Season

• 1. Introduction

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  A season is a division of the year, and the line between one and the next is not always a date on a calendar. In Sweden and Finland, meteorologists do not announce spring on a fixed day. They wait for the mean daily temperature to rise permanently above zero degrees Celsius, and in Finland that means above the line for seven straight days. The seasons there cannot be printed in advance. They are known only after the fact, and they begin on different dates in different parts of the country. That single idea, that a season can be a relative quarter on a calendar or an absolute event you must observe, runs underneath everything that follows. Why do the two hemispheres always disagree about which season it is? Why does the warmest month arrive weeks after the day of strongest sunlight? And why do some peoples count two seasons, while others count six or eight?

• 2. The Tilted Axis

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  Roughly 23.4 degrees of tilt explains most of it. The Earth's axis of rotation leans against its orbital plane by that angle, a quantity also called the obliquity of the ecliptic. For about half the year, from around the 20th of March to around the 22nd of September, the Northern Hemisphere tips toward the Sun, reaching its maximum lean near the 21st of June. For the other half, the Southern Hemisphere does the same, peaking near the 21st of December. This is why the two hemispheres always experience opposite seasons.

  Polaris stays fixed in the sky all year, and that steadiness is the second half of the cause. The Earth's orbit holds approximate axial parallelism, keeping its axis pointed toward the North Star throughout its circuit. A slow drift called axial precession does shift the direction, but it completes one cycle over 26,000 years, far too gradual for any person to notice.

  The low winter Sun spreads its light thin. When the Sun sits low at solar noon, incoming solar radiation falls across a larger patch of ground, so the light is more indirect and weaker. Combine that with shorter daylight hours, and the axial tilt accounts for most of the climate's swing in both hemispheres.

  The distance to the Sun, by contrast, barely matters here. Earth reaches perihelion, its closest point, in January, and aphelion, its farthest, in July. So orbital eccentricity actually works against the Northern Hemisphere's temperature trends, contributing only a 7% variation in sunlight received. Research even shows the planet as a whole runs slightly warmer when farther from the Sun, because the Northern Hemisphere holds more land than the Southern, and land warms more readily than sea.

• 3. Lag And Geography

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  Up to seven weeks separate the strongest sunlight from the hottest weather. The solstices mark the maximum and minimum insolation, yet the heights of summer and winter arrive later because of seasonal lag, the slow absorption and release of heat by the oceans. That same lag makes June, July, and August the warmest months in the Northern Hemisphere, and December, January, and February the warmest in the Southern.

  Proximity to water reshapes the extremes. The South Pole sits in the middle of the Antarctic continent, far from the moderating reach of the southern oceans. The North Pole floats on the Arctic Ocean, its temperature swings buffered by the surrounding water. The result is that the South Pole stays consistently colder during the southern winter than the North Pole during the northern winter.

  The Southern Hemisphere's water blunts what its orbit might sharpen. Earth's elliptical path could intensify southern winters and summers, but the abundance of sea across that hemisphere mitigates the effect. Oceans, their currents, El Nino and other oceanic cycles, and prevailing winds all bend the seasonal pattern from place to place.

  Near the equator, temperature stops being the story. The 23.4-degree tilt is too modest to change the strength of the Sun's rays much across the tropics. Instead, a rainy low-pressure belt called the Intertropical Convergence Zone migrates north and south. When it sits north of the equator, the northern tropics get their wet season while the southern tropics go dry, and the pattern flips when the belt crosses south.

• 4. Counting By Calendar

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  Spring, summer, autumn, and winter have been marked off this way since at least Roman times. In his Rerum rusticarum, Varro placed the start of each season on the 23rd day of the Sun's passage through Aquarius, Taurus, Leo, and Scorpio. Nine years before he wrote, Julius Caesar had reformed the calendar, which let Varro assign concrete dates: the 7th of February, the 9th of May, the 11th of August, and the 10th of November.

  The meteorological scheme groups whole months instead. In 1780 the Societas Meteorologica Palatina, an early international meteorology organization that became defunct in 1795, defined seasons as three-month blocks on the Gregorian calendar. By that reckoning, northern spring begins on the 1st of March, summer on the 1st of June, autumn on the 1st of September, and winter on the 1st of December, with the Southern Hemisphere shifted six months opposite.

  The astronomical scheme ties the start of each season to a solstice or an equinox. Pliny the Elder, in his Natural History, named the two equinoxes and two solstices and gave the lengths between them, and he placed the seasons starting halfway through those intervals. He listed the eighth day to the Kalends of January, the 25th of December, as the winter solstice, though in truth it then fell on the 22nd or 23rd.

  These astronomical seasons are not equal in length, a fact rooted in the work of Johannes Kepler and the elliptical orbit. From the March equinox it currently takes 92.75 days to the June solstice, then 93.65 days to the September equinox, 89.85 days to the December solstice, and 88.99 days back to the March equinox. The stretch from March to September equinox runs 7.56 days longer than the return trip.

• 5. The Wandering Dates

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  Every year the equinoxes and solstices arrive about six hours later, adding up to a full day in four years before a leap year resets them. The Gregorian calendar was built to keep the March equinox no later than the 21st of March, as accurately as practical. Its calendar equinox of the 21st of March, used to calculate Easter, matches the date in the Easter tables current at the Council of Nicaea in AD 325.

  The most common dates today are the 20th of March, the 21st of June, the 22nd or the 23rd of September, and the 21st of December. The four-year average slowly drifts to earlier times as a century progresses, a shift of a full day in about 128 years. Because 2000 was a leap year, that drift has run since the start of the last century, so older books still teach the 21st of March, the 22nd of June, the 23rd of September, and the 22nd of December.

  Clocks decide which calendar day the season lands on. All these times are given in UTC, roughly the time at Greenwich. People to the east see the seasons start later, and in Tonga at UTC plus 13, an equinox fell on the 24th of September 1999, a date it will not repeat until 2103. People far to the west may meet an equinox as early as the 19th of March.

  The deeper rhythm shifts across millennia. Over the Milankovitch cycles, Earth's axial tilt and orbital eccentricity both vary, and the axis fluctuates between 22.1 and 24.5 degrees. Ten thousand years from now, the northern winter will fall at aphelion and the northern summer at perihelion, reversing the present arrangement.

• 6. Reading The Land

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  Crocus blooms tell the season more honestly than any printed date. In ecological terms, a season is a stretch when only certain floral and animal events occur, so a change in those daily events is the season changing. These seasons are absolute, not relative. If the conditions for a season do not normally happen in a region, that area cannot be said to experience it.

  Six ecological seasons can be drawn without any fixed dates. Prevernal begins as deciduous tree buds swell, in February in mild temperate zones and March in cooler ones. Vernal follows as buds burst into leaf and birds claim territory and nest. Estival brings trees in full leaf and hatching birds, serotinal turns the leaves in higher latitudes and opens the harvest by early September, autumnal sends the leaves brown to the ground, and hibernal leaves the trees bare with migrating birds settled in winter habitats.

  The ocean shifts these markers by up to a month. Coastal and oceanic regions reach hibernal as much as a month later than continental climates, while prevernal and vernal arrive up to a month earlier near the sea. Prevernal crocus blooms appear as early as February along coastal British Columbia and the British Isles, but hold off until March or April across the Midwestern United States and parts of eastern Europe.

  Indigenous peoples have long read these signs into their own calendars. Across northern Eurasia, the Americas, Africa, Oceania, and Australia, separate tribal groups marked seasons by local criteria, from the hibernation of polar bears on the arctic tundra to the growing cycles of tropical rainforest plants. In Australia some tribes count up to eight seasons, as do the Sami of Scandinavia.

• 7. Many Names For The Year

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  The Noongar people of South-West Western Australia recognize six seasons they call maar-keyen bonar. Each arrives not by a date but by changing winds, flowering plants, temperature, and migration, lasting about two calendar months. The seasons map onto the human condition: Birak, the first summer, is the season of the young; Bunuru, the second summer, the season of adolescence; Djeran the season of adulthood; Makuru the fertility season; Djilba the season of conception; and Kambarang, the wildflower season, the season of birth.

  The North American Cree count six as well, adding two the others lack. Their system names Pipon for winter, Nepin for summer, and the two extra seasons, Sekwun the break-up and Mikiskaw the freeze-up, for the freezing and breaking of the ice on rivers and lakes.

  In the Hindu calendar of tropical and subtropical India, six Ritu carry fixed dates, two months each. Vasanta is spring, Grishma summer, Varsha the monsoon, Sharada autumn, Hemanta early winter, and Shishira the prevernal or late winter. The Bengali, Odia, and Tamil calendars follow the same six-fold shape with different start and end times.

  Some traditions stop at two seasons, and the names can invert. In Nicaragua, in the Northern Hemisphere, the dry season from November to April is called summer, while the rainy season from May to October is called winter. Thailand instead recognizes three: Ruedu nao, the cold season, from mid October to mid February; Ruedu ron, the hot season, to mid May; and Ruedu fon, the rainy season, to mid October.

• 8. Where Seasons Decide Survival

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  At the weather station Alert, on the northern tip of Ellesmere Island at 82 degrees 30 minutes north, the Sun does not rise at all for months. About 830 kilometers from the North Pole, it climbs above the horizon by over 12 hours by the 21st of March. On the 6th of April it rises and stays up, not setting again until the 6th of September. From mid-November to mid-January there is no twilight, only continuous dark, until a faint wash of light returns in mid-January.

  War has always bent itself to this calendar of light, mud, and ice. Pre-modern armies in Europe campaigned in summer, because peasant conscripts melted away at harvest time and no agricultural society could neglect the sowing season. Firm ground favors a war of manoeuvre, so frozen winter snow can carry an army while spring thaws and autumn rains bog it down. Rainy-season floods can make rivers impassable, and winter snow blocks mountain passes, the reason Taliban offensives kept to the Afghanistan fighting season.

  For navies, the season is decided by the port. Ice-free or warm-water harbors make a fleet far more effective, and storm seasons or polar winter weather can pin surface warships in place. Confined for centuries to Arkhangelsk before the 18th century, and even to Kronstadt, Russia held a lasting interest in keeping access to Baltiysk, Vladivostok, and Sevastopol, three harbors whose seasons, more than any equinox, shaped where its power could reach.