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NatureSpeak Articles

Equinoxes, Seasons, and Solstices

PHOTO BY MALLORY LAKINS. RISING SUN: The equinox is a reason to celebrate.

PERSONAL THOUGHTS about Daylight Savings Time aside, I love this time of year. March 20 saw the first of 2019's two equinoxes, this one heralding the spring freshet, warmer weather, and longer days. We Canadians know this equinox more colloquially as the first day of spring.

The progression of equinoxes, solstices, and seasons in general are caused by the tilt of the Earth away from vertical (relative to the plane on which we orbit the sun). This roughly-23.5-degree tilt means that between the March and September equinoxes, the Northern Hemisphere tilts towards the sun, bringing warmer weather for us and cooler weather for those "down under" the equator. Similarly, from the September to March equinoxes, the Northern Hemisphere points away from the sun, bringing shorter days and cooler weather to us northerners.

The word equinox derives from the Latin words aequus, meaning equal, and nox, for night. At the equinox, the Earth tilts neither toward nor away from the sun and everywhere on Earth experiences an approximately equal 12-hour night and day. During the period around equinoxes, compass bearers north of the equator will note the sun rising due east and setting due west instead of north of these ordinates in the summer and south in winter. From the spring equinox to the solstice on June 29, days lengthen—by four minutes each initially, then more slowly as summer solstice approaches.

The June solstice is the longest day of the year and is embraced by northerners as the first day of summer. At this point, the Southern Hemisphere is farthest from the sun, experiencing its longest night. The northern winter solstice—our longest night of the year—occurs on Dec. 21.

Some may remember from science class that the Earth's orbit around the sun isn't a perfect circle, but an ellipse. During this yearly circuit Earth passes a "perihelion," the point where it comes closest to the sun, and an "aphelion," at its farthest. Although it seems counterintuitive, the planet was at perihelion on Jan. 3, 2019, about three per cent closer to the sun than at aphelion. To complicate things further, even this revolution isn't set in stone—perihelion and aphelion drift by about a day every 58 years.

While equinoxes and solstices are mainly cultural markers for humans, many creatures use these yearly changes in the sun's position to trigger reproductive activity and annual migrations: Monarch butterflies appear to use a combination of the sun's position and magnetism as directional aids in their biannual migrations; longer days and shorter nights spur Saw-Whet owls to vocalize for mates; and spring equinox results in a surge in activity for some flowering plants—shorter nights stimulate the production of hormones that encourage growth and flowering in a process called photoperiodism.

In case you’re looking for other observable phenomenon in the sky this week, March 21 marks the final in a series of three supermoons, the next full supermoon won’t take place until March of 2020.

Written by: Mallory Lakins


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