At exactly 6:07 a.m. EDT Thursday, the noonday sun’s daily march northward will suddenly halt and reverse direction, an astronomical phenomenon traceable to the tilt of Earth’s axis. We in the northern hemisphere hail that singular moment as the summer solstice – “solstice” coming from the Latin words for sun and stop – but few of us today understand its full significance.
Our ancient ancestors were far more connected than we are to the summer solstice, the longest day of the year. This is evident from the innumerable solstice-related monuments and traditions of nearly every nation and culture on the planet – from the stone temples of the Mayas and Aztecs to the bonfire festivals of northern Europe, practiced even now.
For ancient Egyptians, the summer solstice foreshadowed the annual flooding of the Nile River, whose water and silt nutrified the surrounding land. Thus, the solstice helped the Egyptians figure out the optimal time for planting their crops – a matter of no small importance.
More connected to the solstice than even humans are plants. Their cells contain microscopic amounts of phytochrome, a wondrous, light-sensitive pigment that actually enables plants to reckon the length of daylight.
When the length of daylight – the “photoperiod” – falls below a certain critical level, the leaves of a maple tree spontaneously start changing colors. The result is a breathtaking spectacle, especially in New England, with a scientific explanation I always enjoyed broadcasting during my time at ABC News.
The behavior of animals, too, is strongly tied to the length of daylight. For instance, when the photoperiod reaches 12.5 hours of sunlight per day – which in the U.S. happens in late March – the gonads of male and female hamsters instantly begin revving up. When the photoperiod falls below 12.5 hours of daylight in mid-September, the reproductive machinery begins shrinking and slowing down.
Length of daylight – which maxes out on the summer solstice – also triggers many other important animal traits and behaviors. Everything from the color of animal fur to the timing of animal migrations.
During my lifetime, scientists have also come to learn the huge importance of daylight to our own mental and emotional well-being. A recent, clever experiment evaluated the emotional content of tweets from 2.4 million people worldwide for nearly two years. Researchers found that, all other things being equal, as the length of daylight increases, so does a person’s general level of happiness.
Everything else being equal, therefore, the summer solstice is arguably the happiest day on Earth. The perfect mindset for participating in all those over-the-top solstice celebrations happening globally.
For me, however, it won’t be a happy day. Why in the world not? Call me quirky, but I always mourn the longest day of the year, because it marks the start of everything going downhill. On Thursday, a split-second after the summer solstice at 6:07 am EDT, the days will steadily grow darker and darker.
Until Dec. 21, that is, when the noonday sun stops its daily, southward march and reverses course again, producing more and more daylight. That’s the day – the winter solstice – when I will celebrate.