��������

Some of us—like those who have overlooked the ocean—can see the sky all the way down to the horizon. Of course, most of us have our line of sight blocked at some point. But looking at the horizon can make a huge difference because every celestial object changes greatly when we see very low in the sky.

What Is the Horizon?

The horizon is that magical but invisible line that appears to separate our Earth from the sky. It’s the farthest place that we can see, where the sky and Earth touch. It seems as hard to reach as a rainbow or a pot of gold!

How Far is the Horizon?

The horizon is not as far as you might think. We know Earth is not endlessly flat (sorry, flat Earthers!). See my article on how fast the Earth spins—and why we don’t feel it. Our planet is an imperfect sphere (which bulges slightly at the equator).

So, we can mathematically figure out the distance. It simply depends on 1) your own height and 2) the terrain around you. There are some other factors, but let’s not get complicated.

• For a six-foot-tall person standing at sea level and looking out to the sea, the horizon is about 3 miles away. 
• The same goes for sitting in a canoe if you’re right on the water line. This is why tall ships had crow nests back in the day. 
• Of course, if you’re on the top of a mountain, you could see many more miles away. On a small mountain that’s 1,300 feet tall, you can see 44.2 miles. 

For the math nerds (including yours truly), here is a . 

Observing the Sky From the Horizon

Now, let’s get into observing the sky objects with a clear horizon. The first consequence of a very low elevation is that the star, planet, Sun, or Moon is dimmed big-time. Astronomers call this extinction. To truly make sense of this, consider how the atmosphere changes an object’s appearance. 

You wouldn’t think there’d be much of a difference simply because clean, dry air is wonderfully transparent to visible light. For years, I’d wondered whether astronauts orbiting above our air really get a super-view of the universe. After all, if you’re on the ISS (International Space Station), you’re forced to peer through 5-inch-thick fused silica windows. That much material must surely degrade the view. But does being in space make up for it?

I got my chance 25 years ago when I interviewed astronaut Andy Thomas, the commander of the first Space Shuttle mission that followed the tragic Challenger disaster. I knew that Thomas had been raised in the Australian Outback, far from city lights, and was an amateur astronomer long before becoming a professional pilot. If anyone in the world could answer my question, it would surely be him.

I got straight to the point. “Which gives you the best view of the night sky—the heavens you enjoyed when you lived in the outback or the view from space, looking through the windows of the Space Shuttle?”

He didn’t hesitate. “The number of stars is probably comparable from both vantage points,” he said, “But looking through windows always narrows the view, compared with the natural wide-open sky when you’re far from any city.”

Bottom line: You don’t have to go to space to get the best view of the night sky!

Challenges Observing on the Horizon

The biggest issue about observing on the horizon is that we’re looking through Earth’s atmosphere. The air contains not just water vapor, a transparent gas, but also innumerable tiny liquid droplets. These impart an overall whitening effect. People in the U.S. southeastern states see this routinely. There, a cloudless day sky is not deep blue at all but a pale milky blue or even white. That’s from all the tiny suspended water droplets.  

But let’s back up further and consider just how much extra air—and whatever is mixed with it, such as water droplets, dust, and pollution particles—you’re looking through when you observe a low-down object.

• The minimum amount of air between you and a star is encountered if it’s at the zenith or straight overhead. We call this “one atmosphere.” As we were taught in junior high school science class, each square inch of Earth’s surface sits beneath 15 pounds of air. When you instead gaze at a star that’s halfway up the sky, your slanted line of sight penetrates a longer path through the atmosphere, roughly equal to 1 ½ times as much air as when you gaze straight overhead.        
• When an object is truly low on the horizon—let’s say it’s the Moon about to set—its light must penetrate 40 atmospheres worth of air in order to reach your eyes. The air itself is quite transparent, as we’ve seen, but you’re also sighting through 40 times more water droplets, dust particles, and all the rest, and this dims its light by 11 magnitudes, a huge amount. That’s why it’s safe to watch the Sun setting, but it’ll harm your eyes when it’s higher in the sky.
• Other effects kick in, too. There’s something called Rayleigh Scattering. This is a scattering or bouncing around of the light from molecule to molecule, and it happens far more to shorter light rays (meaning violet and blue colors) than to warmer, redder colors. This helps explain why stars, the Moon, and the Sun all appear reddish when they’re very low. Having the blue light Rayleigh Scattering also explains why distant mountains have a bluish cast.

All told, low-down objects are dimmed so very much that you simply cannot see any stars at all when they’re very near the horizon. Try it the next time you’re camping. Look around the sky. From such a rural setting, the night is wonderfully flooded with stars. But look more closely. There’s a wide band all around the sky, a starless belt above the horizon in all directions. There’s only darkness there. That’s because that 11-magnitude dimming effect renders even bright stars invisible.

Wonders Happening at the Horizon

But it’s not all doom and gloom. Some delightful effects happen on the horizon, strange things you won’t see anywhere else. 

The Moon Illusion

A purely psychological effect befalls everyone who looks just above the horizon. Since you’re now seeing the Moon or constellations alongside earthly skyline objects in the distance, like faraway church steeples and such, the celestial object acquires a large relative size it didn’t have when the immensity of the celestial overhead hemisphere dwarfed it. 

Now, as autumn begins, the Big Dipper is at its lowest position of the year, barely above the northern horizon during the first few hours after nightfall. As a result, the adjective “Big” has become truly appropriate. The giant dipping spoon now looks simply enormous.

Whether applied to constellations or any other celestial object, this increased-size effect is called the Moon Illusion. Maybe because that’s the object people particularly notice when it’s low. After all, when something is high overhead, like winter full Moons at midnight, you’d have to crane your neck to see it, and how many of us walk around at night with heads cocked at a steep angle? But a low Moon hovers directly in front of us where it’s more likely to be noticed.    

Beyond its increased visibility when low, the Moon Illusion kicks in to make every horizon-hugging Moon seem reliably gigantic. Did you see the Harvest Moon in September, when the Moon is occasionally a bit closer to Earth? 

Since the reddening effect is also happening then, it’s common to observe the entire combo plate—a low, huge-looking orange Moon, one of the sky’s most eye-catching spectacles. Learn more about the Moon Illusion in my article.

The Green Flash

Many have heard of the Green Flash, but too few people deliberately look for it. It happens. Yes, it’s right on the horizon—when the Sun has almost finished setting, and that orange ball is almost completely below the horizon. 

The tiny remaining topmost piece of reddish Sun, a mere point-like speck of Sun, will sometimes turn an intense emerald for one to five seconds before it too sets. It doesn’t usually flash, meaning it gets more brilliant. Rather, it’s a quiet phenomenon, with the orange dot merely turning green with no fanfare. 

The key to catching it is having a truly unblocked ocean horizon. I’ve looked for it at every such sunset, perhaps 250 times in total, and clearly seen it about 25 times. If my experience is typical, expect to see the green flash perhaps one out of ten times you look for it.

What causes such a strange effect? Well, the refraction of light causes the Sun’s various colors to bend at slightly different angles. On the horizon, this refraction effect is at its strongest, with the sun’s red image as its lowest-down one, its overlapping orange persona a bit higher, yellow still higher, and so on. The uppermost solar image is blue, but the long path through the air at sunset has totally scattered out the blue light so that the actual uppermost solar image is its green one. 

Hence, if conditions are just right—meaning the air is homogeneous or the same temperature between you and the setting sun—all the other colored versions of the sunset before the final uppermost green one, and it’s this final topmost piece of the green Sun that produces the Green Flash.

Chasing Our Shadow

My favorite horizon phenomenon happens every clear evening. The Sun makes us all cast shadows, and our planet Earth casts a shadow, too. This must, of course, play out opposite from the Sun’s location, and so it does. 

So each day at sunset, if you put your back to the setting Sun so that you’re facing east, you’ll see a dark blue-gray band hugging the horizon. This is Earth’s shadow heading out into space! It’s sometimes called the Twilight Wedge, and the pink sky above it, where the Sun is still shining, is sometimes called the Belt of Venus. 

But never mind those terms. Just be sure to look east any clear evening around sunset and be aware of the truly epic nature of that horizontal dark band overlying the eastern horizon. It’s never absent!

When we gaze at the horizon on the beach or from a mountain, we also widen our peripheral vision, as our ancestors did when scanning the seas or land for threats. Knowing it’s all calm ahead also calms us. 

So, gaze at our horizon and enjoy its natural wonders—conveniently situated straight ahead.