Equation of Time

Definition and Explanation

The Equation of Time refers to the discrepancy between Apparent Solar Time (AST) and Mean Solar Time (MST).
It represents the comparison between the actual path of the Sun in the sky (AST) and the average or ‘mean’ daily path (MST).
The Equation of Time becomes noticeable due to two factors. Firstly, the Earth’s axis tilt relative to the plane of its orbit around the Sun. Secondly, the irregular shape of Earth’s orbit, which is elliptical, not circular.
It can range from about +14 minutes to nearly -16 minutes, and it equals zero on only four days a year.

Charting the Equation of Time

The Equation of Time can be represented graphically, often in the form of an analemma, which visually represents the changing position of the Sun in the sky over the course of a year.
On this graph, the vertical axis represents the declination (latitude) of the Sun, and the horizontal axis represents the Equation of Time.
This irregular figure-eight shape reflects the combined effects of Earth’s axial tilt and eccentric orbital shape on the Sun’s apparent position.

Utilising the Equation of Time

The Equation of Time is crucial for the understanding of sundial readings. A sundial displays apparent solar time that needs to be adjusted according to the Equation of Time to get the mean solar time.
The difference between AST and MST is significant to astronomers for understanding the apparent movement of the Sun and other celestial bodies.

Interesting Facts

Around the shortest and longest days of the year (solstices), the Sun appears to slow down. That’s because of the axial tilt making the solar day longer than 24 hours.
Around the equinoxes, the Sun appears to speed up due to the eccentricity of Earth’s orbit making the solar day shorter than 24 hours.

Understanding the Equation of Time, how it’s derived, and its uses, will give a closer insight into timekeeping, the apparent movement of celestial bodies, and various concepts of astronomy.