Solar Radiation and Earth's Seasons

Earth's axis is tilted 23.5 degrees relative to the plane it orbits in, and that tilt never changes direction as Earth travels around the sun. For half the year the Northern Hemisphere leans toward the sun; for the other half it leans away. When a hemisphere tilts toward the sun, sunlight hits it more directly, concentrating the same amount of energy onto less ground, and the day is longer because more of that hemisphere's rotation happens in sunlight. Both effects pile on top of each other, which is why summer feels so much warmer than winter even though the sun itself hasn't moved closer.

Picture the orbit from the side. At the June solstice the Northern Hemisphere tips hardest toward the sun: longest day, most direct light, start of summer there, while the Southern Hemisphere has its longest night and starts winter. Six months later, at the December solstice, the tilt relationship flips. In between, at the two equinoxes, neither hemisphere leans toward or away from the sun, sunlight spreads evenly, and almost everywhere on Earth gets close to 12 hours of day and 12 of night.

This tilt mechanism is why latitude matters so much for climate. Near the equator the sun stays fairly direct all year, so day length and temperature barely swing with the seasons. Moving toward the Arctic Circle, the same tilt that gives New York mild seasonal swings produces extreme ones: 24 hour daylight in summer, 24 hour darkness in winter. That latitude gradient connects directly to the unit's next ideas about uneven heating driving global wind and ocean circulation patterns.