There's an invisible hill in the sky. And tonight at 10:32 p.m. CDT the sun reaches its summit after a laborious 6-month climb. That pinnacle marks the arrival of summer, with its seemingly endless sunshine, abundant life and drippy ice cream cones. Days are now at their longest and nights shortest. Here in Duluth, Minn., the sun will shine for 15 hours 52 minutes, with just 8 hours 8 minutes allotted to the stars. Accounting for twilight, which lingers for 2 1/2 hours on either side of sunset and sunrise, true darkness amounts to little more than three hours.

Nights contract further as you head north. At the 49th parallel, which defines much of the Canada-U.S. border, twilight lasts all night. Keep traveling poleward to the Arctic Circle (latitude 66.5°), and the sun shines for 24 hours on the solstice.

This photo combines separate time-lapse photographs taken (from left) on the summer solstice, fall equinox and winter solstice from southern California. In summer, the sun rises in the northeastern sky; due east on the first days of fall and spring, and to the southeast during the winter. Solstice suns rise 23.5° north and south, respectively, of the equinox sun. (John Krieger / astronomyforthinkers.com
This photo combines separate time-lapse photographs taken (from left) on the summer solstice, fall equinox and winter solstice from southern California. In summer, the sun rises in the northeastern sky; due east on the first days of fall and spring, and to the southeast during the winter. Solstice suns rise 23.5° north and south, respectively, of the equinox sun. (John Krieger / astronomyforthinkers.com

Today marks the culmination of the sun's climb from its lowest point in the sky at the winter solstice to its highest point on the first day of summer. In winter, the sun rises in the southeastern sky and slowly moves north (left as you face east) in the days and weeks that follow. On the first day of spring, it pops up due east, and by June it's happily settled in the northeast. Its swing from southeast to northeast and back again during the course of a year is caused by the tilt of the Earth's axis.

If the planet rotated straight up and down, the sun would rise at the same point on the horizon every day of the year. Instead, its position varies by 23.5° south of due east at the winter solstice to 23.5° north of due east at the summer solstice. That's a total of 47°. Since 10° is equal to one balled fist held horizontally at arm's length, the sun moves nearly five fists along the horizon from winter to summer. That's a lot of traveling (see photo above)! It's also something you can easily record. Take a photo of the sunrise sun now and again on Dec. 21st, then composite them to see for yourself.

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As the Earth plies its orbit, the orientation of its axis changes to the sun. In summer, it's tipped toward the sun, and in winter it's tipped away. The direction the axis points and its tip remain the same all around its orbit. (Sonoma University)
As the Earth plies its orbit, the orientation of its axis changes to the sun. In summer, it's tipped toward the sun, and in winter it's tipped away. The direction the axis points and its tip remain the same all around its orbit. (Sonoma University)

That number, 23.5°, should ring a bell. That's how much our planet's axis is tilted from the vertical. During the course of a year, as Earth's orientation to the sun varies day by day, that number (Earth's tilt) gets expressed as the continually changing position of the sun along the horizon at sunrise and sunset.

The sun's path across the sky varies with the season, shown here for an observer in the northern hemisphere. In summer it shines at a high angle compared to winter and spends many hours above the horizon. Autumn and spring fall in between. (Slancestoene / GNU public license with additions by the author)
The sun's path across the sky varies with the season, shown here for an observer in the northern hemisphere. In summer it shines at a high angle compared to winter and spends many hours above the horizon. Autumn and spring fall in between. (Slancestoene / GNU public license with additions by the author)

During northern hemisphere winter, the north polar axis is tipped away from the sun, so our favorite star follows a low, short path in the sky. Fewer hours of daylight combine with low elevation to bring on the season's familiar chill. In summer, the axis is oriented toward the sun as if we were bowing before its presence. A high path and long hours of daylight combine to bring on the heat.

I mentioned earlier that the tilt of the axis plays out across the year as Earth orbits the sun. Did you know that the sun is also moving east as it moves north or south? As we circle the sun and look back toward it, it appears to slide about 1° to the east each day against the background constellations. One degree is equal to the amount of sky covered by your little finger held at arm's length. Of course, the sun isn't physically moving; its eastward motion is just a reflection of the Earth's orbital motion.

At the same time the sun is moving east due to Earth's orbital motion, it's also traveling north and south over the year because of the planet's tilted axis. Both motions fashion the sun's tilted, circular path called the ecliptic. It looks like a wave here because it's shown in just two dimensions. (Stellarium with additions by the author)
At the same time the sun is moving east due to Earth's orbital motion, it's also traveling north and south over the year because of the planet's tilted axis. Both motions fashion the sun's tilted, circular path called the ecliptic. It looks like a wave here because it's shown in just two dimensions. (Stellarium with additions by the author)

Here's the cool part. When you combine the sun's up-and-down (north-south) movement, caused by Earth's axial tilt, with the steady eastward drift from Earth's orbital motion, its yearly path becomes a tilted circle around the celestial sphere called the ecliptic. The ecliptic crosses the 12 zodiac constellations, the reason these groups are special to us.

When you take a circle that's tilted in three dimensions like the ecliptic and project it onto a flat map, it looks like an undulating wave (above). The same thing happens when you flatten the circular orbit of the space station onto a piece of paper.

In this fanciful illustration, Carlton Peak in northern Minnesota helps us visualize the sun's travels along the ecliptic. Its changing altitude across the year — caused by Earth's tilt — causes the seasons. (Bob King)
In this fanciful illustration, Carlton Peak in northern Minnesota helps us visualize the sun's travels along the ecliptic. Its changing altitude across the year — caused by Earth's tilt — causes the seasons. (Bob King)

Now that the sun has reached its greatest height in the sky, it can only go one way — back down the ecliptic in the direction of winter. Even the biggest, brightest, hottest thing in the solar system can only be king of the hill for so long.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.