Will There Always Be a North Star?

The star Polaris, when viewed in the Northern Hemisphere, points you almost due north. Learn how it's aided navigators for centuries, but why it won't always do so, in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey,
brain Stuff, Lauren bog Obam here. If you ever looked
at the night sky in the Northern Hemisphere, you've probably
noticed a bright star that the rest of the heavens
appears to move around. What you're seeing is Polaris, also
known as the North Star, which is approximately four and

(00:22):
thirty light years away from Earth and is part of
the constellation Ursa minor. The North Star is thus named
because its location in the night sky is almost directly
over the North Pole. We spoke via email with Rick Feinberg,
a Harvard trained astronomer who is now Press Officer of
the American Astronomical Society. He said, so, if you were

(00:43):
to stand at the North Pole latitude ninety degrees north
at night and look straight up, you'd see Polaris directly overhead.
From other latitudes in the Northern Hemisphere, if you face
due north at night and look at the same angle
above the horizon as your latitude, for example, look about
half way up. That's forty five degrees. If you live
in Portland, Oregon, at latitude forty five degrees north, you'll

(01:05):
see Polaris shining there. Polaris is attention getting because unlike
all the other stars in the sky, Polaris is in
the same location every night from dusk till dawn, neither
rising nor setting. Its presence leads some people to think
of it mistakenly as the brightest star in the sky.
It's actually the brightest even so. It's about two thousand,

(01:27):
five hundred times as luminous as our sun because it's
a massive super giant with a diameter nearly forty times
larger than the Sun and five times the mass. But
Polaris also happens to be far away for a star
that's visible with the naked eye, which reduces its brightness
in our night sky. So who first noticed the north Star?

(01:48):
That's a complicated question. Ancient Egyptian astronomers in the Old
Kingdom between forty one and forty seven hundred years ago
had a north star, which they symbolically represented with a femalehipopotamus,
according to Julia Magli's book Architecture, Astronomy and Sacred Landscape
in Ancient Egypt. But this star was not Polarists. That's

(02:10):
because what humans perceive as the North Star has changed
over time. We also spoke via email with Christopher Palma,
a former teaching professor in astronomy who is currently Associate
Dean of the Eberley College of Science at Penn State University.
He said, if you picture a line connecting Earth's north
and south poles as the axis around which Earth rotates,

(02:31):
that axis is slowly moving in its own circle. Often
this is compared to what happens when a top or
a spinning coin starts to wobble before falling over on
their side. He explained. Because of this wobble, the imaginary
line that goes from the north pole to the South
pole traces out a circle once every twenty six thousand years,

(02:53):
so Palma continued over very long time periods more than
a few thousand years. The north pole moves with respect
to the stars, so thousands of years ago people on
Earth saw the star Thuban in the constellation Draco appear
due north instead of Polaris. Polaris seems to have been
first charted by the astronomer Claudius Ptolemy, who lived from

(03:15):
about a hundred and sixty five to eighty five b C.
The star's location close to the celestial north pole eventually
became useful to navigators Feinberg said, at night in the
northern hemisphere, if you can see Polaris, you can always
tell which way is north, and by extension, which ways
are southeast and west. It's true now, It's been true

(03:36):
for hundreds of years, including during the age of exploration
in the fifteenth through the seventeen centuries, and it will
be true for hundreds more years. You can also tell
your latitude, since the angle from the horizon to Polaris
is the same as your latitude to within a degree anyway.
Once you travel south of the equator, though, Polaris drops
below the horizon, so it's no longer useful as a

(03:57):
navigation aid. Additionally, a navigator using Polarist has to take
into account that the star isn't precisely over the north pole,
but instead has an offset of thirty nine arc minutes
that corresponds to an error of about forty five miles
or seventy two kilometers. One of the other things that's
intriguing about polarists is that it pulsates. Palma explained, this

(04:20):
star pulsates because it's in a state that's unstable. It
will swell up, and when it does, an outer layer
of the star becomes transparent, which then makes the star
cool off. As a result of it cooling off, it
will shrink until it becomes opaque again, which causes it
to heat up and swell again. It will do this
over and over, pulsating in and out, which causes its
brightness to fluctuate. But Polaris won't be the north star forever.

(04:46):
Feinberg said, if you look at the fourteen thousand CE point,
you'll see a star that's much much brighter than Polarists,
but farther from the circle that's Vega, which are descend
in some twelve thousand years from now, if humans are
still around, will consider their north star. And as Fineberg explains, quote,
it's just a coincidence that at this point in Earth's history,

(05:08):
the north facing end of the axis happens to point
almost directly at a bright, naked eye star. The same
is not currently true for the south facing end of
the axis. In other words, there is no south star.
Today's episode was written by Patrick J. Kaiger and produced
by Tyler Clang. Brain Stuff is production of I Heart

(05:31):
Radio's How Stuff Works. For more in this months of
other topics to help navigate the world around you, visit
our home planet, how stuff Works dot com. And for
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