Robin Song, Slanted Earth, Must Be Spring

A robin dashes across a snowy yard in Duluth, Minn. in search of open ground and food. Credit: Bob King

I really, truly heard my first robin today, the first day of spring. The lilting, see-saw song made me stop in my tracks; I just listened, soaking in the melody of the new season. Spring began overcast and early at 5:28 a.m. (CDT) in my neighborhood but has since turned sunny and surprisingly warm. I fear the snow on the ground won’t be with us long. Part of me still hopes to cross-country ski one last time.

Earth’s axis maintains a 23.5° tilt as it orbits the sun, but its changing position in orbit causes the axis to point toward, away and sideways to the sun during the year. Days and nights aren’t precisely equal at the spring and fall equinoxes because atmospheric refraction can extend daylight a bit at the expense of night. Credit: Sonoma University with additions by the author

As the Earth makes its yearly orbit around the sun, each pole is sometimes tilted toward the sun and sometimes tilted away. In summer, the north pole is tilted toward the sun. We feel the heat because the tilt causes the sun to rise high in the sky. Its light is not only more direct — it’s practically shining over our heads! — but it takes many more hours to travel that high arc between rising and setting, making for long days.

In winter, the north pole is titled away from the sun, so it appears lower in the sky and describes a much shorter arc between sunrise and sunset. Low altitude spreads its light out, making for both less intense sunlight less intense and short days. When one pole is tipped toward the sun, the other is tipped away and vice versa. But not today!

Because of the 23.5° tilt of Earth’s axis, the altitude of the sun varies cyclically across a year. In winter (lowest arc) it’s 23.5° below the celestial equator, while in summer it’s 23.5° above. At the spring equinox, it crosses the equator moving north; in fall it crosses the equator again headed south. Illustration: Bob King, Source: Stellarium

On the spring and fall equinoxes, the north and south poles are tipped neither toward nor away from the sun but instead receive its light equally from the side. Except for the north and south poles, where the sun crawls around 360° of horizon and remains visible for 24 hours, day and night are approximately equal across the whole planet: 12 hours daylight and 12 hours night. That’s why the first day of spring is called the equinox, a Latin mashup of “equal” and “night.”

On the first day of spring, the sun, which has been moving northward and higher in the sky (as seen from the northern hemisphere), crosses what’s called the celestial equator. The celestial equator is an imaginary circle across the heavens directly above the Earth’s equator. Picture it as a projection of our equator into the sky.

Standing on Earth’s equator, the celestial version starts at the eastern horizon, climbs to cross the zenith — the point directly overhead — and drops down to meet the western horizon. It then continues below the horizon to the point directly beneath your feet before arching back up to the eastern point on the horizon. One complete circle.

I couldn’t find a full-Earth color photo for March 20 (not available yet!), but this is our lovely planet on March 18 on the cusp of the new season as photographed by the DSCOVR satellite. The bright spot at center is the sun’s reflection in the Pacific Ocean. Credit: NASA

So if you’re standing somewhere along the equator on the spring equinox, when the sun crosses the celestial equator, the sun rises due east, passes directly over your head and sets at the due west point. From a point midway between the equator and north pole (45° latitude north), the sun still rises due east and sets due west, but the celestial equator is likewise midway between the zenith and the southern horizon.

Things get more interesting at the north pole. There the celestial equator runs all along the horizon for 360°. On the first day of spring, the sun, which has been south of the equator since the start of fall and invisible for six months, climbs northward to sit exactly on the celestial equator. A north pole observer would see it scrape its way around the horizon and never set — the start of the 24/7 daylight season.

The sun continues to move northward and higher in the sky for northern hemisphere skywatchers until the first day of summer, June 20.

A simple song, the robin’s, but rich with celestial portent!