Equinox in the Tropics

Snowy Egrets gathering in Maine for migration. [hr gap=”1″]

September 22 was the autumn equinox. In the northeast the days are gradually getting cooler. The birds are gathering to make their way south and trees are beginning to drop their leaves in preparation for winter.

What does it mean for St. John? There aren’t the same sorts of seasonal changes here affecting the birds and trees. Really, the most important climate factor is the end of hurricane season, and that isn’t until November.

One thing I like to notice is migrating birds coming down from the north – some staying for the winter and others heading further south.

Lesser Yellowlegs in Fish Bay

Birds get ready to migrate due to genetic predispositions, plus environmental triggers. I recently learned that the most important trigger is the length of the day, not falling temperatures. Interestingly, birds perceive changes in daylight not through their eyes but through sensors buried deep in their brains that respond to variations in the tiny amounts of light that penetrate their skulls.

Trees, too, take their cues from changes in the length of the day in the north, since the earth’s orbit is more reliable than yearly temperature patterns as an indicator of the coming change in the seasons.

At the equinox, the length of the day and night is pretty much the same all over the world.

I had to review the science of how that works. The earth’s axis is tilted at a fixed angle of about 23.5° as it travels around the sun. That means the northern part of the earth is sometimes tipped towards the sun (summer), and sometimes away (winter). As the orbit continues, there are two points when the earth’s tilt is sideways to the sun, and then both northern and southern hemispheres receive roughly equal amounts of sunlight. After the September equinox, it is the southern hemisphere that will be tilted more and more towards the sun, while the north heads towards winter.

Photo courtesy of http://spaceplace.nasa.gov/seasons/en/.
Photo courtesy of http://spaceplace.nasa.gov/seasons/en/.

At the Equator the amount of sunlight and the length of the days and nights are pretty much always the same.

St. John is 18.3° north of the Equator. That puts it within what is called the tropical zone, which extends 23.5° north and south of the equator. This is the area of the earth’s surface that gets the most direct sunlight all year.

Starting in late September, St. John does experience some changes due to the shifts in the earth’s orbit. The days are about two hours shorter in the winter, and the water temperature can go as low as 78°F. Brrrr.

But St. John’s seasonal climate changes are more affected by other factors than the amount of sunlight – like the easterly flow of the trade winds carrying hot Saharan air off the coast of Africa, interacting with warm ocean temperatures in the Atlantic to cause tropical storms. And in the winter the Bermuda High, a semi-permanent area of high pressure to the north, sends down cool ‘Christmas winds’ between December and February.

The northern experience of the autumn equinox involves a certain amount of anxiety about adjusting to the dark and cold. Winter is coming! Yes, yes, there is the magic of rebirth, flower buds and birds singing in the spring, but I am sure my Swedish ancestors did not relish those long months without daylight, wondering if the sun, and the birds, would really come back this time.

Maybe it is better to live in a climate with fewer spiritual lessons based on cycles of loss and renewal.

I like to think of the birds as returning to St. John. At least some of them spend more time here than in the north – only leaving when the spring explosion of insects and plant growth in the north provides them with abundant new resources for breeding. A long trip, a lot of hustle, and then happy to be back when they are done. Coming where the weather suits their souls…

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Gail Karlsson HeadshotGail is an environmental writer and photographer, and author of The Wild Life in an Island House. gkarlsson@att.net. Photos provided by Gail Karlsson.