How Seasonal Light Shifts Shape Life on Earth

Field report

Celebrate the longest day of the year with a crash course in how changing light drives everything from blooming flowers to bird song.

Ecology Correspondent has covered seasonal ecology and wildlife behavior for outlets including National Geographic, The Guardian, and Audubon Magazine.

Set your alarm clock, have a blanket ready, and check out some of July's celestial highlights below. Wishing you clear skies!

Take in the Beauty of the Full Thunder Supermoon (July 2/3)

It's hard to believe that a mere six months ago, those of us in the Northern Hemisphere were watching the last leaves fall and birds depart—and now suddenly, the sun is setting at bedtime and the woods are thick with birdsong and the unofficial peak of summer is upon us. How did that happen?! The winter and summer solstices sometimes feel out of whack with the biological seasons they represent—shouldn't the longest day of the year be the greenest and occur at the height of summer growth? Answers to that phenomenon and others are explained below.

(Note: For the Southern Hemisphere, this is all reversed, thanks to our topsy-turvy world.)

1. When Does the Summer Solstice Happen?

In North America, we can revel in the longest period of sunlight in 2023, on Wednesday, June 21, at 10:57 a.m. EDT (or the equivalent in your time zone). This is the precise moment when the sun stands still at its northernmost point as seen from Earth. Its zenith doesn't teeter north or south but sits patiently at the Tropic of Cancer before switching directions and heading south again. This is where the word solstice comes from; the Latin "solstitium"—from sol (sun) and stitium (to stop). For plants and animals, this celestial pause marks a turning point: growth begins to slow, energy shifts from leaves to seeds, and migration instincts start to stir.

2. There Will Be SO MUCH SUNLIGHT

Get your sunglasses out, feel the warmth on your shoulders, watch the garden respond! The meteorologists willing, we will have loads of sunlight. In New York City, we will have a soul-affirming 15 hours and 8 minutes between sunrise and set—and add a few hours of light on either end for when dawn actually breaks and darkness descends. (You can check your day length at the Farmer's Almanac sunrise and sunset calculator, to see what to expect in your neck of the woods.) For wildflowers, this means peak photosynthesis—fields of clover, yarrow, and vetch converting light into nectar at a furious pace, fueling the second generation of bees and butterflies.

4. The North Gets Short-Changed on Summer Sun

While it certainly may not feel like it, in the Northern Hemisphere's summer we are actually farthest from the sun thanks to the planet's tilt; we get 7% less sunlight than the Southern Hemisphere does during their summer. This is why northern ecosystems—boreal forests, tundra, and subarctic peatlands—have such short, intense growing seasons, cramming a year's worth of plant growth into barely 90 days. Something we'll be grateful for in a few billion years (see #7).

5. Despite the Sunlight, It Was Once a Dark Period for Understanding Nature

For centuries, the solstice was shrouded in superstition rather than science. People who studied the natural cycles of light and season were sometimes accused of heresy or witchcraft for suggesting that the Earth moved around the sun rather than the other way around. While early naturalists were eventually cleared of the most serious charges, they were often instructed to abandon their observations. In 1632, one such thinker published findings once again demonstrating that Earth orbited the sun. And once again was called before the Inquisition—this time found guilty. According to legend, the sentencing occurred on the summer solstice of 1633. The study of phenology—the timing of biological events—would have to wait centuries for proper scientific rigor.

6. It's a Day of Ecological Abundance

Comet C/2023 E1 ATLAS will make its closest approach to the sun on July 1 and then begin making its way back through the inner solar system. It will reach its closest point to Earth on Aug. 18, but will have significantly dimmed by then.To catch it at its brightest, use a pair of binoculars or a small telescope andlook toward the constellation Cetusin the evenings up to and after the 17th.

Stay Up for the Delta Aquarids Meteor Shower (July 28-29)

The solstice has been such a vastly important day throughout natural history that it would be impossible to list all the significant biological events here. From ancient harvest festivals onward, the day is more-so-than-not marked by abundance—including no shortage of ripe berries, blooming meadows, frog choruses at dusk, firefly spectacles, and the general hum of every living thing making the most of the light.

7. The Future of Seasonal Light Cycles Is Changing. Really, Really Changing

The gifts of the sun have driven the rhythm of life for millennia—and as it turns out, based on models of stellar evolution, the sun is about 40 percent more luminous today than it was when the Earth was born some 4.5 billion years ago. And it doesn't look like it's going to slow down. Scientists estimate that in another 1 billion to 3 billion years, the sun's looming intensity will "boil away Earth's oceans, turning our planet into an endless desert." In which case, the winter solstice will surely become the day every remaining organism huddles in the shade.

While the annual Alpha Capricornids meteor shower isn’t known for its prolific numbers of shooting stars, what it lacks in quantity it more than makes up for in quality. This shower, originating from parent body Comet 169P/NEAT, has a habit of producing exceptionally bright fireballs. On the evening of July 31, the storm will reach its peak, with viewing equally good on both sides of the equator. With the moon only 5% full, dark skies should make any Earth-grazing fireballs really pop.

According to a 2010 study of the Alpha Capricornids, it’s estimated that in 300 years time the bulk of the dust left behind by Comet 169P/NEAT will fully intersect Earth’s orbit, transforming it into “a major annual shower in 2220–2420 a.d., stronger than any current annual shower.”

The Return of the 'Ghost of the Summer Dawn' (July 30)

Orion the Hunter is a distinctive constellation during the winter months thanks to the three bright stars, Mintaka, Alnitak, and Alnilam, that make up its belt. On July 30, this constellation will make its eastern return in the early morning hours, an event beautifully nicknamed "the ghost of the shimmering summer dawn." The photo above shows a visualization of the constellation on the morning of July 31 just after 5 a.m. EST.