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Period _______ Name:__________________________________
___________________________________
ALTITUDE AND AZIMUTH (Fall)
I. Introduction
Astronomers require a coordinate system for the sky to find objects in the same way that a navigator or geographer
needs a coordinate system to find objects on the Earth. Navigators and geographers use latitude and longitude to find
their way around on the surface of the Earth. Astronomers, though, have one problem that Earth-oriented people don't
have. Objects in the sky appear to move over time with respect to an Earth-bound observer.
Astronomers therefore have to make a choice: should their coordinate system be fixed on the sky so that it moves
along with the stars and other objects? This is called the "Equatorial System" and has the advantage of having
coordinates that do not change with time (much) with respect to the objects in the sky. Or should astronomers fix
their coordinate system to their local horizon (the "Horizon System") which does not move with respect to
themselves? This system has the advantage of not changing with respect to the astronomers no matter what time of
day it is, but the coordinates of the objects are then continually changing. Note that these coordinates will also change
if the observer changes location.
It turns out that astronomers use both methods depending on what the circumstances are.
how to use the Horizon System of Altitude and Azimuth.
In this lab you will learn
II. Terms and Definitions of the Horizon System
ALTITUDE: The angle of an object above the horizon. Altitude is measured in degrees along a line perpendicular to
the horizon, to the object of interest. An object on the horizon has an Altitude of 0 o, an object directly overhead has
an Altitude of 90o.
AZIMUTH: The angle from North clockwise along the Horizon to the perpendicular line you used to measure
Altitude. Therefore, North has an Azimuth of 0o and South has an Azimuth of 180o.
MERIDIAN: An imaginary line which passes through the NCP, the observer's Zenith, and intersects the Northern
horizon at an Azimuth of 0o and the Southern horizon at an Azimuth of 180o.
NORTH CELESTIAL POLE (NCP): The point in the sky directly above the Earth's geographic North Pole. All of
the stars appear to rotate around this point because of the Earth's rotation on its axis. This apparent motion is similar
to the effects of being on a merry-go-round. Your surroundings appear to be moving around you when it is really the
motion of the merry-go-round causing the effect. Because the Earth is a sphere, the Altitude of the NCP is equivalent
to the observer's latitude. Note that Polaris lies within 1o of the NCP and so is nicknamed the "pole star."
ZENITH: The point directly overhead. It has an Altitude of 90o. Azimuth is not defined for the Zenith.
Study the diagram below. Note the positions of the terms defined on the previous page with respect to the
observer.
Celestial
Equator
Meridian
W (270 degrees)
S
(180 degrees)
Observer
To NCP
N (0 degrees)
E (90 degrees)
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III. Estimating Angular Measurements
You can estimate the angular distances between objects or the angular sizes of objects by using a few simple
tricks:
1. The angular size of your fist held at arm's length is approximately 10o.
2. Your hand, with fingers spread, held at arm's length is approximately 20o from the tip of your thumb to
the tip of the little finger.
3. The width of your little finger, again held at arm's length, is approximately 1o.
4. You can also use the Big Dipper (Ursa Major) to estimate and calibrate angular distances and sizes. The
two stars forming the top of the ladle of the Big Dipper are 10o apart. The distance between the pointer stars
of the Big Dipper are 5o apart. The distance from Polaris to a point midway between the pointer stars is
about 30o.
Polaris
28
10
o
o
5
o
IV. Observing Exercises
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1. Calibrate the size of your fist with the Big Dipper. Is your fist 10o in size? List and discuss at least 3
reasons explaining why it may be different.
2. Determine the Altitude and Azimuth of Polaris. Explain your method.
3. Observe a bright star and determine its altitude and azimuth. Record your results below. Observe the
same star again later in the night (approximately 1 hour later). Again, record your results below and
determine the differences in the star's altitude and azimuth.
NAME OF STAR _______________________
Observation #1
Observation #2
Difference
Time of Observation
Altitude
Azimuth
In what direction did this star appear to move?
4. Estimate the angular size of the constellation Cygnus (length and width).
5. Estimate the angular separation between Vega and Deneb.
6. Estimate the altitude and azimuth of the following objects:
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Object
Time
Altitude
Azimuth
Polaris
Cassiopeia
Vega
Deneb
Arcturus
7. List at least 3 sources of error in your calculations for this lab.
8. What causes a star to change its altitude and azimuth during the night?
9. Will the separation of Vega and Deneb change during the night? Why or why not?
10. What would be the altitude of Polaris for an observer at the Equator? ___________
What would it be at the North Pole? _____________
11. What happens to the altitude and azimuth of Polaris during the night? Why?
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