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Today on Crash Course Astronomy, Phil invites you to head outside and take a look at all the incredible things you can see with your naked eye.
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Transcript Provided by YouTube:
00:02
Hey, Phil Plait here. Welcome to episode 2 of Crash Course Astronomy: Naked Eye Observations.
00:08
Despite the salacious title, nudity is not required.
00:12
In fact, given that a lot of astronomical observations are done at night, you may want to bundle up.
00:17
[Theme Music] “One giant leap for mankind”
00:26
As it relates to astronomy, “naked eye” means no binoculars, no telescope.
00:31
Just you, your eyeballs, and a nice, dark site from which to view the heavens.
00:35
After all, that’s how we did astronomy for thousands of years,
00:38
and it’s actually pretty amazing what you can figure out about the Universe just by looking at it.
00:42
Imagine you’re somewhere far away from city lights, where you have an unobstructed view of the cloudless sky.
00:48
The Sun sets, and for a few minutes you just watch as the sky darkens.
00:53
Then, you notice a star appear in the east, just over a tree.
00:56
Then another, and another, and within an hour or so you are standing beneath an incredible display,
01:02
the sky spangled with stars.
01:03
What do you notice right away? First, there are a lot of stars.
01:07
People with normal vision can see a few thousand stars at any given time, and if you want a round number,
01:12
there are very roughly six to ten thousand stars in total that are bright enough to detect by eye alone,
01:17
depending on how good your sight is.
01:19
The next thing you’ll notice is that they’re not all the same brightness.
01:22
A handful are very bright, a few more are a bit fainter but still pretty bright, and so on.
01:27
The faintest stars you can see are the most abundant, vastly outnumbering the bright ones.
01:31
This is due to a combination of two effects.
01:33
One is that stars aren’t all the same intrinsic, physical brightness.
01:37
Some are dim bulbs, while others are monsters, blasting out as much light in one second as the Sun does in a day.
01:44
The second factor is that not all stars are the same distance from us.
01:47
The farther away a star is, the fainter it is.
01:50
Interestingly, of the two dozen or so brightest stars in the sky, half are bright because they’re close to Earth,
01:55
and half are much farther away but incredibly luminous, so they still appear bright to us.
02:00
This is a running theme in astronomy, and science in general.
02:03
Some effects you see have more than one cause.
02:06
Things aren’t always as simple as they seem.
02:08
The ancient Greek astronomer Hipparchus is generally credited for creating the first catalog of stars,
02:14
ranking them by brightness.
02:15
He came up with a system called magnitudes, where the brightest stars were 1st magnitude,
02:20
the next brightest were 2nd magnitude, down to 6th magnitude.
02:24
We still use a variation of this system today, thousands of years later.
02:27
The faintest stars ever seen (using Hubble Space Telescope) are about magnitude 31 –
02:32
the faintest star you can see with your eye is about 10 billion times brighter!
02:37
The brightest star in the night sky — called Sirius, the Dog Star —
02:41
is about 1000 times brighter than the faintest star you can see.
02:44
Let’s take a closer look at some of those bright stars, like, say, Vega.
02:48
Notice anything about it? Yeah, it looks blue. And Betelgeuse looks red.
02:53
Arcturus is orange, Capella yellow. Those stars really are those colors.
02:57
By eye, only the brightest stars seem have color, while the fainter ones all just look white.
03:02
That’s because the color receptors in your eye aren’t very light-sensitive,
03:06
and only the brightest stars can trigger them.
03:08
Another thing you’ll notice is that stars aren’t scattered evenly across the sky.
03:13
They form patterns, shapes.
03:14
This is mostly coincidence, but humans are pattern-recognizing animals,
03:18
so it’s totally understandable that ancient astronomers divided the skies up into constellations
03:23
(literally sets or groups of stars), and named them after familiar objects.
03:27
Orion is probably the most famous constellation;
03:30
it really does look like a person, arms raised up, and most civilizations saw it that way.
03:34
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.
03:40
And Scorpius, which isn’t hard to imagine as a venomous arthropod.
03:43
Others, well, not so much. Pisces is a fish? Yeah, OK. Cancer is a crab? If you say so.
03:52
Although they were rather arbitrarily defined in ancient times, today we recognize 88 official constellations,
03:58
and their boundaries are carefully delineated on the sky.
04:01
When we say a star is in the constellation of Ophiuchus,
04:03
it’s because the location of the star puts it inside that constellation’s boundaries.
04:08
Think of them like states in the US:
04:10
the state lines are decided upon by mutual agreement, and a city can be in one state or the other.
04:15
Mind you, not every group of stars makes a constellation.
04:18
The Big Dipper, for example, is only one part of the constellation of Ursa Major, the Big Bear.
04:24
The bowl of the dipper is the bear’s haunches, and the handle is its tail.
04:27
But! Bears don’t have tails!
04:29
So astronomers might be great at pattern recognition, but they’re terrible at zoology.
04:34
Most of the brightest stars have proper names, usually Arabic.
04:37
During the Dark Ages, when Europe wasn’t so scientifically minded,
04:41
it was the Persian astronomer Abd al-Rahman al-Sufi who translated ancient Greek astronomy texts into Arabic,
04:47
and those names have stuck with us ever since.
04:49
However there are a lot more stars than there are proper names, so astronomers use other designations for them.
04:55
The stars in any constellation are given Greek letters in order of their brightness,
04:59
so we have Alpha Orionis, the brightest star in Orion, then Beta, and so.
05:04
Of course, you run out of letters quickly, too, so most modern catalogs just use numbers;
05:09
it’s a lot harder to run out of those.
05:10
Of course, just seeing all those faint stars can be tough, which brings us to this week’s “Focus On.”
05:15
Light pollution is a serious problem for astronomers.
05:19
This is light from street lamps, shopping centers, or wherever,
05:22
where the light gets blasted up into the sky instead of toward the ground.
05:26
This lights the up the sky, making fainter objects much more difficult to see.
05:30
That’s why observatories tend to be built in remote areas, as far from cities as possible.
05:35
Trying to observe faint galaxies under bright sky conditions is like trying to listen to
05:39
someone 50 feet away whispering at you at a rock concert.
05:42
This affects the sky you see as well.
05:44
From within a big city, it’s impossible to see the Milky Way,
05:48
the faint streak of across the sky that’s actually the combined light of billions of stars.
05:53
It gets washed out with even mild light pollution.
05:55
Your view of Orion probably looks like this:
05:58
When from a dark site it looks like this:
06:00
It’s not just people who are affected by this, either.
06:03
Light pollution affects the way nocturnal animals hunt, how insects breed,
06:07
and more, by disrupting their normal daily cycles.
06:10
Cutting back light pollution is mostly just a matter of using the right kind of light fixtures outside,
06:15
directing the light down to the ground.
06:17
A lot of towns have worked to use better lighting, and have met with success.
06:21
This is due in large part to groups like the International Dark-Sky Association, GLOBE at Night, The World at Night,
06:27
and many more, who advocate using more intelligent lighting, and to help preserve our night sky.
06:33
The sky belongs to everyone, and we should do what we can to make sure it’s the best possible sky we can see.
06:39
Even if you don’t have dark skies, there’s another thing you can notice when you look up.
06:43
If you look carefully, you might see that a couple of the brightest stars look different than the others.
06:47
They don’t twinkle! That’s because they aren’t stars, they’re planets.
06:51
Twinkling happens because the air over our heads is turbulent,
06:54
and as it blows past, it distorts the incoming light from stars,
06:58
making them appear to slightly shift position and brightness several times per second.
07:02
But planets are much closer to us, and appear bigger, so the distortion doesn’t affect them as much.
07:07
There are five naked eye planets (not counting Earth): Mercury, Venus, Mars, Jupiter, and Saturn.
07:13
Uranus is right on the edge of visibility, and people with keen eyesight might be able to spot it.
07:18
Venus is actually the third brightest natural object in the sky, after the Sun and Moon.
07:22
Jupiter and Mars are frequently brighter than the brightest stars, too.
07:25
If you stay outside for an hour or two, you’ll notice something else that’s pretty obvious:
07:29
the stars move, like the sky is a gigantic sphere wheeling around you over the course of the night.
07:35
In fact, that’s how the ancients thought of it.
07:37
If you could measure it, you’d find this celestial sphere spins once every day.
07:42
Stars toward the east are rising over the horizon, and stars in the west are setting,
07:46
making a big circle over the course of the night (and presumably, day).
07:49
This is really just a reflection of the Earth spinning, of course.
07:52
The Earth rotates once a day, and we’re stuck to it,
07:55
so it looks like the sky is spinning around us in the opposite direction.
07:58
There’s an interesting thing that happens because of this. Look at a spinning globe.
08:01
It rotates on an axis that goes through the poles, and halfway between them is the Equator.
08:06
If you stand on the Equator, you make a big circle around the center of the Earth over a day.
08:10
But if you move north or south, toward one pole or the other, that circle gets smaller.
08:15
When you stand on the pole, you don’t make a circle at all; you just spin around in the same spot.
08:19
It’s the same thing with the sky.
08:21
As the sky spins over us, just like with the Earth, it has two poles and an Equator.
08:25
A star on the celestial Equator makes a big circle around the sky, and stars to the north or south make smaller ones.
08:31
A star right on the celestial pole wouldn’t appear to move at all, and would just hang there,
08:36
like it was nailed to that spot, all night long.
08:38
And this is just what we see! Photographic time exposures show it best.
08:41
The motions of the stars show up as streaks.
08:44
The longer the exposure, the longer the streaks as the stars rise and set, making their circular arcs in the sky.
08:50
You can see stars near the celestial equator making their big circles.
08:53
And, by coincidence, there’s also a middling-bright star that sits very close to the north celestial pole.
08:59
That’s called Polaris, the north or pole star.
09:01
Because of that, it doesn’t appear to rise or set, and is always to the north, motionless.
09:06
It really is coincidence; there’s no southern pole star, unless you count Sigma Octans,
09:10
a dim bulb barley visible by eye that’s not all that close to the south pole of the sky anyway.
09:14
But even Polaris isn’t exactly on the pole — it’s offset a teeny bit.
09:19
So it does make a circle in the sky, but one so small you’d never notice.
09:22
By eye, night after night, Polaris is the constant in the sky, always there, never moving.
09:28
Remember, the sky’s motion is a reflection of the Earth’s motion.
09:32
If you were standing on the north pole of the Earth, you’d see Polaris at the sky’s zenith
09:36
— that is, straight overhead — fixed and unmoving.
09:39
Stars on the celestial equator would appear to circle the horizon once per day.
09:44
But this also means that stars south of the celestial equator can’t be seen from the Earth’s north pole!
09:49
They’re always below the horizon.
09:51
So this in turn means that which stars you see depends on where you are on Earth.
09:56
At the north pole, you only see stars north of the celestial equator.
09:59
At the Earth’s south pole, you only see stars south of the celestial equator.
10:03
From Antarctica, Polaris is forever hidden from view.
10:06
Standing on the Earth’s equator, you’d see Polaris on the horizon to the north,
10:10
and Sigma Octans on the horizon to the south,
10:13
and over the course of the day the entire celestial sphere would spin around you;
10:17
every star in the sky is eventually visible.
10:19
While Polaris may be constant, not everything is.
10:22
Sometimes you just have to wait a while to notice.
10:24
And to that point, you’ll have to wait a while to find out what I mean by this,
10:28
because we’ll be covering that in next week’s episode.
10:30
Today we talked about what you can see on a clear dark night with just your eyes:
10:34
thousands of stars, some brighter than others, arranged into patterns called constellations.
10:40
Stars have colors, even if we can’t see them with our eyes alone, and they rise and set as the Earth spins.
10:45
You can see different stars depending on where you are on Earth,
10:48
and if you’re in the northern hemisphere, Polaris will always point you toward north.
10:52
Crash Course is produced in association with PBS Digital Studios.
10:55
This episode was written by me, Phil Plait.
10:57
The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
11:01
It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.