1. Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
The Ecliptic
Have you ever noticed how the planets, the sun and the moon seem to
align in the sky, such that you could draw a straight line through all of them?
All the visible bodies in the solar system travel along this line, and it’s called
the ecliptic.
For thousands of years, human cultures have attached special meaning to objects
that move around the ecliptic. The Greeks, for instance, identified their gods with
these objects. While the rest of the stars in the sky remain still relative to each other,
only the seven objects along the ecliptic have the appearance of changing in their rela-
tionships from night to night. The rest of the night sky appears static in relation to itself.
This static background is often called the celestial sphere. This is why we can’t put the
planets into defined constellations like we can with the rest of the sky--their positions are
shifting too often, whereas constellations retain their figures.
The seven objects that move independently of the celestial sphere include the five planets Mer-
cury, Venus, Mars, Jupiter and Saturn, as well as the sun and the moon. So important were these
objects to ancient humans that it is because of these seven celestial bodies that we have seven
days in a week.
Why do these seven bodies follow
roughly the same path, the thing
we’ve termed the “ecliptic?” Well,
the ecliptic exists because of the
way that planets orbit the sun. We
usually picture the solar system
as being a bunch of circles or el-
lipses representing the path that
the planets travel (as shown in the
figure to the right). This is what
the solar system would look like if
viewed from many billions of miles
above. (And assuming we could
magically see the paths of planets
and moons, of course.)
Moon
Mars
Now, imagine if you were to view
the visible solar system from the side (edge-on), rather than from above. What might you see?
Sun Mercury Venus
Earth &
Moon Mars Jupiter Saturn
If you drew a line between each planet above (try it!), you would see that they roughly create a straight line.
This is because all the planets in the ecliptic orbit the sun in the same plane. This means that if you had a flat
piece of cardboard that stretched across the whole solar system, all the planets would be physically on top
of the cardboard, like peas on a flat plate.
But this doesn’t quite explain why the planets appear where they do in the sky. For that, we’ll have to think
more deeply about our physical place in the solar system.
A view of the night
sky, with two lines
showing the
path that all
ecliptic bodies
travel inside.
Venus
Figure 1. A partial map of
the the solar system (not to
scale), viewed from above,
showing only the objects
in the solar system that we
can see with the naked eye.
What is missing?
Figure 2. One possible side
view of the visible solar
system. The size of the planets
and their relative distances
are not to scale.

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Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
Worksheet: Thinking About the Ecliptic
The bodies that move through the ecliptic have fascinated humans for thousands of years. Though early hu-
mans didn’t know that these were planets, they realized quickly that these bodies were not like other stars.
The path that the ecliptic weaves through the sky passes through thirteen constellations. These constella-
tions, collectively, are known as the Zodiac. Many people attach special significance to these stars, even going
so far as to claim that they can influence our lives.
Why do the planets appear where they do in the sky? Sometimes the way the planets line up along the ecliptic
can be counterintuitive. Sometimes Jupiter looks closer to the sun than Venus, but we know that Jupiter is
much further away from the sun than Venus!
Deceptive appearances
Though the way the planets line up may seem random, there is actually a very logical reason for their order. It
has to do with our sense of perspective.
Forget that we are talking about planets for a moment and consider the three shapes below: cube, sphere,
pyramid.
For the sake of this example, these three shapes are arranged around a circle. Viewed from above, they look
like this:
1
2
3
Fig. 1.1: An overhead view of our three
shapes.
This is an overhead view of these shapes. This means that figure 1.1 is what you’ll see when you look down at
these shapes as they sit on the ground.
What would a side view look like? Imagine first that you were sitting down at point 1 on the circle. From this
angle, you would see this:
From the vantage point of point 1, we see, from left to right, the cube, then the sphere, then the pyramid.
Fig. 1.2: the view from point 1.

3. 2
3
1
Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
What do we see from another vantage point? Let’s try point 2. Looking out from point 2, towards the inside
of the circle, we would see this:
Going across, we now see sphere, triangle, cube. Notice how the triangle is elevated a bit in the background,
because our viewing angle is elevated. Note the perspective we have now: the circle around which we’ve ar-
ranged these shapes, viewed from the side, doesn’t look like a circle anymore. What if we put our head against
the ground so that our eyes were so close to the ground that the circle appeared just as a line? At this extreme
angle, we would see this:
Viewed edge-on, it’s hard to get a sense of perspective. Yet this is similar to the way that we see stars and
planets from Earth, since the Earth is in the same plane as the other planets, and because all the planets are
so far away..
For instance, Venus and Saturn sometimes appear next to each other in the sky, but we can’t tell just by look-
ing which is further away. Additionally, brightness doesn’t always correspond to nearness. Sirius, for instance,
is one of the brightest stars in the sky, brighter even than Mars, but Sirius is hundreds of thousands of times
farther away from Earth.
Exercise 1: What would we see if we looked out at our shapes from point 3? Draw a diagram below, similar to
figures 1.2 and 1.3, that shows the order in which we would see the shapes.
Fig 1.3 : the view from point 2.

4. 1
Mercury
Venus
Earth
Moon
Mars
Sun
Mercury
Venus
Earth
Moon
Mars
Sun
Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
Making it planetary
Now that we’re getting used to thinking about
anglesandperspective,let’strytomakeananalogy
to the shape example, except this time with real
planets. For our first example, we’ll consider only
what are called the terrestrial planets: Mercury,
Venus, Earth and Mars. These planets are called
terrestrial because they are composed primarily
of solid, rocky matter like the Earth (terre being
Latin for “ground” or “earth.”) Contrast these with
gas giants like Jupiter and Neptune.
We’ll start with another “above view” of the inner
solar system. Consider the image to the right: a
possible (yet unlikely) arrangement of planets, all
in line.
Though it is very rare for the planets to align so
nicely, we’ll consider this possibility for the sake of
this example. From the perspective of someone
standing at point 1, just outside the orbit of Mars,
looking towards the inner solar system, they would see something like this:
A very straight line of planets, ordered by distance from the sun. This is a very familiar arrangement we are
used to seeing on posters and in diagrams.
There are several reasons we’ll never see the planets aligned like this. First of all, we are not standing at point
1. Instead, we are stuck on the Earth (unless you are one of the lucky few humans who is orbiting the earth
in a spacecraft at this instant). Secondly, viewing
both the sun and the planets at the same time is
very difficult, because the sun’s light blots out our
ability to see other objects in the sky!
A more likely orbital arrangement of the terrestri-
al planets might be something like the following
image on the right. Notice that the planets seem
much more randomly oriented than before.
Let’s move our focus away from point 1--a point in
empty space that we’ll probably never visit--and
think instead about the Earth, a place we’ve spent
our whole lives.Take a look again at the frozen mo-
ment in time to the right. If you looked up from the
Earth on this day (assuming you could magically
see the planets in daylight) what would you see in
the sky?
Sun Mercury Venus
Earth &
Moon Mars

5. Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
Think back to the shape example. This one’s a bit harder--there are several circles to think about, as well as
more than three objects. It might help to shift our focus to the Earth--as you’ve noticed, we had a sun-centered
model before. Let’s leave our orbital trace lines but shift the focus to the Earth:
This is the same image as before, but rotated at a 55 degree angle, so that the Earth is at the bottom now.
Remember, this is still an overhead view. The planets are all in the same plane, like the shapes were. From the
perspective of an earthling, these celestial bodies would appear in the sky like this:
Though not precisely to scale, I’ve adjusted the size of the moon to reflect the way it actually appears in the sky.
After looking at the above, consider how strange this ordering of bodies is--Venus, Mercury, Sun, Moon, Mars.
It’s kind of confusing, given what we know about the actual location of these planets in the solar system.
If you had never seen a model of the solar system before, how would you know which planets were closer and
which planets were further away? Do you understand why it took so long for humans to figure this out?
There is one more issue to discuss in considering the appearance of the ecliptic. As mentioned before, you can’t
see planets during the day time. The sun blots out all other bodies except for the moon, although sometimes
at dawn and dusk you can see Mercury or Venus.
How do we consider the course of a day on Earth? Well, you might recall that it is always day on half of the
planet, and night on the other half. If we were drawing the solar
system from above, we could divy up the Earth as follows: the half
facing the sun is always lit, and the half away from the sun is always
dim. However, this does not take into account that it is a different
time everywhere on the planet. When we divy up the planet into
day and night “views,” we are simplifying the Earth into only two
different time zones.
This is a convenient simplification because half the sky is always
visible from wherever you are on Earth, assuming there are no clouds are mountains. We have simply chosen
to divide it into mid-day and midnight views for convenience’s sake, though we could make other divisions
(dawn/dusk, for instance).
If we draw a line extending out from the night-day line, then we can assume that everything on the night
side is visible, and nothing on the day side is visible, except for the moon and sun. You can imagine this day-
night line as follows: First, draw a line from the center of the sun to the center of the earth. Then, draw a line
perpendicular to that line, which also touches the center of the Earth. The dotted line above simulates this
line. Now, let’s reconsider our earlier example with this in mind...
MercuryVenus
Moon
Mars
Sun
Sun
Earth
Daytime
(night sky
not visible)
Night
Mid-day Midnight
Dawn
Looking down on Earth, from above, half
would appear lit and half dimmed (day
and night).

6. View A (darkness)
View B (light)
Astrosheets © 2008 Keith Spencer / / / Intended for ages 14-140
Mercury
Venus
Earth
Moon
Mars
Sun
Why does the sky appear as it does?
Reconsider the example from the last page, the image of the solar system showing only the terrestrial planets.
Let’s look at this image again, but take into account day and night.
1. What object would you see from the night side of Earth (in the middle of the night)?
2. What objects would you see from the lit side of Earth (at noon)? In what order?
2a. If you could see planets while the sun was shining, what would you see on the lit side of Earth?
Now, consider this model of the entire visible solar system. Also, we’re going to look at Earth a little differently
now. Whereas before we were dividing our skies precisely into a midnight sky on one side, and a noon sky
on the other, now we’re going to look at
the sky at more arbitrary times.
1. Standing on Earth, what would you see
in the night sky from the vantage point of
view“A?”
2. What would you see from view“B?”
3. Viewed from above, the planets rotate
counter-clockwise; the Earth, for instance,
makes one counter-clockwise revolution
every 24 hours. Knowing this, what time of
day would you estimate it is on earth from
view“A” (dusk, dawn, afternoon, morning,
noon, or midnight)?
(Hint: Think about which way the Earth
turns. Where is the sun going to appear in
the sky? Is it about to rise, or about to set?