Vernal Equinox Means Equal Night

Mar 21, 2014

The Vernal Equinox has arrived! For one brief moment, everywhere on planet Earth, day and night are equal: 12 hours from sunrise to sunset and sunset to sunrise.

Sunset over Mt. Major.
Credit Abigail via flickr Creative Commons

The length of daylight compared to dark, is known as photoperiod. Seasonal changes in photoperiod  trigger a lot of changes in plants and animals. Many plants are known as short-day species; they flower after the summer solstice when days are getting shorter. Plants that bloom in spring are known as long-day species.

Geranium buds.
Credit s58y via flickr Creative Commons

But, not all organisms are driven by cycles of day and night. Geraniums that bloom year-round on kitchen windowsills are photoperiod-neutral.

In the animal world, photoperiod triggers hormonal responses that determine the timing of breeding, migration and hibernation. Birds mate in spring as longer days trigger reproductive systems, while deer and moose are short-day, late fall breeders. Photoperiod determines when moose antlers begin to grow in the spring and are shed in fall, about when snowshoe hares shed their brown fur for the white-tipped fur of winter.

Credit DenaliNPS via flickr Creative Commons

After a long winter of shorter days, the arrival of spring and later sunsets is a welcome reprieve. As a result, daylight gets most of our focus and that's how photoperiod is usually defined: as lengthening or shortening days. Throughout history we've celebrated the return of the sun, spring rebirth, the growing season and harvest that sustains us.

However, there is evidence, often overlooked, that length of dark for many plants and animals is more important. Dark triggers photoreceptor proteins in plants and melatonin in mammals. These communicate the duration of the dark—not daylight—to plants and animals that are photoperiod sensitive. 

Remember Equinox, translated as 'equal night', because that really sheds some light on the night.