Unless you've spent quality time on the International Space Station,

this is probably not a view you are super familiar with.

This is the east coast of the United States.

That's New York down there in the lower right,

and it's a band of light all the way up through Washington DC.

Those cities are shining like jewels,

highways are traced by webs of light.

And all of that light is super photogenic.

But there's a problem.

That light is meant to be illuminating our sidewalks,

and our streets and our houses.

Instead, it's actually going up into the sky

and out into the universe,

where it's not doing any of us any good.

When I see photos of this, of the Earth,

I see environmental catastrophe.

Those aren't jewels,

those are tumors.

I'm an astronomer,

so it's really no surprise probably to anyone

that I've always loved the night sky.

I'm kind of a walking cliché.

But when I was growing up in Minnesota,

one of my favorite things to do on a summer evening

was grab my old Raggedy Ann sleeping bag

and take it out into a field behind my house,

where I would spend hours looking at the night sky.

And to do this, I had to brave not only the darkness,

but also swarms of mosquitoes,

and my sleeping bag really didn't smell very good.

(Laughter)

But there was one particular star

that I would look for, night after night.

And then I would play this game

where I would try to focus on that star so intensely,

that everything else would fade from my view

and that single star would be all that I could see.

I could only ever hold on to that focus

for a few fleeting moments.

But when I did,

I felt this deep sense of connection to the universe.

And almost a sense of vertigo,

like I was going to fall into space.

And when this happened --

I know this sounds kind of ridiculous,

but I would simultaneously feel unfathomably insignificant

and also kind of weirdly important.

That star I looked to night after night was called Vega.

Vega is the brightest star in the constellation Lyra,

which is not coincidentally the name of one of my dogs.

(Laughter)

But this experience is being lost.

My favorite constellation, Lyra,

this is what it would look like from Manhattan.

For people who live in urban and suburban environments,

if they go outside at night and look up,

instead of being awestruck by the majesty of the universe,

they see pretty much nothing.

These unremarkable, completely blank night skies,

of course are due to all of the light we produce at night.

Those very same lights we see all the way from space

are shining up into the atmosphere,

where they bounce around and create this featureless smog of light.

And that featureless smog of light has a name.

It's called light pollution.

As an astronomer,

I can actually tell how bad light pollution is

by the brightness of stars I can see in the sky.

And it turns out

that when you're trying to unlock the secrets of the cosmos,

it's really helpful to be able to see the cosmos.

And --

[Laughs]

Truth.

And this light that we're trying to detect

is coming from millions or billions of light-years away,

and so it's generally pretty faint.

And as an astronomer,

I fight with this every day to do my job,

and I have to tell you, it is a really big problem.

But the problem is far worse

than just losing some whimsical ability to gaze at the stars.

For example,

countless plant and animal species are affected.

So we could talk about sea turtles

or pollinators

or any of these super important species

that are also cute.

Instead, I want to talk

about these quietly unassuming dog whelks.

You may have seen them around

and not given them really a whole lot of thought.

But they're pretty cool.

So in an entire year

a dog whelk will rarely move more than about 10 meters.

That means that when they are attacking their prey

they can hit this brisk pace of about a millimeter an hour.

And --

(Laughter)

This works out OK,

because they attack things like barnacles.

(Laughter)

So these dog whelks live in the intertidal area of coasts,

where, it turns out,

they're a pretty key part of the ecosystem.

Not only are they one of the most dominant invertebrate predators,

but other animals, like crabs and birds, think they're pretty tasty.

So that leaves these poor snails in a kind of precarious situation,

because if they go too low in the water,

then crabs are a threat,

but if they come out of the water too far,

birds are going to have a feast.

Why is an astronomer telling you about dog whelks?

I ask that myself.

Because their behavior is impacted by light pollution.

For example,

if dog whelks are subjected to artificial light at night

they're about twice as likely to stay under the water with a predator.

And that puts them at increased risk.

And it's not like they can make a speedy escape.

And so these --

(Laughter)

And the other issue is because they literally move

at a snail's pace.

If a population is wiped out,

it can take decades to replenish.

And that, in turn, affects the rest of their ecosystem

and the other species, like the birds and the barnacles and the crabs.

So this is just one small and slimy example

of how light pollution can unleash a cascade effect

on an entire ecosystem.

Virtually every species that has been studied to date

is impacted by light pollution.

And that includes humans.

So let's talk about us.

You are probably not surprised to hear

that light pollution can affect your ability to sleep well at night.

But you might be surprised to hear that light pollution is linked to obesity.

In fact, in a recent study

they found that light pollution contributed to over 70 percent

of the obesity rates in 80 countries.

More than that,

light pollution actually contributed about the same amount to excess weight

as eating junk food.

And it gets worse.

For people who are subjected

to significant amounts of artificial light at night

they're about 50 percent more likely to get breast cancer.

And in fact, light pollution is correlated with types of cancer

across the board.

And in controlled lab experiments

there's a direct link between increased artificial light at night

and a rate of tumor growth.

You might be wondering how normal light

could possibly impact cancer rates.

It likely all comes down to the super important hormone

called melatonin,

which we have evolved over millions of years

to produce on a day-night cycle, or a circadian rhythm.

What happens is that when light impacts

the retina at the back of our eye at night

it can disrupt melatonin production,

and when melatonin production is disrupted,

a whole chain of other chemical processes are affected,

and that includes estrogen production.

And when we throw this chemical balance out of whack,

really bad things can happen.

In fact, things are so bad,

that the International Agency for Cancer Research

has said that disrupting the human circadian rhythm

is a probable carcinogen.

Also, for fun, I want to let you know

that light pollution has been linked to,

let's see -- headaches, anxiety, depression, diabetes,

cardiovascular disease and the list goes on.

But maybe you don't care about your health.

We're all going to die anyway,

you might as well die in a brightly lit room.

(Laughter)

The fact that you're laughing about death is kind of amazing.

(Laughter)

You might still care about money.

The money that's spent on that wasted light,

and I mean just the light that's going out into the universe,

and not doing us any good,

is three billion dollars a year.

That's enough money to build, like,

1,000 utility-grade windmills,

or fund the entire DC public-school system for over two years,

or -- this is my favorite, because I really want one

but I can't afford one --

buy 30,000 Tesla Model X SUVs.

(Laughter)

And that includes the electric car tax credit.

And then there are the existential costs.

I don't have any data

on how losing touch with our place in the cosmos

impacts us.

But I believe

that this probably impacts our humanity

more than any of the other scary statistics I can share with you.

And it's getting worse with time.

The amount of light pollution is doubling roughly every 35 years.

That means that within the next decade

virtually the entire eastern half of the United States

will be perpetually brighter than twilight.

And there's another issue with light pollution.

The problem is way worse than we can see with our own eyes.

Our eyes have evolved to just detect this tiny range

of the full spectrum of light.

All of this other light that we can't see,

this invisible light,

also has a pollution problem.

Mostly it's from modern technology,

things like cell phones or car-to-car radar,

or now apparently we need appliances that can talk to each other.

All of this modern technology is putting out strong signals

that can completely swamp

this exceedingly faint light we're trying to detect

from the rest of the universe outside Earth,

which just for the record, is most of the universe.

(Laughter)

And then, there are satellites.

Satellites are a problem at both visible and invisible wavelengths.

A host of private companies have plans to deploy tens of thousands of satellites

into Earth orbit,

where they will not only outnumber,

literally outnumber the visible stars in the sky,

while also beaming invisible light back to Earth.

So for astronomers like me,

who use invisible light to study the universe,

it's going to be like staring at the Sun

and trying to see a birthday candle behind it.

Alright, I want to be clear

that there's nothing inherently wrong with any of this modern technology.

With cell phones or satellites or car radar.

I'm not sure about kitchen appliances.

(Laughter)

I haven't broken down

and gotten an oven that talks to my cell phone yet.

And I use lights at night like everybody else.

But here's the thing.

Some problems in the world,

like we've heard about today and you'll hear more about,

are overwhelming and they seem intractable.

Visible light pollution is not one of these problems.

This is actually stupidly simple, OK?

So here are five super simple things you can do.

Don't use lights brighter than you need to.

Don't use lights when you don't need them.