Bible verses, physiology

Light Pollution

My favorite part of my work day in summer is my walk home to my car at night. After 12 hours of non-stop action in the ER, I revel in the silent moments outside in the dark, interrupted only the soft noise of crickets and the occasional car.

On particularly difficult days, I look up at the sky in prayer during my walk. My heart searches for answers to the unanswerable.  All I see are a few scattered stars in the sky. As almost a mockery, my mind almost always thinks of the Lord’s promise to Abraham:

Look up at the sky and count the stars, if you can. Just so will your descendants be.

Genesis 15:5

“If you can?” I think, despondently. I can count the stars, Lord. There’s about 25 on a good night, and that’s not counting the moon. That’s not a very good promise, I grumble.

Too bad my vision is incapable of perceiving all the stars actually in the sky. Too bad my vision is clouded with light pollution.

Astronomers estimate that the Milky Way has 400 billions stars, that the Milky Way is one of 170 billion galaxies, and that the observable universe has septillion stars. The naked eye, however, can only perceive on average 2500 stars at night. The Earth itself blocks about 1/2 the stars that would be visible to the naked eye, and the eye itself is only capable of seeing a fraction (a very small fraction) of the stars in the universe.


We do not have the visual acuity to see every single star in the sky. That should be a fairly obvious given. Dim stars that are billions of lightyears away just cannot be perceived by the eye. Oh well. 2500 seems like an adequate amount to be amazed.

But then why on the average night do I see a handful of stars when I am capable of seeing 2500?

The human eye uses different receptors on the retina during well-lit and poorly-lit circumstances. During the day and other well-lit times, the eye uses cone cells. The eye’s three different types of cone cells detect light in three different spectra (red, green, and blue). This vision is called photopic vision.

During the night and other poorly-lit times, the eyes uses rod cells. Rod cells are more sensitive to light than cone cells and can detect single photons of light. However, rod cells are limited to the short blue-green spectrum and not the longer red spectrum. Vision at this time is called scotopic vision.

When the eye perceives enough light to utilize cone and rod cells, it is called mesopic vision. In natural light, we use mesopic vision during twilight hours like the time around sunrises and sunsets.

However, under typical street lighting, we use mesopic vision. For the typical city dweller, from street lamps to advertisements, we have enough light sources to allow our eyes’ cone cells to continue to work. In essence, we’re not in enough darkness to see the light of the stars. Light itself is polluting our vision.

City lights are polluting our eyes’ natural ability to see stars in a recent phenomenon called light pollution. Its effects are still being studied, but if you look at satellite images of Earth at night, it’s obvious we’re bathed in light 100% of the hours of the day. Heck, it’s even difficult to see some of the tiny cities because the light of the big cities is so overwhelming.

In nature, full moons reduce our ability to see stars because of the overwhelming light of the moon. But a full moon is nothing compared to the limiting magnitude of city lights. In a large metropolitan city, the limiting magnitude can be +4, reducing the eye’s ability to see 2500 stars to the few brightest, even with the aid of a telescope.

To the naked eye at a limiting magnitude of +2 to +3, you might be able to see 25-50 stars. Remember, your eye is capable of seeing about 2500. Under city lights, you see 1-2% of what you’re naturally able to see, an even smaller fraction of the septillion stars in the universe.

But even the small fraction we can see in all its glory is amazing. 

My college roommate recently worked in Chile. During her time there, she visited a deserted place to stargaze with her co-workers. She raved about the sky and described seeing seemingly billions of stars in the sky. Scientifically, it was probably at most about 7000 with a limiting magnitude of +7. Too, the world still has a handful of places for authentic stargazing left in the world.

I’ve never had the opportunity to visit such a place, but last summer my brother and I took a boat across Lake Michigan. Under the few lights of the boat, I saw seemingly thousands of stars. Scientifically, it was probably at most about 800 with a limiting magnitude of +5.

Regardless of how many stars I actually saw, I was captivated by the sight. Despite the temperature and a small amount of rain, I spent a long time on the deck, admiring the skyline. The same line played through my head:

Look up at the sky and count the stars, if you can. Just so will your descendants be.

Genesis 15:5

I was in awe. I looked at the sky and could only imagine the consolation of God’s promise to Abraham.

But God’s promise still took faith.

First of all, Abraham was childless. His wife Sarah was barren. They had been trying for years to have a child. Everything looked hopeless, and here God was promising a child, children’s children, thousands of eventual offspring, an innumerable nation.

Second of all, the thing about this passage is God actually told Abraham to look at the night sky during the day. After God comes to him in a vision, delivers His promises to Abraham that he will have a son, offspring, a nation, and tells him to look at the stars in beginning lines of Genesis 15, line 17 starts off: “When the sun had set and it was dark.” So, clearly, everything else had happened during daylight hours.

Yes. God told Abraham to count the stars during the day.

During the day, during a well-lit circumstance when his cones were firing, his photopic vision was fully operating, and his eyes were probably addled by old age, scattering light and blurring his vision even more. Regardless if Abraham had cataracts or not, he did not see stars. Maybe the moon. I’ll allow that maybe Abraham saw the moon, but there’s no physical nor scientific way Abraham saw the septillion stars of the universe and could fully understand God’s promise to him.

And neither can we.

Light is constantly polluting our eyes. Even if we had a perfect night sky with no moon and could see thousands of stars, we’re still not even seeing a significant fraction of the septillion stars in the universe.

That’s Our Heavenly Father’s love for us. It’s so abundant, we cannot even begin to fathom it. That’s Our Heavenly Father’s promise for our future. It’s so overwhelming, we cannot even stare in awe at all of it.

Yet that’s Our Heavenly Father’s promise to us. We have to trust, hope, believe our promises are coming, even when it’s the middle of the day and night seems so far away, even when light at night clouds our vision, even when our human eyes cannot perceive all of it.

We have to believe there’s more love, more hope, more good in our future than we can both see and imagine.

We have to believe because we both incapable of perceiving all of it and our environment pollutes our vision.

And we can believe because we have a Father in heaven who loves us more than we’ll even truly know.

Look up at the sky and count the stars, if you can. Just so will your descendants be.

Genesis 15:5

Most nights, I am able to count the stars. The lamplights around the hospital pollute my vision, and my eyes are only so capable of seeing in the dark. But I know what I can see is an insignificant portion of the septillion stars in the universe. Even more, I know love I experience is a only a painfully small portion of the Love that awaits me both on this earth and in heaven.

For more information on photopic and scotopic vision, check out this interactive link.

For the background picture and more night sky pictures, visit Thierry Cohen’s “Darkened Cities” gallery.

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