Astro Bob: Sun shouts spring, but Minnesota keeps winter close
Spring officially begins on Monday, March 20, at 4:24 p.m. Central Time. Not that you'd know it in Minnesota.
DULUTH — Winter's my favorite season. I love to cross-country ski and warm up by a fire. Nothing beats the glitter of the season's constellations or the ferocity of a snowstorm. Melancholy, gray skies bring quietude and relaxation. But my arms ache from shoveling and piloting a wayward snowblower. Snow limits where I can set up a telescope or chase the northern lights. I'm ready for a rescue. I'm ready for spring.
Here in northern Minnesota, three feet of snow still cover my yard and fill the woods on this final day of winter. On March 20 at 4:24 p.m. CDT, the sun crosses the celestial equator headed north and spring begins. The celestial equator is an extension of Earth's equator into space. If you're standing on the equator, its celestial analog passes directly overhead. Someone standing on the equator at local noon on March 20 will see the sun shine straight overhead. Flagpoles cast no shadows.
The start of spring is also called the vernal equinox from vernal (spring) and the Latin aequus (equal) and nox (night). At the equinoxes day and night are approximately of equal length everywhere because Earth is sidelong to the sun (see diagram). Its axes point neither toward nor away from our star. In summer, the north polar axis points towards the sun; in winter it points away.
Notice I used the word approximately. Small differences in daylight length arise because the sun is a disk, not a point. Days are actually a bit longer than nights on the first day of spring because the sun's disk takes a couple minutes to completely clear the horizon at sunrise and an additional couple minutes to fully set. If it were a point instead of a circle it would rise and set instantly.
Further, the atmosphere bends sunlight especially at sunrise and sunset when the sun is at the horizon. The thicker air in the horizon direction "lifts" the sun into view a couple minutes before it actually rises and also keeps it in view a couple extra minutes at sunset, adding even more time to the day. Because the amount of bending depends upon air temperature and pressure, which varies from place to place, day-length at the equinox is unique from everyplace on Earth!
For Duluth, Minnesota, these effects combine to make daylight 12 hours 9 minutes long and night 11 hours 51 minutes. The two are only equal (or nearly so) on March 17. There's a special term for this day-night parity — equilux. Equilux depends upon your latitude. In New Orleans, for example, it occurs on March 16.
Other fun things happen at the equinox. Since the Earth is sideways to the sun, we see it rise due east and set due west. This makes it an excellent natural direction indicator. Ancient peoples built structures such as the temple at Chichen Itza in Mexico that may have taken advantage of the sun's position at the equinoxes. One of the most famous is a large sandstone slab etched with a spiral petroglyph at Fajada Butte in New Mexico's Chaco Canyon. Sunlight shines through an opening in a cliff on both the solstices and equinoxes to form brilliant daggers of light that slowly slice across the spiral.
The sun has been moving north and higher in the sky since the first day of winter. I can feel the difference on my face anytime it shines. With increasing altitude solar heating intensifies and becomes more effective at melting snow and heating the ground. While the snow around here looks formidable for now, it can only fight the sun's gaze for so long.
And if you'll excuse me, it's time to go skiing. Again.