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ASTRONOMY 130
COORDINATE SYSTEMS
PURPOSE: To introduce you to the two major coordinate systems so that you will be able to
locate objects in the sky.
PROCEDURE: The coordinate systems will be introduced in the planetarium and measurements
then will be made with an equatorial and a quadrent.
COORDINATE SYSTEMS:
The two major coordinate systems are known as the horizon system and the equatorial
system. The horizon system uses the zenith and the horizon as its basic point and plane. The
equatorial system uses the north and south celestial poles and the celestial equator as its basic
point and plane.
Horizon System: In this system two measurements are made. The first is the angular
distance (altitude) that the object is above the horizon. Altitude ranges from 0o (object on the
horizon) to 90o (object at the zenith). Altitude is always the shortest distance from the horizon to
the object. As there are many objects that could have the same altitude, to find the one you want
requires the second measurement called azimuth. Azimuth is the angular distance measured from
the north cardinal point towards the east to the object's vertical circle. A vertical circle is a line
from the zenith through the object perpendicular to the horizon. Azimuth can range from 0o
(north cardinal point) to 90o (cardinal point east) to 180o (cardinal point south) to 270o (cardinal
point west) back to 360o (cardinal point north), where all cardinal points are on the horizon.
Given the altitude and azimuth of an object then allows you to locate it in the sky. As we have
seen from the previous exercise, the position of the stars change with time so an object's altitude
and azimuth are only good for a certain time. This is the major disadvantage of this coordinate
system.
Estimates of angular distance can be made by using your hand held at arms length.
Width of thumb
Width of fist
Span of hand
= 1o
= 8o
= 18o
Equatorial System: Although this system is more useful than the horizon system, it is
more difficult to make measurements with little of no equipment. This system is based on the
north and south celestial poles (extension of the north and south poles of the earth onto the
celestial sphere). The north star (Polaris) is located close to the location of the NCP (about 1o
away) whereas the SCP cannot be seen from our location. The celestial equator is the extension
of the earth's equator onto the celestial sphere. At our location it crosses the horizon at the east
and west cardinal points and intersects the celestial meridian 52o above the south cardinal point.
If you are acquainted with the measurement of latitude and longitude here on earth, then using
the equatorial system is very similar.
The angular distance an object is above or below the celestial equator is called
declination. Declination ranges from 0o (object on the celestial equator) to +90o (object at the
NCP) or to -90o (object at the SCP). Many objects could be located at the same declination, so we
must now make a measurement around the celestial equator. This angular measurement is
measured in units of hours and minutes in an eastward (counter-clockwise) direction from the
vernal equinox to the object's hour circle. An hour circle is a line from the celestial pole through
the object crossing the celestial equator at right angles on to the other celestial pole. The vernal
equinox is the starting point for measuring right ascension and is determined by the intersection
of the sun's path (ecliptic) and the celestial equator. It is located on the celestial equator in a line
with the left side of the Great Square of Pegasus. As time passes and the stars move, their
position with respect to the celestial equator does not change. This is the advantage of the
equatorial over the horizon system.
You will be given a list of five object to observe. For each object estimate its altitude and
azimuth. Record the data and time of the observation. Next observe the altitude of each object
using a quadrent as described in figure 1. Record the data and time of the observation. Then
measure the altitude and azimuth of each object using the equatorial. Operation of the equatorial
will be explained in class. Record the data and time of the observation. ALL DATA TO BE
RECORDED IN TABLE FORM. Repeat the observations made with the quadrent one
hour later. If you are working with a partner, record both sets of observations and list a
column of the average values.
Figure 1
For a second list of five objects, use the equatorial to determine their right ascension and
declination. When using the equatorial, it must be set for the proper star time, ask the lab
assistant to do this for you. Record the data in table form. Repeat the observations one hour
later.
Questions:
1. How do the values of altitude as determined by estimate, quadrent, and equatorial compare?
Explain the cause for any differences.
2. How do the values of altitude compare with those determined one hour later? Explain.
3. In what part of the sky (high, low, east, southwest, etc.) would you find objects with the
following coordinates?
(a)
(b)
(c)
(d)
(e)
A = 140o h = 30o
A = 295o h = 75o
A = 30o
h = 50o
o
A = 360 h = 38o
A = 322o h = 90o
4. How do the two sets of right ascension and declination measurements compare?
5. Compare your right ascension and declination measurements with the catalog values. Explain
the reasons for any differences.