- Hey smart people, Joe, here.

How do we figure out how far away things are?

If you're measuring the distance to an object nearby, it's pretty easy.

We take another object whose length that we know, and we use it as a reference to find that distance.

What about an object that's beyond our physical reach?

Well, we can literally walk to that object and count our steps to calculate the distance.

Even something as far away as the moon, we can shine a beam of light, bounce it off.

Special mirrors left on the lunar surface by Apollo.

Astronauts count how long that light takes to make the round trip and calculate distance.

These are all ways of measuring using things, physical objects in the world around us.

But how do we measure how big a universe is?

This is a video about the tools we use to answer that question.

Since the time of the ancient Greeks, scientists have been constructing a cosmic measuring tape to measure the universe from our own backyard all the way to its ever expanding edge.

The cosmic distance ladder.

Let's climb it and learn our true place in space.

The first ancient sky watchers knew that our moon, the planets, and the stars were incredibly far away.

And before they had the technology to even try measuring these distances, precisely.

Early astronomers in ancient Greece figured out one cool trick based on basic geometry that would become the key to measuring thousands of distances in our own galaxy.

It's called parallax.

Parallax is why when you look out the window of a moving car, things closer to you, like trees seem to move more than far away.

Mountains and things that are super far away don't appear to move at all.

It's also why when you look at something right in front of you and then close one eye than the other, the distance between your eyes makes the object appear in a different place against the background.

Chances are many of you are doing this right now, holding something up and opening one eye and then the other.

That's parallax.

The key is the closer an object is, the more its position seems to change as you look at it from a different perspective.

Astronomers use this same trick to measure distances to planets and stars by looking at things with two different eyes.

Only the eyes they use are thousands of miles apart.

Way back in the second century BC the Greek astronomer Hipparchus used this exact trick to figure the distance to the moon.

He compared observations of a solar eclipse from two separate locations.

At one location, people saw a total eclipse and the other saw four fifths of the sun covered.

Hipparchus knew the distance between the two locations, his two eyes, and the angle that the moon appeared to move.

So he could calculate the distance to the moon with some basic geometry.

And in 1672, Giovanni Cassini used the same trick to calculate the distance to Mars.

What's amazing is Hipparchus and Cassini were both really close to the actual distances we know today without any of our newfangled modern technology.

Parallax is a great way to measure distances to objects in our solar system, but we can even pick up the parallax of some stars.

To do this, astronomers use earth's position at different times of year as two different eyes.

By looking at a closer star several months apart, they can measure how the background stars appear to move.

Do a little math and calculate distances.

A satellite named after Hipparchus used super precise angular measurements to map the distances to some two and a half million stars.

But parallax only works up to a certain distance.

To go deeper into the universe, we need to add another rung to the ladder.

By the 1920s, astronomers had a pretty decent map of our galaxy.

The distances weren't spot on, but the spiral arms were there.

The nearby objects were in place, and astronomers had a rough idea of how big our galaxy was.

The numbers were so huge, we stopped measuring in terms of distance and started measuring in terms of time -- how long it would take light to travel that far.

But as this vast picture was coming together, there was a hint that there might be way more to the universe than anyone had imagined.

In the 19th century, astronomers had started noticing mysterious fuzzy objects they called "spiral nebulae."

Some people thought these were just gas clouds within the Milky Way, but others believed they were so-called island universes, whole galaxies outside our own.

This was a huge debate among astronomers.

Could there really be galaxies besides ours?

On the night of October 5th, 1923, astronomer Edwin Hubble aimed a hundred inch telescope toward a mysterious fuzzy spot in the sky and captured a photograph.

As he poured over the image, comparing it to others from different nights, he noticed one little speck was changing over time -- gradually brightening and then dimming.

This was exactly what he was looking for.

A slowly blinking star called a Cepheid variable.

Years earlier, astronomers had learned that the brightness of these stars, along with how fast they pulse could tell us how far away they are.

The astronomer Henrietta Leavitt had figured out the relationship between a cepheid's pulsing and its true brightness.

For instance, a star that takes 30 days to go through one cycle of light and dim is going to be bright -- around 10,000 times brighter than our sun.

Meanwhile, a star that goes through one cycle every day or so is going to be on the dimmer side -- only a few hundred times as bright as the sun.

Now, if you know how fast a Cepheid star blinks, now you know how bright it actually is.

But depending on how far away it is, that blinking star will appear dimmer.

So when we measure how much it dims when we actually look at it, we can calculate the distance to that.

Star objects like this are called standard candles.

There are lights that we know how bright they actually are so that we can reveal the distances between us and them.

Doing that would require a new rung on our cosmic distance measuring ladder.

And for this one, we need to split light into a rainbow.

If you look at any star's light like this, you'll find that a bunch of lines are missing, much like a barcode.

These missing lines are a fingerprint of the elements that star is made of.

But astronomers noticed that the lines in these barcodes weren't always showing up at the wavelengths that they expected.

Sometimes they were shifted towards the blue end of the spectrum and sometimes towards the red.

They realized that they were looking at a weird but familiar phenomenon -- the Doppler effect.

This is an effect that happens when something that's emitting waves is also moving relative to the observer.

You've almost certainly experienced the doppler effect here on earth, but astronomers can also use it to map our universe.

In 1912, an astronomer named Vesto Slipher started using spectroscopy to study those mysterious spiral nebulae, the ones that turned out to be other galaxies.

And he noticed that most of their color fingerprints were extremely shifted to the red side, or redshifted, meaning these things were moving away from us at unbelievable speeds too.

We're talking hundreds of kilometers a second.

Then in 1929, Hubble realized something even more mind blowing.

The farthest galaxies seemed to be moving away the fastest.

That could only mean one thing, the universe was expanding.

Suddenly the picture of the universe that astronomers had been after for so long wasn't a picture after all.

It was a movie.

But that raised another new question -- Just how fast was it expanding?

And what did this mean for the fate of the universe?

Well, to answer that, astronomers would need to peer even deeper into space.

And they'd need to add yet another rung to our cosmic distance ladder.

The key to these questions were supernovae -- powerful explosions of massive dying stars that for a short instant, flash as bright as an entire galaxy.

A certain supernova called type Ias all max out at a similar brightness, which means whenever one goes off, we can use it as a standard candle just like Cepheids.

But since supernovae are so bright, we can detect these standard candles halfway across the visible universe.

And that lets us take one more big step on the cosmic distance ladder.

And by the 1990s, astronomers had used this ladder to put together a detailed picture of an unthinkably vast universe -- one that seemed to be expanding as far as the eye could see.

When astronomers looked at light from the most distant supernovae, they could see they found something completely surprising, something that didn't fit any of their previous ideas.

The expansion of the universe was not slowing down.

Not even a little.

It was accelerating.

What was pulling our universe apart?

And how had this tug of war between gravity and whatever this mysterious expanding force is shaped the universe over time.

Once again, climbing the highest rung of the cosmic distance ladder had uncovered previously unthinkable questions and opened up entire areas of research that didn't exist before.

Today, over 2000 years Have past since Hipparchus first measured the distance to the moon Our view of the universe has gone from this .

.

.

to this.

We've figured out that the universe has way more than one galaxy, that the edge of the observable universe is billions of light years away and that our universe is not a photograph, but rather a movie playing out over time.

But perhaps most importantly, we've learned that sometimes the most fundamental questions -- like how far away is that thing?

-- can lead to surprising new questions that transform the way we see the world.