axial tilt changes the angle and hours of sunlight; distance from the sun is not the reason
Summer is not hotter because Earth is closer to the Sun. It is hotter because Earth’s axis is tilted, and during summer your hemisphere leans toward the Sun, so sunlight arrives more directly and daylight lasts longer.
Earth’s tilt is about 23.5 degrees from vertical. That tilt changes the angle at which sunlight hits the surface. When the Sun is high in the sky, the same beam of light is concentrated over a smaller patch of ground. When the Sun is low, the same beam spreads across a larger area, so each square meter receives less energy. The tilt also gives your hemisphere more hours of daylight in summer, adding the Sun’s heat over a longer period each day. Both effects are caused by the same tilt, and they work together to produce warm months.
Consider a location at 40° north latitude. At the summer solstice, the Sun’s noon elevation is 90° − 40° + 23.5° = 73.5°. At the winter solstice, it is 90° − 40° − 23.5° = 26.5°. The intensity of sunlight on a flat horizontal surface is proportional to the sine of the solar elevation angle. The sine of 73.5° is about 0.96, while the sine of 26.5° is about 0.45. So at noon, summer sunlight delivers more than twice as much heat per square meter as winter sunlight. Day length adds to the difference: at 40° latitude, a summer day lasts roughly 15 hours, while a winter day lasts about 9 hours. Multiply the intensity by the daylight hours, and summer receives roughly three and a half times as much total solar energy per day as winter. That gap shrinks closer to the equator and grows toward the poles.
Many people believe the Earth’s elliptical orbit is responsible for seasons. It is not, for two reasons. First, the Earth-Sun distance changes by only about 3 percent throughout the year, which has a small effect on solar heating. Second, the timing is backward: Earth is closest to the Sun in early January, during the Northern Hemisphere winter. If distance were the main driver, January would be the warmest month in the north, and it is not. The tilt overrides any small distance effect.
The tilt explanation works best for middle latitudes, where the four seasons are clearly defined. At the equator, the Sun is high all year and day length barely changes, so seasons are controlled more by rainfall patterns than by temperature. At the poles, the tilt produces months of continuous daylight and darkness, making the seasonal swing far more extreme than anywhere else. The tilt also interacts with the atmosphere and oceans, which redistribute heat, so a specific place’s climate can be warmer or cooler than the simple angle calculation would suggest. But the basic rule remains: axial tilt determines the Sun’s direction, and the Sun’s direction determines how much heat your part of the planet receives.
Cram I always thought summer is hotter because Earth is closer to the sun.
Rep Common belief. And it is completely wrong. Earth's orbit is nearly circular.
Cram Wait, it is not? I learned the orbit is an ellipse.
Rep It is slightly elliptical. But the distance change is only three percent. That barely affects temperature.
Cram So what actually makes it hotter?
Rep The tilt of Earth's axis. Twenty three point five degrees off vertical.
Cram How does leaning change the temperature?
Rep In summer your hemisphere tilts toward the sun. Sunlight hits at a steeper angle, more concentrated.
Cram So the same energy hits a smaller area?
Rep Exactly. Direct light is intense. Angled light spreads out, so each patch of ground gets less heat.
Cram And the sun is up longer in summer too.
Rep Right. Longer days mean more hours of heating. More total heat added every day.
Cram OK so it is the angle of sunlight and the daylight hours combined.
Rep Both come from the same source: axial tilt. The tilt gives you direct rays and long days. Distance from the sun is irrelevant.