The Earth’s longest day is not the hottest day because a significant area of the Earth’s surface is water, and water takes a long time to heat up or cool down. This causes what astronomers refer to as “Seasonal lag,” where the warmest day lags behind the longest day. On Earth the lag varies from 15 days to as much as two months.
On other planets in the solar system, particularly the gas giants, there is also a great deal of seasonal lag. But on Mars, where there’s not much atmosphere and no liquid water, there’s basically no seasonal lag. The hottest day on Mars is the longest day.
The sun shining more directly into the hemisphere heats it up gradually. The heat buildup doesn’t dissipate completely at night, so the next day is a little hotter as a result. By the time you are at the longest day, only half of your summer days have come, so there is still more time for heat to build up until the sun isn’t shining as directly such that the heat can dissipate at night.
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