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  • Hey, Phil Plait here. Welcome to episode 2 of Crash Course Astronomy: Naked Eye Observations.

  • Despite the salacious title, nudity is not required.

  • In fact, given that a lot of astronomical observations are done at night, you may want to bundle up.

  • [Theme Music] "One giant leap for mankind"

  • As it relates to astronomy, “naked eyemeans no binoculars, no telescope.

  • Just you, your eyeballs, and a nice, dark site from which to view the heavens.

  • After all, that’s how we did astronomy for thousands of years,

  • and it’s actually pretty amazing what you can figure out about the Universe just by looking at it.

  • Imagine youre somewhere far away from city lights, where you have an unobstructed view of the cloudless sky.

  • The Sun sets, and for a few minutes you just watch as the sky darkens.

  • Then, you notice a star appear in the east, just over a tree.

  • Then another, and another, and within an hour or so you are standing beneath an incredible display,

  • the sky spangled with stars.

  • What do you notice right away? First, there are a lot of stars.

  • People with normal vision can see a few thousand stars at any given time, and if you want a round number,

  • there are very roughly six to ten thousand stars in total that are bright enough to detect by eye alone,

  • depending on how good your sight is.

  • The next thing youll notice is that theyre not all the same brightness.

  • A handful are very bright, a few more are a bit fainter but still pretty bright, and so on.

  • The faintest stars you can see are the most abundant, vastly outnumbering the bright ones.

  • This is due to a combination of two effects.

  • One is that stars aren’t all the same intrinsic, physical brightness.

  • Some are dim bulbs, while others are monsters, blasting out as much light in one second as the Sun does in a day.

  • The second factor is that not all stars are the same distance from us.

  • The farther away a star is, the fainter it is.

  • Interestingly, of the two dozen or so brightest stars in the sky, half are bright because theyre close to Earth,

  • and half are much farther away but incredibly luminous, so they still appear bright to us.

  • This is a running theme in astronomy, and science in general.

  • Some effects you see have more than one cause.

  • Things aren’t always as simple as they seem.

  • The ancient Greek astronomer Hipparchus is generally credited for creating the first catalog of stars,

  • ranking them by brightness.

  • He came up with a system called magnitudes, where the brightest stars were 1st magnitude,

  • the next brightest were 2nd magnitude, down to 6th magnitude.

  • We still use a variation of this system today, thousands of years later.

  • The faintest stars ever seen (using Hubble Space Telescope) are about magnitude 31 –

  • the faintest star you can see with your eye is about 10 billion times brighter!

  • The brightest star in the night skycalled Sirius, the Dog Star

  • is about 1000 times brighter than the faintest star you can see.

  • Let’s take a closer look at some of those bright stars, like, say, Vega.

  • Notice anything about it? Yeah, it looks blue. And Betelgeuse looks red.

  • Arcturus is orange, Capella yellow. Those stars really are those colors.

  • By eye, only the brightest stars seem have color, while the fainter ones all just look white.

  • That’s because the color receptors in your eye aren’t very light-sensitive,

  • and only the brightest stars can trigger them.

  • Another thing youll notice is that stars aren’t scattered evenly across the sky.

  • They form patterns, shapes.

  • This is mostly coincidence, but humans are pattern-recognizing animals,

  • so it’s totally understandable that ancient astronomers divided the skies up into constellations

  • (literally sets or groups of stars), and named them after familiar objects.

  • Orion is probably the most famous constellation;

  • it really does look like a person, arms raised up, and most civilizations saw it that way.

  • There’s also tiny Delphinus; it’s only 5 stars, but it’s easy to see it as a dolphin jumping out of the water.

  • And Scorpius, which isn’t hard to imagine as a venomous arthropod.

  • Others, well, not so much. Pisces is a fish? Yeah, OK. Cancer is a crab? If you say so.

  • Although they were rather arbitrarily defined in ancient times, today we recognize 88 official constellations,

  • and their boundaries are carefully delineated on the sky.

  • When we say a star is in the constellation of Ophiuchus,

  • it’s because the location of the star puts it inside that constellation’s boundaries.

  • Think of them like states in the US:

  • the state lines are decided upon by mutual agreement, and a city can be in one state or the other.

  • Mind you, not every group of stars makes a constellation.

  • The Big Dipper, for example, is only one part of the constellation of Ursa Major, the Big Bear.

  • The bowl of the dipper is the bear’s haunches, and the handle is its tail.

  • But! Bears don't have tails!

  • So astronomers might be great at pattern recognition, but they're terrible at zoology.

  • Most of the brightest stars have proper names, usually Arabic.

  • During the Dark Ages, when Europe wasn’t so scientifically minded,

  • it was the Persian astronomer Abd al-Rahman al-Sufi who translated ancient Greek astronomy texts into Arabic,

  • and those names have stuck with us ever since.

  • However there are a lot more stars than there are proper names, so astronomers use other designations for them.

  • The stars in any constellation are given Greek letters in order of their brightness,

  • so we have Alpha Orionis, the brightest star in Orion, then Beta, and so.

  • Of course, you run out of letters quickly, too, so most modern catalogs just use numbers;

  • it’s a lot harder to run out of those.

  • Of course, just seeing all those faint stars can be tough, which brings us to this week’s “Focus On.”

  • Light pollution is a serious problem for astronomers.

  • This is light from street lamps, shopping centers, or wherever,

  • where the light gets blasted up into the sky instead of toward the ground.

  • This lights the up the sky, making fainter objects much more difficult to see.

  • That’s why observatories tend to be built in remote areas, as far from cities as possible.

  • Trying to observe faint galaxies under bright sky conditions is like trying to listen to

  • someone 50 feet away whispering at you at a rock concert.

  • This affects the sky you see as well.

  • From within a big city, it's impossible to see the Milky Way,

  • the faint streak of across the sky that’s actually the combined light of billions of stars.

  • It gets washed out with even mild light pollution.

  • Your view of Orion probably looks like this:

  • When from a dark site it looks like this:

  • It’s not just people who are affected by this, either.

  • Light pollution affects the way nocturnal animals hunt, how insects breed,

  • and more, by disrupting their normal daily cycles.

  • Cutting back light pollution is mostly just a matter of using the right kind of light fixtures outside,

  • directing the light down to the ground.

  • A lot of towns have worked to use better lighting, and have met with success.

  • This is due in large part to groups like the International Dark-Sky Association, GLOBE at Night, The World at Night,

  • and many more, who advocate using more intelligent lighting, and to help preserve our night sky.

  • The sky belongs to everyone, and we should do what we can to make sure it’s the best possible sky we can see.

  • Even if you don’t have dark skies, there’s another thing you can notice when you look up.

  • If you look carefully, you might see that a couple of the brightest stars look different than the others.

  • They don’t twinkle! That’s because they aren’t stars, theyre planets.

  • Twinkling happens because the air over our heads is turbulent,

  • and as it blows past, it distorts the incoming light from stars,

  • making them appear to slightly shift position and brightness several times per second.

  • But planets are much closer to us, and appear bigger, so the distortion doesn’t affect them as much.

  • There are five naked eye planets (not counting Earth): Mercury, Venus, Mars, Jupiter, and Saturn.

  • Uranus is right on the edge of visibility, and people with keen eyesight might be able to spot it.

  • Venus is actually the third brightest natural object in the sky, after the Sun and Moon.

  • Jupiter and Mars are frequently brighter than the brightest stars, too.

  • If you stay outside for an hour or two, youll notice something else that’s pretty obvious:

  • the stars move, like the sky is a gigantic sphere wheeling around you over the course of the night.

  • In fact, that’s how the ancients thought of it.

  • If you could measure it, you’d find this celestial sphere spins once every day.

  • Stars toward the east are rising over the horizon, and stars in the west are setting,

  • making a big circle over the course of the night (and presumably, day).

  • This is really just a reflection of the Earth spinning, of course.

  • The Earth rotates once a day, and were stuck to it,

  • so it looks like the sky is spinning around us in the opposite direction.

  • There’s an interesting thing that happens because of this. Look at a spinning globe.

  • It rotates on an axis that goes through the poles, and halfway between them is the Equator.

  • If you stand on the Equator, you make a big circle around the center of the Earth over a day.

  • But if you move north or south, toward one pole or the other, that circle gets smaller.

  • When you stand on the pole, you don’t make a circle at all; you just spin around in the same spot.

  • It’s the same thing with the sky.

  • As the sky spins over us, just like with the Earth, it has two poles and an Equator.

  • A star on the celestial Equator makes a big circle around the sky, and stars to the north or south make smaller ones.

  • A star right on the celestial pole wouldn’t appear to move at all, and would just hang there,

  • like it was nailed to that spot, all night long.

  • And this is just what we see! Photographic time exposures show it best.

  • The motions of the stars show up as streaks.

  • The longer the exposure, the longer the streaks as the stars rise and set, making their circular arcs in the sky.

  • You can see stars near the celestial equator making their big circles.

  • And, by coincidence, there’s also a middling-bright star that sits very close to the north celestial pole.

  • That’s called Polaris, the north or pole star.

  • Because of that, it doesn’t appear to rise or set, and is always to the north, motionless.

  • It really is coincidence; there’s no southern pole star, unless you count Sigma Octans,

  • a dim bulb barley visible by eye that’s not all that close to the south pole of the sky anyway.

  • But even Polaris isn’t exactly on the pole -- it’s offset a teeny bit.

  • So it does make a circle in the sky, but one so small you’d never notice.

  • By eye, night after night, Polaris is the constant in the sky, always there, never moving.

  • Remember, the sky’s motion is a reflection of the Earth’s motion.

  • If you were standing on the north pole of the Earth, you’d see Polaris at the sky’s zenith

  • that is, straight overheadfixed and unmoving.

  • Stars on the celestial equator would appear to circle the horizon once per day.

  • But this also means that stars south of the celestial equator can’t be seen from the Earth’s north pole!

  • Theyre always below the horizon.

  • So this in turn means that which stars you see depends on where you are on Earth.

  • At the north pole, you only see stars north of the celestial equator.

  • At the Earth’s south pole, you only see stars south of the celestial equator.

  • From Antarctica, Polaris is forever hidden from view.

  • Standing on the Earth’s equator, you’d see Polaris on the horizon to the north,

  • and Sigma Octans on the horizon to the south,

  • and over the course of the day the entire celestial sphere would spin around you;

  • every star in the sky is eventually visible.

  • While Polaris may be constant, not everything is.

  • Sometimes you just have to wait a while to notice.

  • And to that point, youll have to wait a while to find out what I mean by this,

  • because well be covering that in next week’s episode.

  • Today we talked about what you can see on a clear dark night with just your eyes:

  • thousands of stars, some brighter than others, arranged into patterns called constellations.

  • Stars have colors, even if we can’t see them with our eyes alone, and they rise and set as the Earth spins.

  • You can see different stars depending on where you are on Earth,

  • and if youre in the northern hemisphere, Polaris will always point you toward north.

  • 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.

Hey, Phil Plait here. Welcome to episode 2 of Crash Course Astronomy: Naked Eye Observations.

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Naked Eye Observations: Crash Course Astronomy #2

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    王政弘 posted on 2016/11/09
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