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The Summer Solstice


Last updated 7/18/2023 at 12:22pm

Though the sun spends the longest time in the sky on the day of the summer solstice, that is not the hottest day of the year.

Long ago, when I was in grade school, I counted the days until the start of summer. There was always something exciting to me about that time of year. Perhaps it was that I would have no homework to do for nearly three months or that daylight would be longer so I could play outdoors later in the evenings.

Whatever the reason, summer has always been special to me. Back then I lived on the U.S. East Coast, where a hot summer day might occasionally reach 90 degrees F with a dripping wet 80% relative humidity. Today I live in the deserts of Southern California, where summer temperatures regularly exceed 110 degrees F. And, yes, the dry heat here is much more pleasant!

With the first day of summer coming this week, I'm absolutely giddy with delight! The moment it begins – known to astronomers as the summer solstice – occurs this year at 10:58 a.m. EDT (7:58 a.m. PDT) on June 21.

The summer solstice marks the time when the sun reaches its northernmost position over our planet and takes its longest path across the daytime sky. Depending on your latitude, the sun can remain in your daytime sky for 15 hours or more – plenty of time to heat our air to summertime temperatures.

You might ask, if the sun spends the most time in our sky around the solstice, why isn't that day the hottest of the year? And that's a legitimate question. The answer is more intuitive than you might imagine. It's simply because our atmosphere doesn't respond to temperature variations instantly.

Imagine placing a pot of cool water on a hot burner; you know that it won't heat up instantly and you make allowances for that when cooking. The same is true of our atmosphere, so the hottest days in the Northern Hemisphere typically occur not around the solstice, but in late July or August.

We owe all this seasonal stuff to the fact that our planet's equator is tipped by about 23.4 degrees to the plane of its orbit around the sun. This means that, during this time of year, the Earth's Northern Hemisphere is tilted toward the sun, causing the sun's rays to shine down on us more directly. Six months – and half an orbit – later, our planet's tilt aims the Northern Hemisphere away from the sun and the Solar rays impact us at a shallower angle.

The summer solstice marks the end of the sun's northerly climb in the midday sky, and the beginning of our star's midday descent once again. The term "solstice" comes from two Latin words: "sol" (meaning "sun") and "sistere" (meaning "to stand still"). And, if you were to watch the midday sun for an entire year, you'd see it stop its annual climb.

This time has been celebrated by cultures throughout the ages. Even today, farmers appreciate it as a time when they're busy harvesting those tasty summer fruits and veggies we all love.

So, even though I no longer have my summers off, nor do I get to play outside later in the evenings, I still get excited about the arrival of summer!

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