Solstice Brings Late Nights, Bright Sights

Celebrate the June 20th solstice, when the Sun and the full Strawberry Moon combine their powers to illuminate both day and night.

Summer officially starts on Monday June 20th, the summer solstice, when the Sun crests to its highest point in the sky, making for the longest day of the year in the northern hemisphere.

Bob King

Welcome, summer! Welcome, late sunsets, long twilights, fierce heat, lazy afternoons, and the return of the sumptuous summertime Milky Way. The season begins on Monday June 20th at 22:34 UT, the moment when the Sun reaches its northernmost point in the sky. For North American time zones that's 6:34 p.m. EDT, 5:34 p.m. CDT, 4:34 p.m. MDT, and 3:34 p.m. PDT.

Since December, you and I have been on a journey of more than 292 million miles (470 million km) — half an orbit — from winter to summer. We've endured frozen fingers and cracked pipes to finally enjoy the simple pleasure of walking out the door in shirtsleeves and shorts every June. I love the ease of the season, and it all boils down to a simple astronomical fact — the tilt of Earth's axis. Whatever protoplanet came along billions of years ago to give ours a proper whack and tip us over on our side 23.5° — thank you.

During Earth’s yearly revolution, the northern hemisphere tilts toward the Sun at the summer solstice (left). The Sun rides high in the sky and days are long. At the same time, the southern hemisphere tilts away from the Sun at the start of its winter season or winter solstice. The seasons are caused by the tilt of the Earth’s axis and its changing orientation to the Sun during its yearly orbit.

Thomas G. Andrews / NOAA

During summer, the Earth’s northern hemisphere tilts toward the Sun, so the daytime star appears higher in the sky. Higher means it spends more time traveling across the sky from east to west, increasing the length of day at the expense of the night. Indeed, the June solstice marks the longest day and shortest night of the year for the northern hemisphere.

A higher sun also means that solar rays beam nearly straight down instead of from a low angle as they do in winter. Direct rays concentrate the Sun’s energy, heat the air and ground more intensely, and keep temperatures warm. All this extra energy and light is put to good use coaxing leaves from trees and making our gardens grow.

This map depicts circles of declination intersecting the western horizon in late June. True night begins when the Sun dips 18° below the horizon. Because the circles of declination intersect the horizon at a shallow angle around the solstice, Earth must spin a while longer before the Sun reaches the –18° mark. This makes for long twilights. The Sun doesn't stay at –18° (or lower) for very long because the lines of declination curve upward, leading the Sun back toward dawn after only a few hours of darkness.
Source: Chris Marriott's SkyMap

In winter, the Sun’s path follows strongly downward-curving declination lines. It reaches –18° much sooner than in the summer, the reason twilight is so much shorter.

Source: Chris Marriott's SkyMap

For amateurs, summer's late sunsets combined with lengthy twilights mean staying up late to observe under a dark sky. Where I live in Minnesota at 47° N, the Sun goes down at 9:06 p.m. on the solstice and rises just 8 hours later at 5:14 a.m. Subtracting the ‘tween time’ of twilight, true darkness lasts no more than 3 hours. The situation improves further south, but the difference between day and twilight length in summer vs. winter is dramatic.

On the first day of summer, the Sun reaches its maximum northerly declination; at the winter solstice, it reaches its minimum and appears furthest south and lowest in the sky. These seasonal ups and downs are a reflection of Earth's tipped axis.

Diagram: Bob King; Source: Stellarium

Our star travels along an invisible highway in the sky called the ecliptic defined by Earth's orbit around the Sun. You can picture the ecliptic by shooting a line from Earth through the Sun and extending into the starry sky beyond. As the Earth circles the Sun in a year's time, the line describes a circle in the sky. If the Earth rotated straight up and down, the Sun's path would never vary over the year, always rising in the east, reaching peak altitude and then setting in the west. Yawn.

But the planet's tilted axis guarantees that as we travel along our orbit, the Sun climbs to a maximum declination of +23.5° (in Taurus) on the first day of summer and sinks to a minimum declination of –23.5° (in Sagittarius) on the winter solstice. Taken together — axial tilt and revolution — the Sun cycles north and south in the sky as the seasons march along.

Einstein once said: "Reality is merely an illusion, albeit a very persistent one." The rising of the Sun, its steady stride across the zodiac during the course of a year, the seasonal teeter-tottering — all illusion. We might better describe the Sun, at least from Earth's perspective, as standing still.

Combine the 23.5° tilt of our planet's axis with its annual motion around the Sun, and the Sun appears to bob up and down from high to low (summer to winter) during the course of a year as viewed from Earth. At winter and summer, the Sun appears to stand still for a time as it reverses direction either north or south.

Diagram: Bob King; Source: Stellarium

Stop Earth's rotation and halt its revolution, and the Sun would sit still in exactly one spot in the sky, the location of which would depend on your latitude and longitude. Half the Earth would be in sunlight, while the other half would never see the Sun. Hours, days, years and centuries would pass with no apparent change in its position. Over the course of millennia, only the molasses-slow, shape shifting of the constellations on the planet's nightside would give away the Sun's true motion around the center of the Milky Way galaxy to the skywatcher's eyes.

"Solstice" comes from Latin and means "standing still." At both the summer and winter solstices, the Sun appears to briefly stand still in declination as it reverses seasonal directions. In winter, it stops traveling south in the sky and begins its northward arc; in summer, the Sun arrives at the top of its arc and begins moving south again.

If we could make the atmosphere disappear on the summer solstice, we'd see the Sun in Taurus above Orion.

Diagram: Bob King; Source: Stellarium

My old landlord Ed could always be relied on to remind everyone that whatever good weather coming would inevitably give way to bad. “Better start countin’ ’em,” he'd say, referring to a spell of pleasant days. Ed would fully appreciate the true meaning of the summer solstice as the time when the Sun begins its downward spiral toward winter.

Ever get asked why we can't see Orion in the summer? There's no better time than now to explain why. Around the time of solstice, the Sun shares the sky with the winter constellations, so they're lost in the glare of day. If we could magically remove the atmosphere and carefully block the Sun from view, we'd spot Orion on the meridian at lunchtime.

Watch for the full solstice Moon Monday, too! Click the photo to find the time the Moon rises for your city.

Bob King

If you like the flood of light that comes with the start of summer, you're going to love solstice 2016. By happy coincidence, the Moon will be full on the same day at 6:02 a.m. CDT (11:02 UT). Since the full Strawberry Moon will rise at the same time the Sun sets, we'll be chased by spheres of light both day and night.

Wishing you a happy and radiant solstice!