Astronomy & Celestial Mechanics
the sky, computed
Concept

Solar noon — where it appears

The moment the Sun crosses the local meridian and stands highest in the sky, when a shadow is shortest. By the clock it rarely falls at twelve, because of the equation of time, the place's longitude inside its time zone and summer time.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Solar noon by the clock, in four places on ordinary time. The clock time at which the Sun crosses the meridian, through a year, for London (0.13°W, clocks on UTC+0), Madrid (3.70°W, clocks on UTC+1), Vigo (8.72°W, clocks on UTC+1), Kashgar (75.99°E, clocks on UTC+8), with summer time where it is kept, from the end of March to the end of October. In London noon falls between 11:44 (4 November) and 13:07 (27 July); in Madrid noon falls between 12:58 (4 November) and 14:21 (27 July); in Vigo noon falls between 13:18 (4 November) and 14:41 (27 July); in Kashgar noon falls between 14:40 (4 November) and 15:10 (12 February). The smooth wave on each curve is the equation of time, the same ±16 minutes everywhere. The steps are summer time, a whole hour. And the vertical offset of each curve is where the place sits inside its time zone, which for a city in the far west of a wide zone is larger than both: noon near three in the afternoon, by the clock, is an ordinary consequence of one time zone spanning a large country.

The smallest term between a clock and the Sun

A clock and a sundial disagree for three reasons, and the one astronomy supplies — the equation of time, sixteen minutes at most — is usually the smallest of them. Where a place sits inside its time zone can put noon three hours after twelve, and summer time adds a whole hour on top. The equation of time is visible in ordinary life only where the other two happen to vanish.

sky · Equation of time An hour that was a twelfth of the daylight. The length of a daylight hour when the time from sunrise to sunset is divided into twelve, through a year, at Alexandria (31.2°N), Rome (41.9°N), London (51.5°N), Stockholm (59.3°N). At Alexandria the hour runs from 51 minutes at the winter solstice to 71 at the summer solstice; at Rome the hour runs from 46 minutes at the winter solstice to 76 at the summer solstice; at London the hour runs from 39 minutes at the winter solstice to 83 at the summer solstice; at Stockholm the hour runs from 30 minutes at the winter solstice to 93 at the summer solstice. The dashed line is sixty minutes, the length it has at both equinoxes everywhere. This is the hour of the ancient Mediterranean world and of medieval Europe until mechanical clocks: an hour defined by the Sun, which a sundial with suitably drawn lines reads exactly, and in which the equation of time does not exist, because nothing is being compared with a uniform clock. Sunrise, sunset and noon are each defined by the Sun, and the clock that would disagree with them had not been built.

An hour that stretched with the season

For most of recorded history an hour was a twelfth of the daylight — seventy-six minutes at a Roman midsummer and forty-six in midwinter — and a sundial read it exactly. In that system there was no equation of time, because nothing uniform was being compared with the Sun. The sixteen-minute correction became real only when the hour was made equal, and measurable only when clocks could keep time more steadily than the Sun by more than it.

sky · Equation of time

Named alongside it

The objects these essays reach for when they reach for this one.

Equation of timeMean solar timeApparent solar timeChronometerDay lengthDaylight saving timeDeclinationHour angleLatitudeLocal mean timeLongitudePendulum clock

All concepts