LESSON 4
THE MOVEMENT OF THE SKY
How the Sky Appears to Move
The movement of the sky, remember, is really just caused by the rotation of the Earth.
Just as the Earth rotates around its axis, so does the sky. The axis of the celestial sphere lines up
with the Earth’s axis, and the sky rotates around this axis. Providentially, the North Pole is lined
up very closely with the star Polaris, in the constellation Ursa Minor. You have probably heard
Ursa Minor more commonly called the “Little Dipper.” From the Northern Hemisphere, the
whole sky seems to rotate around the star Polaris.
4.1 Ursa minor.
This also means that the celestial sphere has north and south poles, and an equator as well.
Now, you should recall that you can only see half of the celestial sphere at a time, simply
because the rest is below the horizon from your perspective – the Earth itself is in your way. As
the Earth rotates, hidden portions will rise into your sky from the west, while the eastern portion
of the sky will slowly set below the horizon. The angle at which this rotation occurs, however,
will depend on your latitude (just as the angle of the Sun’s path depended on latitude, as we saw
in the last lesson.)
Let’s look at some examples to see how this works. First, imagine you are standing on the
Earth’s equator. Now, the celestial poles lie directly north and south of you, right on the horizon.
The celestial equator will pass straight over your head, running from east to west. As the night
progresses, you will see stars rising straight up in the east and following a line across the sky to
set straight down in the west.
4.2 The view from the Earth’s equator. Note that the celestial pole lies right on the horizon,
and the celestial equator passes directly overhead.
Also importantly, from the equator you can see the entire celestial sphere, from the North Pole to
the South Pole. If you wait long enough, the entire celestial sphere will pass overhead. As you
will see in a minute, this is not true elsewhere on the Earth.
Now, let’s go to the opposite extreme and look at the sky from the North Pole. Here, the celestial
north pole will be straight overhead, and the celestial equator will like right along the horizon. As
the night progresses, you will not see any stars rise or set, they will all just rotate in great circles
around the celestial north pole.
Notice that from the North Pole, you can only see the northern celestial hemisphere. The celestial
equator lies on the horizon, and the southern celestial hemisphere will never rise above it.
4.3 The view from the Earth’s North Pole. The celestial pole lies directly overhead, and the
celestial equator runs along the horizon.
Of course, in reality you will be observing from somewhere in between the poles and the
equator. Let’s choose a location like St. Paul, Minnesota, which lies at about 45 degrees north
latitude, about halfway between the North Pole and the equator.
4.4 The view from the northern hemisphere between the North Pole and the equator. Note that
the north celestial pole lies between the horizon and overhead. Some of the southern celestial
hemisphere is also visible.
From this location, the sky of course will continue to rotate around Polaris, and the celestial
equator will be visible. but as we moved south away from the North Pole, the equator rose above
the southern horizon, and Polaris sank towards the northern horizon. Now, the sky continues to
rotate around Polaris, but it now does so at an angle. Just as at the North Pole, Polaris and the
stars near it do not ever sink below the horizon, but stars farther away do. These stars rise in the
east, take a curved path across the sky, and set in the west.
Also, you will notice that from this middle latitude you can see much, but not all, of the celestial
sphere. There are portions of the southern celestial hemisphere that will never rise above your
horizon, though you can see some of it.
The further south you move towards the equator, the higher the celestial equator will rise, and the
lower Polaris will sink.
Finding Polaris
Finally, we’ll wrap up this lesson by talking about the first two constellations we will learn in
this course. Constellations are imaginary figures in the sky made by connecting stars together.
By grouping stars into constellations, they are easier to find in the sky. Astronomers have
divided the sky into 88 recognized constellations. We will learn to identify several of these in the
night sky.
Polaris, the North Star, is part of the constellation Ursa Minor, or “the Little Bear” in Latin. Part
of this constellation is the asterism called “the Little Dipper.” An asterism is a group of stars that
make up a recognizable figure, but which is not an official constellation. Polaris is the star right
at the end of the Little Dipper’s “handle.”
Ursa Minor is not always the easiest constellation to find, so it’s wiser to star by finding Ursa
Major, “The Big Bear”, instead. Ursa Major contains the asterism known as the “Big Dipper”,
which is brighter and larger than the Little Dipper. Once you have found the Big Dipper, draw a
line from the two stars that make up its bowl – this line will point straight to Polaris.
4.5 Finding Polaris. First, look north and find the Big Dipper (Ursa major.) Then draw a line
from the stars at the end of the dipper – they will point to Polaris, at the end of the “Little
Dipper’s” handle.
Weather permitting, try to see if you can find these constellations tonight, and then watch for a
while to notice the path the stars are following across the sky. You should see the stars near
Polaris making small circles around it, and the stars farther away following longer curved paths
from horizon to horizon.