Seasons come from Earth's tilt, and the warmest months lag the sunniest
The Northern Hemisphere gets its strongest sunshine in May, June and July, yet its hottest months are June, July and August. That seasonal lag is one surprise. Another is how many ways people divide the year: ecologists use six seasons, much of the tropics two, and ancient Egypt counted three tied to the Nile.
Earth's axis leans about 23.4 degrees from the plane of its orbit, a tilt called the obliquity of the ecliptic, and keeps pointing toward Polaris all year. As the planet circles the Sun, first one hemisphere and then the other faces it more directly, so the Sun climbs higher, sunlight concentrates and days lengthen. That is why the two hemispheres always have opposite seasons. The axis does wobble, a motion called precession, but one cycle takes 26,000 years, far too slow for anyone to notice.
From roughly 20 March to 22 September the north leans sunward, most strongly around 21 June, while the south's turn peaks around 21 December. At the equinoxes the Sun stands directly above the equator, both poles sit on the line between day and night, and daylight is shared equally. Around the March equinox the north enters spring as days lengthen, while the south slides into autumn.
In temperate and polar zones, changing sunlight drives animals to hibernate or migrate and plants to go dormant. Calendars there recognise spring, summer, autumn and winter, but ecologists often prefer six seasons that float free of the calendar, running from prevernal through estival and serotinal to hibernal. A lot of tropical places have just a wet or monsoon season and a dry one, sometimes with a cool or harmattan season added. Hazards define seasons too, such as hurricanes and wildfires.
Culture shapes the count. Farming societies organise life around planting and harvest and often mark the turn of the seasons with ritual; Midsummer celebrations in June follow the sunniest stretch of the northern year. India has long recognised six seasons, or Ritu, for farming and trade, and ancient Egyptians divided the year into flood, growth and low water according to the Nile's annual rise.
Source: Season
