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Under a dark sky, you can see thousands of
stars. If you watch for a few hours, you can
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see them rise and set as the Earth rotates
once a day.
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And if you go outside the next night at the
same time, you’ll see that things’ll look
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pretty much the same as they did the night
before. The stars rise and set, Polaris hangs
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to the north, and so on.
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One day hardly makes any difference to the
sky’s appearance.
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But what if you wait for another night? Or
a week? If you’re that patient, and observant,
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you can spot subtle changes in the sky.
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Let’s say a couple of weeks have passed.
Remember that star that was just over a tree
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in the east when the Sun set -- the one that
made you first notice the stars are rising
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and setting? Go look at it again.
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If you happen to be out at the same time,
you’d expect that star to be in the same
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place. But it’s not. It’s actually a bit
higher above the tree. And if you look west,
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stars that were well off the horizon just
after sunset last week are now lower.
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If you wait a month, this effect is even more
pronounced; all new constellations will be
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visible in the sky after sunset. This is because
the Earth is going around the Sun, literally
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changing our viewpoint on the sky.
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The Earth takes a year to orbit the Sun once.
Every day, it moves a little bit along its
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orbit. And as it does, from Earth’s perspective,
distant stars appear to move their positions
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relative to the sun. So, one day we might
see a star very near the Sun, but the next
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day the angle is a bit bigger.
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At some point, about six months after we first
saw it, the star is directly opposite the
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Sun in the sky. Then the angle starts to shrink
again as the star approaches the Sun from
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the opposite side, until, after a full year,
the cycle repeats.
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What this means to you, the naked eye observer,
is that the stars appear to rise and set at
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different times over the course of the year.
Stars in the east rise about four minutes
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earlier every night, and stars in the west
set four minutes earlier. A constellation
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that was entirely below the eastern horizon
at sunset one month might be completely visible
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after sunset the next month.
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Another way to think about it is that the
stars appear to be fixed, and as the Earth
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circles the Sun, the Sun moves through the
stars over the course of the year, making
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a complete circle around the sky once per
year.
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The path it takes is a reflection of the Earth’s
path around the Sun, a line in the sky. We
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call that line the ecliptic.
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That means the Sun passes through the same
constellations in the sky every year. We give
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those constellations a special name: the zodiac.
Every year, during a given month, the Sun
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will appear to be in a certain zodiacal constellation,
from Sagittarius through Scorpius, Libra,
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Virgo, Leo, Cancer, and the rest.
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Eventually, over a year, the Sun returns to
Sagittarius, and the cycle starts again. But
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even though we talk about this process in
terms of the sun’s movement, it’s really
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the path traveled by the Earth that creates
this effect, as our perspective moves with it.
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And of course, the planets move in the sky
as well. Mercury, Venus, Mars… they orbit
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the Sun, too, and they do so in approximately
the same plane the Earth does. If you could
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see the solar system from the side, it would
look flat!
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So to us, on Earth, the planets go around
the sky over the course of a year, and they
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also appear to change their positions relative
to the Sun and the stars.
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The inner planets, Mercury and Venus, move
so rapidly you can see their motion after
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a single night. The outer planets are more
leisurely, but wait long enough and they too
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will be seen to move, sliding through the
constellations.
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By the way, the word “planet” is Greek
for “wanderer."
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There’s another aspect of all this you might
notice over time.
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You’ve probably seen a globe, and noticed
that the axis of it is tilted; that is,
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it’s not straight up-and-down, perpendicular
to how it sits. That’s because a globe is
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modeling the Earth and the Earth is tilted.
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The Earth spins on its axis once per day,
and orbits the Sun once per year. But the
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Earth’s axis is tilted with respect to its
orbital plane by 23.5 degrees. And this has
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a profound effect on our planet.
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Imagine for a moment that the Earth’s axis
were exactly perpendicular to its orbit, straight
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up and down. If that were the case, every
day, the Sun would take the same path across
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the sky. If you were on the equator the Sun
would rise, go exactly overhead, and then
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set. If you’re on the pole, the Sun will
appear to go around the horizon every day,
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neither rising nor setting — it would always
be twilight.
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But that’s not the case. The Earth is tilted.
In the months of June and July, the Earth’s
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north pole is tipped toward the Sun. Six months
later it’s pointed away. This affects the
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path the Sun takes across our sky.
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Instead of it taking the same path every day,
in the northern summer, when we’re tipped
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toward the Sun, the Sun takes a higher path
in the sky. Because that path is longer,
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the days are longer, too.
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Six months later, in December and January,
the Earth’s pole is tipped away. The Sun
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takes a lower path in the sky, and because
the path is shorter days are shorter too.
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That’s why we have seasons! When the Sun
is up high in the sky it shines straight down
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on the ground, heating it better, and days
are longer so it has more time to heat us
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up. It gets hot.
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In the winter, it’s the reverse: The Sun
is lower so it can’t warm us up as efficiently,
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and it has less time to do so. It gets cold.
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There you go: seasons. The Earth’s axis
is tipped. If it weren’t, the seasons wouldn’t
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occur, and the temperature of the Earth wouldn’t
change month to month.
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There’s a common misconception that the
Earth has seasons because it orbits the Sun
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on an ellipse, and so it’s closer to the
Sun in summer and farther in winter. While
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it’s true the orbit is elliptical, Earth
is closer to the sun in January -- on the
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order of 5 million kilometers or so -- than
it is in July. It’s the angle of the sun’s
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rays that makes winter cold and summer hot,
not our distance from the sun.
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Also, you may know that when it’s summer
in the northern hemisphere, it’s winter
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in the southern. When the north pole is tipped
toward the Sun, the south pole is tipped away,
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so northern and southern hemisphere seasons
are opposite each other.
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But nothing in astronomy is permanent. The
north pole’s not always going to point toward
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the sun in June, and Polaris is not always
going to be the North Star.
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That’s because our planet’s axis is actually
moving.
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Have you ever seen a spinning top start to
wobble, its axis moving in a slow circle even
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as the top itself spins? This is called precession,
and the Earth does it too! Our planet spins
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on its axis once per day, but the axis wobbles,
making a very slow circle that takes
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26,000 years to complete.
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This affects a lot of what we see in the sky.
For example, Polaris won’t always be the
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pole star! Every year, the pole points a little
farther from that star, making a big circle
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47 degrees across. For ancient Egyptians,
the star Thuban was the pole star, and in
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about 11,000 years that position will be held
by the bright star Vega.
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Also, the date the Sun is in a particular
zodiac constellation changes slowly due to
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precession as well. When the ancients first
thought up this idea, the Sun was in Aries
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on March 22, the vernal equinox (what some
people call the first day of spring). But
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due to precession, it’s now in Pisces! That’s
why your astrological sign doesn’t match
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where the Sun actually is in the sky; 2000
years of precession has changed them…one
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of the many reasons astrology is silly.
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It’s incredible to think about: The Earth,
the Sun, the stars: they allow us to tell
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the time and time of year just by looking
up and paying attention. This is why the stars
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were so important to ancient humans. The stars
were like a clock and a calendar in the sky,
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long before we had invented either.
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We’ve actually learned a lot about the sky
just by looking at it. Of course, some of
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the stuff I’ve explained we’ve learned
through other means – the Earth is spinning,
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stars have different intrinsic brightnesses,
and so on. But all of that knowledge, and
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far more, got its start by people who went
outside and looked up. Later, as we applied
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math and physics to what we observed we
learned even more, and could then go back
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and explain what we saw.
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So don’t discount naked eye astronomy; it’s
all we had for thousands of years.
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In fact, I think we lost something when we
started using clocks and calendars, and moving
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to cities with bright lights that washed away
the stars from the sky. Those folks long ago
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were tied to the sky; they knew it like you
know the streets in your neighborhood. They
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could see the stars rise and set, they knew
the glory of the Milky Way sprawled across
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the heavens, even if they didn’t know exactly
what it was.
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We do know, now, with our knowledge gained
over the centuries. But it comes at the cost
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of losing touch with the sky, not living under
it as much as we once did. I’ve spent thousands
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of hours over my life at night just simply
looking up, watching the stars, appreciating
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the Universe as I can see it. The things I
have witnessed have shaped my life, and instilled
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in me a permanent and endless sense of wonder
and joy.
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The Universe belongs to everyone. Go outside
and, if you can, soak up your share.
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Today we talked about cycles: As the Earth
goes around the Sun we see stars rising and
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setting at different times, the Sun moves
along a line in the sky called the ecliptic,
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through a set of constellations called the
zodiac — really a reflection of the Earth’s
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motion around the Sun — that the planets
move more or less along the ecliptic as well,
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and that seasons are caused by the tilt of
the Earth’s axis together with its annual
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orbit around the Sun.
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Crash Course is produced in association with
PBS Digital Studios. This episode was written
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by me, Phil Plait. The script was edited by
Blake de Pastino, and our consultant is Dr.
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Michelle Thaller. It was co-directed by Nicholas
Jenkins and Michael Aranda,
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and the graphics team is Thought Café.
