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PRE-LAB
THE SKY AND CONSTELLATION TOUR:
Read the next pages, answer the questions on the next page, and hand in that page.
Keep pages 1 to 7; you will need these for the main Lab.
DO THE PRE-LAB BEFORE ARRIVING TO THE LAB
ALWAYS COME PREPARED
OTHERWISE YOU WILL NOT BE ABLE TO FINISH THE LAB IN TIME
IMAGE TAKEN FROM HTTP://WWW.FREEWEBS.COM/STARSURF/CONSTELLATIONS.HTM
The Sky #1  Pre-Lab 3  1
HAND IN THIS PAGE ONLY; KEEP PAGES 3 TO 8 FOR THE MAIN LAB.
1) Answer the following questions:
a) What do you call the angle between your HORIZON and the POLE STAR?
____________
b) What do you call the angle between the CELESTIAL EQUATOR and the POLE STAR?____________
c) How big is this angle (in degrees)?
____________
d) How big is the angle between your HORIZON and your ZENITH?
____________
2) Explain the difference between Horizon and Celestial Equator
3) Explain the difference between Zenith and Polaris. Under which conditions are they the same?
4) Explain the difference right ascension and longitude?
5) What are the units of right ascension and longitude?
6) Explain the difference declination and latitude?
7) What are the units of declination and latitude?
8) Why do we bother with two coordinate systems? In what sense do they differ from each other, and in
what sense are they similar?
2  Pre-Lab 3  The Sky #1
FINDING THE POLE STAR
Fig 1
Fig 2: The image to the left which is taken from
http://www.absoluteaxarquia.com/nightsky/cons
tellations.html.
That constellation chart shows a constellation
map that also includes other constellations. The
constellations lines are drawn in for easier
recognition. The Big Dipper and Cassiopeia are
highlighted in red along with Polaris.
To find Polaris, first locate the Big Dipper and
Cassiopeia. The Pole Star is between the Big
Dipper and Cassiopeia. Draw an imaginary line
between the two stars farthest from the handle
of the Big Dipper (these are called the pointer
stars) and extend this line roughly 3 times
towards Cassiopeia. The pointer stars always
point to the Pole star — no matter how the
Dipper is positioned. The Pole star is actually
the tail star of the Little Dipper. Since the Little
Dipper has fainter stars, you cannot always
make it out, particularly if you are in a big city.
The Sky #1  Pre-Lab 3  3
THE CELESTIAL SPHERE
Imagine the sky to be a dome, and like in antiquity, imagine all stars, galaxies, planets, etc., to be
painted on the inside of this dome. You see the EARTH only as far as the HORIZON and to you it looks
like a circular flat disk. The point directly above you (the top of the dome) is called the ZENITH. Once
you locate and face the POLE STAR, you also know that that direction is NORTH. SOUTH is opposite that
(behind you); WEST is to you left and EAST is to your right. The height of the POLE STAR (or any star)
above the HORIZON is called ALTITUDE. This is basically an imaginary line drawn from the HORIZON to
the POLE STAR. Although it looks like a line, it is actually an angle. Recall that distances on the sky are
always measured in angles! Below is an artist’s impression of the sky. (Remember that this is a
perspective drawing; the horizon is actually a circle extending all around you, not an oval.) Please learn
and know these terms.
Fig 2
ZENITH
The point directly above you.
HORIZON
The line in the distance where EARTH and sky meet. It makes the EARTH look like a flat,
circular disk.
MERIDIAN The imaginary line in the sky connecting
ZENITH.
NORTH
with
SOUTH
and passing through the
ALTITUDE The height of a star above your HORIZON. It is an angle.
AZIMUTH
The angle along the HORIZON from NORTH to the star. This is also an angle.
4  Pre-Lab 3  The Sky #1
THE APPARENT MOTION OF STARS IN THE SKY
During the night, all stars seem to move on the sky. Only one star does not move. That star is the POLE
STAR (Polaris). The other stars rotate counter-clockwise around the POLE STAR. To visualize this
rotation, imagine a giant umbrella like below, with the POLE STAR at its center and you at the handle.
The Big Dipper and Cassiopeia are drawn on this umbrella. As the umbrella turns, you see the stars
move around the POLE STAR in a circular manner.
Fig 3
The Sky #1  Pre-Lab 3  5
Now translate this motion onto your dome as shown below. The line from the POLE STAR to you
corresponds to the handle of the umbrella - it is the AXIS of the CELESTIAL SPHERE as well as the axis of
the EARTH.
Consider the motion of the star from the previous drawing of the dome (Fig. 2 of the upper half of the
CELESTIAL SPHERE). Let’s figure out how that star will appear to move throughout the sky. Like the
Big Dipper and Cassiopeia it will rotate counter clockwise around the POLE STAR in a circular manner
(indicated by the arrows). The star rises in the EAST and sets in the WEST, as all objects (sun, moon,
planets, stars) do. It reaches its highest point in the sky when it crosses the MERIDIAN (the imaginary
line connecting NORTH and SOUTH). This is when the star TRANSITS.
Fig 4
6  Pre-Lab 3  The Sky #1
HOW DO YOU FIND STARS IN THE SKY?
Well, you could specify the position of a star in terms of ALTITUDE and AZIMUTH. However, this is
messy because ALTITUDE and AZIMUTH change throughout the night. Also, they depend on your
position , (i.e., your HORIZON and your latitude) on the EARTH.
Clearly, we need a better method to describe the positions of stars on the sky. Astronomers therefore
invented sky charts that are comparable to maps. Maps have a coordinate system — you can find any
place on EARTH by specifying its LONGITUDE and LATITUDE (the x and y axis). Below you can see
what we mean by LONGITUDE and LATITUDE.
The positions of the stars in the sky are described in the same manner as cities on the globe. However,
instead of using the word LONGITUDE we refer to it as RIGHT ASCENSION and to the LATITUDE as
DECLINATION. This can be seen in the diagram below. The DECLINATION is measured in degrees, just
like the LATITUDE. However, RIGHT ASCENSION is measured in hours. This means that 360o in
LONGITUDE corresponds to 24 hours in RIGHT ASCENSION — i.e., the time taken for the EARTH to
complete one rotation around its own axis.
Fig 5
The Sky #1  Pre-Lab 3  7
KEEP THESE DEFINITIONS HANDY FOR THE REST OF THE LAB!
Fig 6
LATITUDE
The angle from the EARTH’s equator to a place on EARTH.
LONGITUDE
The angle along the EARTH’s equator from the PRIME MERIDIAN to the
MERIDIAN (NORTH-SOUTH line) of the city.
RIGHT ASCENSION
The angle along the CELESTIAL EQUATOR from the zero point to the
MERIDIAN (NORTH-SOUTH line) of the star.
DECLINATION
The angle from the CELESTIAL EQUATOR to the star.
CELESTIAL EQUATOR
An imaginary circle, similar to the EARTH’s equator, but drawn onto the
CELESTIAL SPHERE. It is perpendicular (i.e., 90o) to the axis.
CELESTIAL NORTH POLE
The point of the CELESTIAL SPHERE; it corresponds to the EARTH’s NORTH
o
POLE. This point almost coincides with the position of Polaris (it is 1 off).
CELESTIAL SPHERE
The imagined sphere surrounding the EARTH on which all celestial objects
seem to be drawn.
8  Pre-Lab 3  The Sky #1