To help us learn about the celestial sphere let's talk a little bit about how
we define locations on Earth. Knowing what we know today, that the Earth is in
fact rotating, we can define the rotation axis as piercing through the Earth's
North and South Poles and we can divide the earth into northern and southern
hemispheres from the equator.
And we can draw parallels to the equator so we
define the equator as having zero degrees of latitude going all the way up
to 90 degrees north latitude at the North Pole and define southern latitudes
going all the way to 90 degrees south latitude at the South Pole. Most of us
live somewhere in-between.
We can define longitude, first by starting off with the
prime meridian which goes from the North Pole through Greenwich England to the
South Pole, and from there we can define what are called "meridians of longitude",
and this is what divides the Earth into eastern and western hemispheres. So I
live near Baltimore that is somewhere about 39 degrees north latitude and 76
degrees west [longitude]. The Cerro Paranal, Chile is home to the European Southern
Observatory, and that's at 24 degrees south latitude 70 degrees west [longitude],
and Rome Italy, for example is at 42 degrees north latitude and 12 degrees east so that
puts Rome on the same latitude approximately as Boston Massachusetts
than in the United States. Okay, well, going back to this idea of the
celestial sphere.
Let's start by taking the Earth's poles and projecting them on to the sky and
that gives us the North and South celestial poles and we'll take the
Earth's equator and project that onto the sky giving us the celestial equator.
So this in total is the celestial sphere. Now we know today that no, the stars are
not really imprinted on an actual sphere, but it's a useful fiction; it allows us
to identify not only the positions of stars in the sky but to help us
understand their apparent motion around us. So now we can depict the sky above us
I've positioned us here in Baltimore Maryland so if you look down here I've
got our location on the map you'll see that we're at 39 degrees north latitude
and about 76 degrees west longitude and here we are on earth as well so we have
the North celestial pole and the South celestial pole represented. The Earth's
equator has been projected onto the sky to give us the celestial equator. And
I've gone ahead and just added a few stars at random. On the right side of the
diagram is our horizon illustration so here we are standing on our horizon. The
celestial equator is projected onto the sky so the celestial equator is...well I
don't want to say "visible" because we don't actually see the celestial equator,
but it does pass through our sky here in Baltimore. The zenith is directly
overhead, the meridian goes from due north through the zenith to due south
and back around again, and the nadir is beneath us. So the celestial poles are
presented in both diagrams as well and you notice that the North celestial pole
is some altitude above our horizon meanwhile the South celestial Pole is
beneath our horizon hidden from our view.
So now let us imagine "riding", if you will, along different parts of the Earth.
And this is going to help us understand the apparent motion of stars. So let's go
ahead and begin our animation, and you'll see that the Earth is rotating; in
reality the stars are "fixed", if you will, on the celestial sphere. But from our
perspective on the ground, the stars appear to be rising somewhere in the
east and for the most part setting somewhere in the West. So to help us
understand this motion better, we're gonna go ahead and jump from Baltimore
all the way to the North Pole. Well what's going on up here? We're standing
at the top of the world so to speak and now the North celestial pole is directly
over our head at the zenith. All of the stars in the northern celestial
hemisphere are rotating around us. In fact, you notice that none of them are
ever setting; none of them are rising, they're just rotating around us carousel style.
It's worth noting that only the northern half of the sky is visible. Somebody at
the south pole by the way on Earth would likewise see nothing but the southern
stars rotating around them.
Now let's go ahead and transition from the North Pole down to the equator. So
I'm gonna set our latitude to zero degrees and notice that once again the
North celestial pole and...or the celestial poles and the equator
celestial equator are still 90 degrees separated from one another as they are
seen from this perspective. But because we're at the equator the
celestial equator is projected onto the sky directly above us in other words it
starts in the east and ends in the west but unlike what we saw at home it passes
directly through the zenith. Now what's interesting about this
perspective is that because the North and South celestial poles are directly
on the horizon every star in the entire celestial sphere is visible if you are
standing at the equator. Now we can't see stars during the daytime we have the Sun
out, but different stars become visible at night throughout the year so that
means that throughout the year you will see every star visible from anywhere on
Earth at the equator. Ok so what then about those of us who live at mid
latitudes like most of us do? Let's go ahead and return back home. So go ahead
and set us back to approximately our latitude on Earth... And here we are, we see
that the celestial sphere continues to appear to rotate around us but the poles
are tilted and well we're midway between the earth's equator and the North Pole
and so the North celestial pole is somewhere partway between the horizon and
zenith. It turns out that any star that is in a certain region surrounding the
North celestial pole if it's within a certain angle which just happens to be
38 degrees so if the stars within 38 degrees of the North celestial pole it
will never appear to rise or set. And these stars are known as "circumpolar
stars." So any star that lies within this red shaded area will never rise or set.
I'll go ahead and drag the star we grab the star really quick...there we go
go place it right in there and you notice that they're never going to set
below the horizon from our latitude here in Baltimore. So let's see what this
looks like from the ground.