在这个免费的课程里学习目前的太阳系探索背后的科学。用物理、化学、生物和地理的法则去理解关于火星的最新的新闻，理解外太阳系，思考太阳系外的行星，寻找附近环境以及更远区域的可居住性。这个课程普遍在本科级别的数学和物理知识上讲授，但是大多数的概念和课程并不需要这些知识就能理解。小测和期末考试会考察你是否能对学习过的材料进行批判性思考，而不是简单的记忆事实性的知识。这个课程应该有些难度，但会很有收获。
Lecture 1.01: Early views of Mars
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来自 Caltech 的课程
太阳系科学
263 个评分
从本节课中
Unit 1: Water on Mars (week 1)
与讲师见面
Mike BrownProfessor
Planetary Astronomy
We're going to start this class with looking at the planet Mars and
trying to understand where and when water has existed on the planet Mars.
We're starting with planet Mars for really good reasons.
Mars has had an extraordinary impact on the history of planetary science,
both a recent scientific history of the field and
really the historic history of the field, back to the,
the earliest times when people tried to understand what planets really were.
One of the reasons that planets have been so important historically is because
they do something different from everything else up in the sky.
Everything else in the sky,
the stars they just move in a nice fixed pattern across the sky.
Planets don't.
Planets move between the stars, among the stars, and they,
they clearly stand out as something different, something interesting to see.
Now, what I want you to do is go find some.
Planets are easy to see.
In this whole class, we're not studying a set of abstract
objects that exist somewhere out there in space.
We're studying objects that you can go outside at night and look at, and
now know that you've learned something about this.
Let's figure out how to find some of the planets so you can go do that right now.
Or when the sun goes down.
A great tool for finding planets, and anything else in the sky, is,
is often your phone.
Many phones these days have a very nice app where you can find sky maps.
On your phone, you point to the position in the sky,
you see through your phone what you're seeing on the other side of your
phone except, through your phone, it has labels on it.
So you can say, where is, where is Mars right now?
And you go around, there's sky.
Oh!
There! Or you say,
what is that bright thing in the sky right there?
You're like, oh, that's Jupiter, or that's Venus, as it might be the case.
These are fantastic.
Sometimes you want something a little bit more sophisticated than just your phone.
There are other nice ways to do it.
There are websites that are available.
Their easy to find just just search for any sort of website.
There's a, there's a nice set of free software, though,
that I really like that we'll use right now to see what's in the sky right now.
Okay, the software is super-simple to find, you can just Google Stellarium,
or even easier, www.stellarium.org.
It's free.
And it installs on most operating systems you can think of.
Click on it, download it, and start to check out what's going on in the sky.
When you do that, you'll get something that looks sort of like this.
And we're now looking at the sky from Earth, Pasadena, zero meters, okay.
I have to tell you this doesn't look anything like Pasadena.
This kind of looks like the French countryside to me, so
they have some default going in.
And I have the date set to April 1st, 2015 at 15:14.
You can see that the sun is just about to set here.
If you want to we can speed things up a little bit by pressing right here.
Press.
Press.
Press.
Faster and faster and faster.
Sun is going down.
Behind at the French countryside here in Pasadena, and
you'll start to see what's in the sky right at sundown.
And what do we see?
Well Venus comes in.
Venus is, if you look out, if this is April 2015 when you're viewing these, and
you look to the west as the sun goes down, you can't miss Venus.
It's spectacular in the sky.
This is the time of year when I get phone calls and
emails all the time saying, what is that bright thing in the sky?
Is it a spaceship?
Is it, is it something else?
And the answer is without even looking in the sky, the answer is, it's Venus.
But if you look really carefully, if you have a nice, western horizon,
go a little bit later into the night, and you can see Mars, and even in,
in this view you can see that Mars glows a little bit yellow compared to Venus,
if you see it in real life, you'll really be able to tell the difference,
Mars is distinctly red compared to stars, compared to Venus.
Let's keep on going th, through the night and
see what else is around tonight as we're going through,
we're only going to be looking west, you're allowed to look straight overhead.
Some of the bright stars, Aldebaran, Betelgeuse, Rigel,
Sirius, some of these beautiful bright stars but
coming up almost now to the west, we'll go a little faster.
Here it is, Jupiter.
Jupiter, at sunset, is nearly straight overhead.
And, you can, if you know your constellations, or
you can use the software to figure it out it is in between Pollux,
Pollux is part of the constellation Gemini.
Regulus is in, in Leo, the Lion.
And since this is not a particularly bright region of the sky, bright meaning,
there are not that many bright stars in the sky, and so you'll see Jupiter.
It will be a quite obvious star sitting out there.
One of the fun things you can deal in real life we don't do it in real life but we'll
do it in fake life right here, is that you can watch the sky night after night,
after night, after night, after night, after night, that gets a little old.
But what you'll find is that all the stars are always in those same fixed patterns,
but the planets are moving around them.
I want you to go try to see.
Mars moves very quickly, Venus moves very quickly,
you have to go look at them, compare them to the background stars.
If you can't see Mars because your horizon doesn't go down far enough,
just watch Venus.
Watch Venus tonight, look at it the next couple of nights,
look at it the next couple of weeks, and
watch how the background stars are moving compared to it.
Look at Jupiter, Jupiter moves more slowly, it's further away.
But again, if you watched Jupiter over the course of the season,
you see that it's moving.
There's no way that you could have been someone who,
who paid any attention to the sky at all, and
would not have noticed that these things are moving differently than all the stars.
And, Mars is one of the most interesting ones.
Unfortunately, you're not going to be able to follow Mars throughout
the sky right now.
But, if you could watch throughout the course of the year,
you would see that Mars makes some really peculiar motions.
Mars sticks out.
It's red, it's interesting, it's moving.
If I were someone in an ancient civilization looking up at the sky,
I would really want to know what the heck is going on with Mars.
So Mars is an obvious, bright, red object in the sky, really stands out because of
its redness, and the redness has been noted since the earliest times, but
the reason that planets have been noticed as interesting
objects from the earliest times is because they move in the night sky.
And of the planets that move in the night sky, which is all of them,
Mars is the one that is the most obvious to watch it move.
In fact, if you find it tonight, make a little note of where it is.
Draw a little picture of precisely where it is with respect to the stars around it.
And then go out tomorrow night and look again, and
it will be in a visibly different location if you can look carefully.
Come back in a week, and it'll be very different location from the other things.
Why is it moving?
Well again, if we have the, the sun here, and we put the Earth, and
we put Mars, we know that Mars is going around the sun,
so you think of it as moving, but it's not just Mars going around the sun.
The Earth, of course, is going around the sun and
as the Earth goes around the sun, you're looking at Mars and you're actually,
we are overtaking Mars, we're going faster so it's like we're in the fast lane and,
and there's this car in the slow lane over here we're overtaking it.
And we're watching the mountains in the background and it almost looks like
the car is going backwards compared to the mountains in the background,
the mountains in the background in this case are the very distant stars,
over here which are, which look fixed.
And so right now we see Mars going essentially backwards compared to
the direction it really moves and that's because the Earth is overtaking it.
This is retrograde motion and if I were to plot the location of Mars on the sky
over the course of one year, or one Martian year,
what you would see is that Mars starts out in some location, let's say right here.
And It moves for awhile then it slows down as the Earth overtakes it,
makes its retrograde loop, and speeds back up again,
and then it keeps doing this year, after year, after year.
You can imagine how intriguing that
behavior is to somebody who is noticing movements in the sky for the first time.
It's not just something that's marching evenly across the sky.
There's clearly a pattern here.
But, and here is where I think Mars was actually one of
the more important things in understanding how planets went around the sun.
Mars not only does this pattern repeat year,
after year, after year, it doesn't exactly repeat year after year.
There are differences every single year.
And the reason there are differences every single year is because,
unlike the Earth which goes in, in orbit around the Sun in a nearly circular orbit,
Mars is, is notably eccentric.
We'll talk more about orbital dynamics and
eccentricity later in particular and more talking about things like comets and
objects in the outer part of the solar system.
But I'll just remind you that if, if Mars
is eccentric sometimes it's further from the sun, sometimes it's closer to the sun.
And that means sometimes when we're looking at it right here, sometimes it
might be a little bit closer, sometimes it might be a little bit further.
And the speed during these retrograde loops and the size of these retrograde
loops depend on how far away Mars is from the Earth at the time.
This is something that drove Kepler crazy, I think to begin with,
he was trying to understand how Mars in particular went around the sun.
Because the data, or the best data,
the discrepancies between all the ideas that people had about circular orbits or,
or, or slightly modified circular orbits, or something, the discrepancies were so
large that you could not explain the positions and the speed of Mars
without finally realizing how Mars is really working in this elliptical orbit.
Finally understanding that Mars was moving in an ellipse with the sun at its focus
and realizing that the speed that it was moving was dependent on how far or
close to the sun it was, enabled Kepler to finally put together a plot of like this,
of the distance of Mars from the Earth.
Now you have to imagine that the Earth is stationary at the center right here.
Here's the Earth right at the, at the middle.
In fact this whole thing will make the,
the Earth and as if you're just sitting on the Earth stationary and
you're watching where Mars goes over the course of the year forgetting that
the Earth is going around the Sun, you see it through something like this.
First Earth is, can be very close to Mars.
And then this would be a year later, a little further away,
a little further away, a lot further away, a lot further away.
These are at all of its closest approaches.
And of course, when the, Mars in on the other side of the sun from you,
it's way out over here.
And this is where Mars was between 1560 and 1596.
And if you know your symbols for the constellations in the Zodiac, here's,
you can even see where Mars really was.
I actually don't know them.
But I guess that's a Gemini.
Doesn't that sort of look like it should be a Gemini?
That's the only one I know.
I only know Gemini because I was born in June, and therefore I know that symbol.
Not surprisingly then Mars is bright in the sky.
It's doing interesting things.
It's red.
There's clearly something special about Mars.
When telescopes were first developed Mars was one of the things that people
were very eager to take a look at.
Galileo looked at Mars through his telescope but his telescopes
were not good enough to really see anything of detail on the surface.
In the next lecture we'll talk about the first telescopic observations where you
could actually see features on the surface,
and finally start to learn something about this object that had up until that point
in human history, had just been this bright spot moving across the sky.
