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  • Under a dark sky, you can see thousands of stars. If you watch for a few hours, you can

  • see them rise and set as the Earth rotates once a day.

  • And if you go outside the next night at the same time, youll see that thingsll look

  • pretty much the same as they did the night before. The stars rise and set, Polaris hangs

  • to the north, and so on.

  • One day hardly makes any difference to the sky’s appearance.

  • But what if you wait for another night? Or a week? If youre that patient, and observant,

  • you can spot subtle changes in the sky.

  • Let’s say a couple of weeks have passed. Remember that star that was just over a tree

  • in the east when the Sun set -- the one that made you first notice the stars are rising

  • and setting? Go look at it again.

  • If you happen to be out at the same time, you’d expect that star to be in the same

  • place. But it’s not. It’s actually a bit higher above the tree. And if you look west,

  • stars that were well off the horizon just after sunset last week are now lower.

  • If you wait a month, this effect is even more pronounced; all new constellations will be

  • visible in the sky after sunset. This is because the Earth is going around the Sun, literally

  • changing our viewpoint on the sky.

  • The Earth takes a year to orbit the Sun once. Every day, it moves a little bit along its

  • orbit. And as it does, from Earth’s perspective, distant stars appear to move their positions

  • relative to the sun. So, one day we might see a star very near the Sun, but the next

  • day the angle is a bit bigger.

  • At some point, about six months after we first saw it, the star is directly opposite the

  • Sun in the sky. Then the angle starts to shrink again as the star approaches the Sun from

  • the opposite side, until, after a full year, the cycle repeats.

  • What this means to you, the naked eye observer, is that the stars appear to rise and set at

  • different times over the course of the year. Stars in the east rise about four minutes

  • earlier every night, and stars in the west set four minutes earlier. A constellation

  • that was entirely below the eastern horizon at sunset one month might be completely visible

  • after sunset the next month.

  • Another way to think about it is that the stars appear to be fixed, and as the Earth

  • circles the Sun, the Sun moves through the stars over the course of the year, making

  • a complete circle around the sky once per year.

  • The path it takes is a reflection of the Earth’s path around the Sun, a line in the sky. We

  • call that line the ecliptic.

  • That means the Sun passes through the same constellations in the sky every year. We give

  • those constellations a special name: the zodiac. Every year, during a given month, the Sun

  • will appear to be in a certain zodiacal constellation, from Sagittarius through Scorpius, Libra,

  • Virgo, Leo, Cancer, and the rest.

  • Eventually, over a year, the Sun returns to Sagittarius, and the cycle starts again. But

  • even though we talk about this process in terms of the sun’s movement, it’s really

  • the path traveled by the Earth that creates this effect, as our perspective moves with it.

  • And of course, the planets move in the sky as well. Mercury, Venus, Marsthey orbit

  • the Sun, too, and they do so in approximately the same plane the Earth does. If you could

  • see the solar system from the side, it would look flat!

  • So to us, on Earth, the planets go around the sky over the course of a year, and they

  • also appear to change their positions relative to the Sun and the stars.

  • The inner planets, Mercury and Venus, move so rapidly you can see their motion after

  • a single night. The outer planets are more leisurely, but wait long enough and they too

  • will be seen to move, sliding through the constellations.

  • By the way, the wordplanetis Greek forwanderer."

  • There’s another aspect of all this you might notice over time.

  • Youve probably seen a globe, and noticed that the axis of it is tilted; that is,

  • it’s not straight up-and-down, perpendicular to how it sits. That’s because a globe is

  • modeling the Earth and the Earth is tilted.

  • The Earth spins on its axis once per day, and orbits the Sun once per year. But the

  • Earth’s axis is tilted with respect to its orbital plane by 23.5 degrees. And this has

  • a profound effect on our planet.

  • Imagine for a moment that the Earth’s axis were exactly perpendicular to its orbit, straight

  • up and down. If that were the case, every day, the Sun would take the same path across

  • the sky. If you were on the equator the Sun would rise, go exactly overhead, and then

  • set. If youre on the pole, the Sun will appear to go around the horizon every day,

  • neither rising nor settingit would always be twilight.

  • But that’s not the case. The Earth is tilted. In the months of June and July, the Earth’s

  • north pole is tipped toward the Sun. Six months later it’s pointed away. This affects the

  • path the Sun takes across our sky.

  • Instead of it taking the same path every day, in the northern summer, when were tipped

  • toward the Sun, the Sun takes a higher path in the sky. Because that path is longer,

  • the days are longer, too.

  • Six months later, in December and January, the Earth’s pole is tipped away. The Sun

  • takes a lower path in the sky, and because the path is shorter days are shorter too.

  • That’s why we have seasons! When the Sun is up high in the sky it shines straight down

  • on the ground, heating it better, and days are longer so it has more time to heat us

  • up. It gets hot.

  • In the winter, it’s the reverse: The Sun is lower so it can’t warm us up as efficiently,

  • and it has less time to do so. It gets cold.

  • There you go: seasons. The Earth’s axis is tipped. If it weren’t, the seasons wouldn’t

  • occur, and the temperature of the Earth wouldn’t change month to month.

  • There’s a common misconception that the Earth has seasons because it orbits the Sun

  • on an ellipse, and so it’s closer to the Sun in summer and farther in winter. While

  • it’s true the orbit is elliptical, Earth is closer to the sun in January -- on the

  • order of 5 million kilometers or so -- than it is in July. It’s the angle of the sun’s

  • rays that makes winter cold and summer hot, not our distance from the sun.

  • Also, you may know that when it’s summer in the northern hemisphere, it’s winter

  • in the southern. When the north pole is tipped toward the Sun, the south pole is tipped away,

  • so northern and southern hemisphere seasons are opposite each other.

  • But nothing in astronomy is permanent. The north pole’s not always going to point toward

  • the sun in June, and Polaris is not always going to be the North Star.

  • That’s because our planet’s axis is actually moving.

  • Have you ever seen a spinning top start to wobble, its axis moving in a slow circle even

  • as the top itself spins? This is called precession, and the Earth does it too! Our planet spins

  • on its axis once per day, but the axis wobbles, making a very slow circle that takes

  • 26,000 years to complete.

  • This affects a lot of what we see in the sky. For example, Polaris won’t always be the

  • pole star! Every year, the pole points a little farther from that star, making a big circle

  • 47 degrees across. For ancient Egyptians, the star Thuban was the pole star, and in

  • about 11,000 years that position will be held by the bright star Vega.

  • Also, the date the Sun is in a particular zodiac constellation changes slowly due to

  • precession as well. When the ancients first thought up this idea, the Sun was in Aries

  • on March 22, the vernal equinox (what some people call the first day of spring). But

  • due to precession, it’s now in Pisces! That’s why your astrological sign doesn’t match

  • where the Sun actually is in the sky; 2000 years of precession has changed themone

  • of the many reasons astrology is silly.

  • It’s incredible to think about: The Earth, the Sun, the stars: they allow us to tell

  • the time and time of year just by looking up and paying attention. This is why the stars

  • were so important to ancient humans. The stars were like a clock and a calendar in the sky,

  • long before we had invented either.

  • Weve actually learned a lot about the sky just by looking at it. Of course, some of

  • the stuff I’ve explained weve learned through other meansthe Earth is spinning,

  • stars have different intrinsic brightnesses, and so on. But all of that knowledge, and

  • far more, got its start by people who went outside and looked up. Later, as we applied

  • math and physics to what we observed we learned even more, and could then go back

  • and explain what we saw.

  • So don’t discount naked eye astronomy; it’s all we had for thousands of years.

  • In fact, I think we lost something when we started using clocks and calendars, and moving

  • to cities with bright lights that washed away the stars from the sky. Those folks long ago

  • were tied to the sky; they knew it like you know the streets in your neighborhood. They

  • could see the stars rise and set, they knew the glory of the Milky Way sprawled across

  • the heavens, even if they didn’t know exactly what it was.

  • We do know, now, with our knowledge gained over the centuries. But it comes at the cost

  • of losing touch with the sky, not living under it as much as we once did. I’ve spent thousands

  • of hours over my life at night just simply looking up, watching the stars, appreciating

  • the Universe as I can see it. The things I have witnessed have shaped my life, and instilled

  • in me a permanent and endless sense of wonder and joy.

  • The Universe belongs to everyone. Go outside and, if you can, soak up your share.

  • Today we talked about cycles: As the Earth goes around the Sun we see stars rising and

  • setting at different times, the Sun moves along a line in the sky called the ecliptic,

  • through a set of constellations called the zodiacreally a reflection of the Earth’s

  • motion around the Sunthat the planets move more or less along the ecliptic as well,

  • and that seasons are caused by the tilt of the Earth’s axis together with its annual

  • orbit around the Sun.

  • Crash Course is produced in association with PBS Digital Studios. This episode was written

  • by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.

  • Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda,

  • and the graphics team is Thought Café.

Under a dark sky, you can see thousands of stars. If you watch for a few hours, you can

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B1 INT US CrashCourse sun sky earth axis pole

Cycles in the Sky: Crash Course Astronomy #3

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    Vicky Wang   posted on 2015/08/05
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