In a world of booked calendars and packed schedules, it's hard to imagine life without them.

But as it turns out, we've been keeping track of the hours long before the clock's invention.

For millennia, people have used the stars to understand and organize the movement of time.

By far the most accessible timekeeper is our nearest star, the Sun.

As early as 3500 BCE, the Egyptians began building obelisks to divide their days into parts

resembling the hours we know today.

The moving shadows created by the Sun hitting the obelisk helped to divide morning from afternoon,

while the length of the noontime shadow showed the year's longest and shortest days.

This is the same principle behind sundials, which you may be more familiar with.

But watching shadows move across the Earth isn't the only way the sky can help us keep time.

Around the same time the Egyptians were building obelisks,

a 366-day calendar structured on the movements of the Sun and the moon was being developed in China.

But after a few centuries of use, astronomers began noticing that the calendar became inaccurate every 300 years or so.

The reason? Well, the stars, including the Sun, aren't as “fixed” in the night sky as they appear to be.

There's movement happening; something that we call precession.

As the Earth's rotational axis slowly moves, the stars shift in our night sky.

About every 26,000 years or so, we get a new view of the stars.

Today, most of us know that Polaris is the North Star.

But years ago, Thuban—a star in the 'tail' of the constellation Draco—was the marker of the poles!

By the 5th century CE, Chinese scholars had figured out the whole precession problem and factored it into their calendar.

And roughly 500 years later, one of the greatest time-keeping achievements of ancient China was unveiled:

a five-story astronomical clock tower.

This mechanical structure ran on a day and night time-keeping wheel that was powered by water!

Astronomical clocks displaying the relative position of the Sun, planets, and even astrological information

also became all the rage in medieval Europe.

Some of these clocks, like the Orloj in Prague, still run to this very day.

But not all of us have fancy clocks nearby to go look at.

Fortunately, using the stars to tell time is as simple as pointing a finger.

Simple being a relative term.

First, find the Big Dipper and the North Star.

Next, trace a line through those last two stars of the Dipper, called the Pointers, towards Polaris.

Imagine that Polaris is the center of a 24-hour clock, with its hour hand passing up to the Pointers.

But instead of turning clockwise, its hour hand turns backwards.

There's another pretty big catch: You can only read this clock directly from the sky on March 6!

On any other night of the year,

take the reading off the "Dipper Clock" and subtract two times the number of months after March.

This system works well, but definitely involves some math.

We've linked a handy reference down in the comments if you're curious to try it out on your own!

Once you get the hang of it, you'll be able to calculate the time on any given solar day!

For those of us here on Earth wondering the time, thankfully the sky offers us a fair number of clocks to use.

But what if you're out in space amongst the stars, with no shadows to read and no ecliptic line to follow?

Well, that's where atomic clocks come in.

They're used by GPS satellites to produce super precise signals

and on the ISS to study the relationship between gravity and time.

But despite their extreme precision, these clocks aren't perfect;

they require constant communication with the more accurate atomic clocks located here on Earth to stay calibrated.

This works fine for now, but as we continue to navigate deep space,

we're going to need ultra-accurate clocks that can run on their own.

That's why NASA engineer Jill Seubert and her team are developing and testing the Deep Space Atomic Clock—

a clock that's about as close to perfect as it gets.

The reason that timekeeping is important for navigation is because we can figure out how far away spacecraft are

by measuring the time it takes to send a signal from the ground station to the spacecraft.

And if we collect those measurements over time,

we can get tracking information that tells us what the trajectory of the spacecraft is,

or what its position and velocity is.

NASA's Deep Space Atomic Clock is a precise instrument for measuring how long it takes

for a signal to travel from point A and B.

Using the frequencies of light emitted by atoms, it's been shown to lose just one second

every 10 million years during controlled tests on Earth.

That's up to 50 times more stable than the atomic clocks used onboard GPS satellites!

Since 2019, the clock has been undergoing a series of tests up in space

to make sure everything is running just as accurately.

Once confirmed, it will be instrumental in helping spacecraft navigate on their own,

without having to rely on directions sent from Earth.

Your computer can actually determine where the spacecraft is, predict where it's headed,

and determine if it needs to fire its thrusters to correct its course and get back on track.

From an ancient sundial used to gauge the length of a day to an atomic clock developed for deep space travel,

humanity continues to rely on the cosmos to make sense of the mysterious flow of time.

But no matter the timekeeping tools we use, one thing is for sure:

the search to find our place in the universe is truly a story as old as time.

I'm Sarafina Nance and this is Seeker Constellations.

If there's another astronomy topic you'd like to see us to cover, let us know in the comments.

Thanks for watching!