You might expect northern summer to be cooler because it occurs when Earth is farther from the Sun. Not so, explains Spencer. The oceans and land on Earth are not evenly distributed around the globe. This tends to moderate the impact of differences in sunlight between perihelion and aphelion. At aphelion the land-crowded northern half of our planet is tilted toward the Sun.
Sunlight is therefore more effective at raising the temperature of the northern hemisphere .
This results in the surprising fact that northern summer is a little warmer than southern summer even though Earth is farther from the Sun in July. On planets like Mars with orbits much more eccentric than ours, the varying distance to the Sun can have a big influence on seasonal conditions. For example, northern summer on Mars lasts 24 days longer than winter because the planet is close to aphelion when the north pole is tilted toward the Sun. This won't happen on Earth because our orbit is more nearly circular than the Red Planet's. Earth has one of the most circular orbits in the solar system.