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  • When you think of a clock, you probably think of that tik-toking device that we use to tell time.

  • 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 asfixedin 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 Dracowas 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!

When you think of a clock, you probably think of that tik-toking device that we use to tell time.

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From Sundials to Atomic Clocks: How the Universe Shapes Our Understanding of Time

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    Summer posted on 2021/04/02
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