If you ask most people why it’s hotter in summer, a surprisingly large percentage will say “because the Earth is closer to the Sun.” It’s a logical-sounding answer. It’s also completely wrong — and the actual answer is one of the most elegant explanations in basic astronomy.
The Wrong Answer (and Why It Seems Right)
The Earth does have an elliptical orbit — it’s not a perfect circle. At its closest point to the Sun (perihelion, around January 3rd), Earth is about 91.4 million miles away. At its farthest (aphelion, around July 4th), it’s about 94.5 million miles away. Notice the dates: Earth is actually farthest from the Sun in early July, in the middle of the Northern Hemisphere’s summer. Distance from the Sun cannot explain summer heat — if anything, it slightly moderates it.
The Real Answer: Axial Tilt
Earth’s axis is tilted at approximately 23.5 degrees relative to its orbital plane. This tilt doesn’t change as Earth orbits the Sun — the axis always points toward Polaris (the North Star). As a result, different hemispheres receive different amounts of sunlight at different times of year.
During Northern Hemisphere summer (roughly June through August):
- The Northern Hemisphere is tilted toward the Sun.
- Sunlight strikes at a more direct (steep) angle.
- Days are longer — more total hours of sunlight per day.
- Both effects together produce significantly more solar energy input per day.
The Angle Effect: Why It Matters More Than Distance
When the Sun is directly overhead (high in the sky), the same amount of solar energy is concentrated on a smaller area of ground. When the Sun is low in the sky (at an oblique angle), the same energy is spread over a much larger area — like the difference between shining a flashlight straight down versus at an angle.
This is why the same city is dramatically hotter on a June afternoon than a December afternoon even though the distance to the Sun barely changes. It’s also why the poles are cold even in their “summer” — the Sun never gets very high in the sky there, so solar energy always arrives at a shallow angle.
The Day Length Effect
In New York City, a summer day in June has about 15 hours of daylight. A December day has about 9 hours. That’s 6 additional hours of solar heating per day, accumulated over the entire summer. Even if each hour of sunlight were identical in intensity, the longer days alone would produce warmer temperatures. Combined with the angle effect, you get summer.
Why the Southern Hemisphere Is the Opposite
When the Northern Hemisphere is tilted toward the Sun (summer), the Southern Hemisphere is tilted away (winter). Seasons are literally the same phenomenon viewed from the opposite side of the planet. This is why Australians have Christmas in summer and ski season in July — their seasons are reversed relative to ours.
Why Is There a Lag?
The longest day (summer solstice) is around June 21st, but the hottest month in most places is July or August. This lag exists because land and water take time to absorb and release heat. The atmosphere and oceans store the summer’s accumulated solar energy and release it gradually — the same reason coastal cities are milder than inland ones.
Sources: NASA Earth Observatory — Milankovitch Cycles and Ice Ages. | NOAA Climate.gov — Why are seasons? | Williams, J. (2009). The AMS Weather Book. University of Chicago Press.