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?
