time and mean solar time at any instant. Apparent solar time is the time that would be shown by a sundial. Mean solar time is the time that is kept by a clock that is adjusted to keep the correct time on average.
The equation of time is caused by the fact that the Earth’s orbit around the Sun is not a perfect circle. The Earth’s orbit is actually an ellipse, which means that the Earth is closer to the Sun at some points in its orbit than it is at others. This difference in distance causes the Earth’s speed around the Sun to vary.
The Earth’s speed is fastest when it is closest to the Sun, and slowest when it is farthest from the Sun. This variation in speed causes the Earth’s day to be slightly longer when it is closest to the Sun than it is when it is farthest from the Sun.
The equation of time is greatest at the equinoxes, when the Earth’s speed around the Sun is changing the most. The equation of time is zero at the solstices, when the Earth’s speed around the Sun is changing the least.
The equation of time can be positive or negative. When the equation of time is positive, apparent solar time is ahead of mean solar time. When the equation of time is negative, apparent solar time is behind mean solar time.
The equation of time is a complex function of the Earth’s position in its orbit around the Sun. It is not possible to give a simple formula for the equation of time. However, the equation of time can be calculated using tables or a computer program.
The equation of time is important for astronomers and navigators. It is also important for people who use sundials.