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Colour version of the “Flammarion engraving,” a wood engraving by an unknown artist. It became associated with Nicolas Camille Flammarion (1842–1925), the French astronomer, meteorologist, and writer, after he reproduced it on page 168 of his 1888 book The Atmosphere: Popular Meteorology.
This article explains a common belief that is correct only to a certain extent.
The common belief is that on the first day of Farvardin—Nowruz—when spring begins at the spring equinox, the lengths of day and night are equal everywhere on Earth. In fact, at the spring equinox the daylight period is not equal to the night; it is somewhat longer. [source]
In astronomical usage, daytime is generally the interval during which the Sun is visible above the horizon at a particular location, assuming no local obstructions. From Earth the Sun is not a point but a luminous disk. Therefore, when the centre of the Sun has dropped below the horizon, its upper portion can still remain visible.
In addition, Earth's atmosphere refracts sunlight. Thus, even when the Sun's upper edge is about 0.4° below the horizon, its light can still reach us along a curved path for a short time after the Sun has geometrically set.
Sunrise and sunset tables conventionally use an apparent solar radius of 16 arcminutes and atmospheric refraction of 34 arcminutes. Together these mean that while the Sun's upper edge appears to remain at the horizon, its geometric centre is actually about 50 arcminutes below it. According to the source, this makes daylight near Earth's equator approximately 14 minutes longer than night.2
True equality of day and night therefore occurs only at locations far enough from the equator for the seasonal difference in day length to offset at least about seven minutes.3 Across most of Earth outside the near-equatorial region, this occurs several days before the spring equinox.
The farther north a location lies, the closer the date of equal day and night falls to the beginning of spring. In Iran, the article states that day and night are approximately equal on 25 or 26 Esfand: southern Iranian cities around 25 Esfand and more northern cities around 26 Esfand. From 27 Esfand onward, daylight is longer than night throughout Iran.4
Notes
1 The Persian term used by the original article for equinox derives from the idea of balance or equality. It refers readers to the earlier article on calculating the precise instant of the Persian New Year.
2 Thus, according to the article's explanation, day and night are never exactly equal at the equator under the sunrise/sunset definition being used. [source]
3 The seven-minute figure can be obtained with a short calculation.
4 A corresponding effect continues for several days after the autumn equinox. When autumn begins in the Northern Hemisphere as the Sun crosses from the northern to the southern celestial hemisphere, daylight remains longer than night for several more days. The article gives approximately 3 Mehr for southern Iranian cities and 4 Mehr for more northern cities as the dates when day and night become equal.