Why solar noon is not at 12:00

scheduleIn short

Three reasons the clock and the sun disagree

If everyone kept their own sun time, solar noon would always be 12:00. Clocks gave that up in the nineteenth century, when railways needed one time for a whole region.

1. Your place in the time zone. The Earth turns 15° an hour, so the sun reaches each degree of longitude four minutes after the degree to its east. A time zone is set by one meridian; the further west of it you live, the later the sun peaks on your clock.
2. Daylight saving time moves the clock an hour forward in summer, and solar noon an hour later with it.
3. The equation of time: the sun itself runs up to 16 minutes ahead of an even clock or 14 minutes behind it, depending on the date, the same everywhere on Earth.

publicThe biggest effect

Where you sit in your time zone

Spain is the classic example. Madrid lies 3.7° west of Greenwich, yet Spain has kept Central European Time, set for the meridian 15° east, since 1940. That puts its clocks 75 minutes ahead of the sun before daylight saving adds another hour, and in June the sun peaks at 14:16 in Madrid and 14:35 in A Coruña. China goes further: one time zone for a country five zones wide, so in Kashgar, in the far west, solar noon comes at about three in the afternoon all year. India’s single zone puts Mumbai 38 minutes behind the sun’s schedule and Kolkata 23 minutes ahead of it.

Two neighbours in the same zone can differ a lot: Detroit sits at the western edge of Eastern Time and sees solar noon 36 minutes later than New York, while Chicago, near the centre of Central Time, keeps close to the clock.

show_chartThe small one that never goes away

The equation of time

Even without time zones, a sundial and a clock agree on only four days a year. The rest of the time the sun is early or late by up to a quarter of an hour, following the curve below. Astronomers call the difference the equation of time.

Two things cause it. The Earth’s orbit is an ellipse, and the Earth moves fastest at its closest point to the sun in early January, so the sun’s daily drift against the stars is uneven. And the Earth’s axis is tilted 23.4°, so even an even drift along the ecliptic projects unevenly onto the equator, where clocks are measured. Added together they give a curve with two peaks and two troughs: the sun is 14.2 minutes late on 11 February and 16.4 minutes early on 3 November, and on time around 16 April, 13 June, 2 September and 25 December.

The columns of the table were chosen to show it: compare London on 11 February (12:14) and 3 November (11:44), both in Greenwich Mean Time. Half an hour separates the two solar noons with no change of clock at all.

helpWhy it matters

What solar noon is good for

• It splits the day in two. Sunrise and sunset sit symmetrically around solar noon, within a minute, not around 12:00. In Madrid on 21 June the sun rises at 06:44 and sets at 21:48, seven and a half hours either side of 14:16; that is why Spanish evenings are so long.
• Shadows are shortest and point due north (south in the southern hemisphere, outside the tropics). That is the basis of the most reliable way to find north with a stick.
• Solar panels peak around solar noon. If the output of a south-facing roof peaks at 13:00 in summer, that is daylight saving time, not a fault.
• It explains odd sunsets. The equation of time is why the earliest sunset comes before the shortest day, and why the sun’s noon positions trace a figure eight over a year.

The sunrise and sunset calculator gives solar noon for any place and date, on your local clock.

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