The Celestial Sphere Part 2

Now, some of these stories are quite good. Here is a representation of a typical winter
sky. And if you look at the stars, here, winter is dominated by the constellation of Orion
the Hunter. This is the winter sky. In mythology, not only is Orion a hunter. You can see
that he's attacking a bull.

And the V of stars here is called the Hyades. That represents the head of the bull. In fact,
this star, Aldebaran-- and the word literally means "eye of the bull." In fact, you can
make the forms of the bull there. So Orion is attacking the bull.

But not only is Orion recognized as being a hunter, he's a bit of a philanderer, as well.
There are seven maidens on the back the bull. And those maidens are called the Pleiades,
seven sisters. Here is a mythological depiction of the seven maidens.

And over here are the actual stars that represent these maidens in the sky. It's actually a
young group of stars. You can see seven of them with the naked eye. There's over 200 or
so, if you look at a small telescope.

Now, in mythology Orion is chasing these maidens across the sky. And you can see that
as you see this sky unfold. As the earth spins on its axis, then these constellations appear
to move in the direction of West. And Orion is constantly chasing them but never
catching them.

Now, if you read the full story then you'll find that Orion eventually dies by being killed
by a scorpion. Now, if you look carefully, the scorpion is not actually in the winter sky.
There's a storm up in the sky. There's the constellation of Orion. But don't worry about
these coordinates, here.

We'll talk about coordinates of declination and Orion ascension in a little bit. But this is
just a map of the sky showing you the different constellations of the seasons. Orion is a
winter consolation. If I draw a line down, you can see the time, this idea of real time,
when that constellation appears.

And if you look for the scorpion, well, the scorpion's actually over here, in the summer
sky. And there's the time that it appears. So why are they linked together in mythology?
Well, if you read the time difference between those two-- and you can see there's
approximately 11 auras difference between those constellations in the sky.

Now, what does that mean? Well, Orion's a constellation for wintertime. So as Orion is
setting in the West, as Orion is getting lower down in the sky, as Orion is fleeing away,
up comes the scorpion from the East, on the opposite side of the sky, rising into the sky,
rising in prominence as the hunter feeds in the West. And so, you can see these
mythological stories played out in the nighttime sky, as the seasons pass by.
OK, so there you have it, the different constellations that you can see throughout some of
the seasons. And from our perspective, we don't really appreciate, fully, what's going on.
For instance, if you look at the famous constellation of the Great Bear, Ursa Major.

You can see a recognized pattern of stars in the sky called the Big Dipper. And I'm going
to mark it. It looks more like a saucepan in the sky. And there's the pattern of the Big
Dipper. Now, each one of those stars is a different distance from the Earth. And we just
really don't get any kind of idea as to the distances of these objects.

Like, for instance, if you start marking off a few of these stars-- now, there are two stars
there. The fainter one is called Alcor. And the brighter one is called Mizar. Now, if I
were to tell you that the distance between Alcor and Mizar was 3 light years, you could
get a sense for how much a light year is on the pattern of stars in the sky.

Now, the next star along, in the handle of the Big Dipper, is the star Alioth. You can see
it's quite bright, there. And I can tell you that Alioth and Alcor are actually 6 light years
apart. And that just doesn't seem to make sense, does it? How can this star and this star be
6 light years apart, whenever these two stars, which appear very close together, are
actually 3 light years apart.

Well, in order to get a sense for what's going on, you need to be able to understand the
full distances. So if we look from the perspective of the Earth-- here's where the Earth is
in space-- it turns out that Alcor-- Hydrae aligned to Alcor. Alcor is here in the sky. And
Alioth is up over here in the sky.

There's separation. There's separation. There's about 6 light years. Now, from our
perspective, we can actually draw a triangle, there. And you'll find out that the angular
separation of those two stars is about 4 degrees. And if you measure that, I know you'll
find it at the time of 4 degrees is equal to 6 divided by 81. And that's because both of
those stars are about 81 light years from the Earth.

Now, why does Mizar appear so close to Alcor? Well, it's actually just in the same line of
sight. And if I were to market it off in this diagram-- let me just change the color so you
can get a better idea-- Mizar is about here. So it appears close together in the sky, but it's
actually only 78 light years away from the Earth.

So it's three light years closer but pretty much in the same direction as Mizar. And we
just don't get that sense when we're looking up into the nighttime sky. Some stars can
appear very close together, even though they are very far apart. But you get that sense of
what's going on.

But the first person to sit down and start to record the stars in the sky, and put some sort
of scientific definition to them, was the first true astronomer. His name was Hipparcos,
and he is recognized in history as the first person to start cataloging the stars. And if you
look up into the heavens, you'll find out that the brightest stars have all got names.
So here is the consternation of Orion. There is the shoulder of Orion. That star is called
Betelgeuse. And then, down over on the other side, you'll see the foot of Orion. That's
called Rigel. Now, the brightest stars in the sky have got names.

The vast majority of those names are actually Arabic names. Betelgeuse doesn't mean
anything in English. But in Arabic, it means armpit of the giant. And Rigel doesn't mean
anything in English, but in Arabic, it actually means foot.

And the reason why they've got Arabic names is because, during the Middle Ages, when
the Greek writings fell away from prominence, the center of learning shifted into the
Arabic world. And so, we get a lot of the ideas from the Greeks, translated through
Arabic astronomers, which is why the vast majority of the names of stars in the sky have
got Arabic names.

Now, what Hipparcos started to do then, was he started to classify stars. And originally,
they were classified according to simply the names of the alphabet, usually being with the
alpha star being the brightest in the constellation, beta star being the next one, so on and
so forth. That's not always the case.

Sometimes the alpha star isn't the brightest. Sometimes it's simply the most prominent
star in the constellation, or it means something in terms of what the constellation
represents. Now, what he did was, he started to classify the stars. And he said that the
brightest stars were stars of first magnitude.

And then he said the ones that weren't just as bright, we'll call those second magnitude,
then third magnitude, fourth, fifth. And then the faintest stars that he could see, he called
those sixth magnitude stars. Now, we still use the same system that Hipparcos used in the
past. It's typically the way science works. But we have made it much more scientific.

We start off with the same idea that the faintest constellations that you can see are sixth
magnitude. But in order to make it much more scientific, you've got to use instruments to
measure the brightnesses of stars, because the human eye is not very good at doing it. In
fact, when you're looking up at the stars, if you were to look at two stars close together,
and you considered them to be about five times difference in brightness, a machine would
actually measure them to be about 100 times difference in brightness.

And so, what astronomers have specified today is that five orders of magnitude equals
exactly 100 times difference in brightness. You can see that if you actually look out in the
nighttime sky. If you pick out the belt stars of Orion, which you can see there-- Now, the
bottom two stars are very, very similar brightnesses.

Now, the bottom one is called Alnitak, and the middle one is called Alnilam. I would
dare you to have a look at them in the nighttime sky and tell me which one you think is
the brightest. It's indiscernible to human eye. The third star, Mintaka, is easy to see. But
it's actually a fainter star.
Now, when we make this scientific system a bit more rigorous, you'll find out that, rather
than going to first magnitude is the brightest object in the sky, you end up going into
negative numbers. So the brightest object in the sky is minus 26.7. That's the sun. There's
nothing in sky that's brighter than the sun.

The next brightest object in the sky is the moon. Its magnitude is about minus 12. Now,
the brightest star in the sky is Sirius, minus 1.45. So we make it much more scientific, but
it makes it a bit more confusing.

Because you can go to zero magnitudes, and then you can actually go into negative
numbers, as well. So if you go into a computer program, like Stellarium, and you click on
a star, it will actually give you as a magnitude number of that star.