Learn & Understand

The Sun Is a Bad Clock: The Equation of Time

If you tracked the Sun's highest point each day against a perfect clock, you would find it drifting — sometimes minutes early, sometimes minutes late, wandering across the year. The Sun, it turns out, keeps irregular time. This drift, called the equation of time, is why a sundial and a wristwatch rarely agree, and why solar noon seldom falls at twelve o'clock.

Sun Time Versus Clock Time

Our clocks tick at a perfectly uniform rate, but the Sun does not cross the sky at a perfectly uniform pace. The moment of solar noon — when the Sun stands highest — shifts earlier and later through the year relative to the steady clock. A sundial, which reads true solar time, therefore runs ahead of or behind a mechanical clock depending on the date.

Two Reasons the Sun Wanders

The drift has two causes. First, Earth's orbit is an ellipse, so the planet moves faster when nearer the Sun and slower when farther, speeding and slowing the Sun's apparent motion. Second, Earth's axis is tilted, which makes the Sun's daily progress along the sky uneven through the seasons. Together these produce the smoothly varying discrepancy across the year.

The Analemma

If you photographed the Sun at the same clock time every day for a year, it would trace a lopsided figure-eight in the sky, called the analemma — a visible portrait of the equation of time combined with the Sun's seasonal rise and fall. This elegant curve is the drift made visible, the record of the Sun's yearly wandering against our uniform clocks.

Where You Stand in Your Zone

A third, simpler reason solar noon rarely hits twelve is that time zones are broad. Unless you stand on your zone's central meridian, the Sun reaches its peak before or after the zone's official noon, shifted by your longitude within the zone. Between the equation of time and your position in the zone, clock noon and true solar noon almost never coincide.

Finding Solar Noon

To find the clock time of solar noon for a place and date, use the Solar Noon Calculator. Convert between clock zones with the Time Zone Calculator, and locate a position with the Latitude Longitude Converter.