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coordinate grid. Thus, object positions on the Earth's surface are specified by their latitude and longitude. For
example, you could locate Los Angeles, California, by its latitude (+34° ) and longitude (1180); similarly, you could
locate the constellation Ursa Major (which includes the Big Dipper) by its general position on the celestial sphere:
1. Declination: The celestial analog to Earth latitude is called Declination, or "Dec", and is measured in
degrees, minutes and seconds (e.g., 15° 27'33"). Declination shown as north of the celestial equator is
indicated with a "+" sign in front of the measurement (e.g., the Declination of the North Celestial Pole is
+90° ), with Declination south of the celestial equator indicated with a "-" sign (e.g., the Declination of the
South Celestial Pole is -90° ). Any point on the celestial equator itself (which, for example, passes through
the constellations Orion, Virgo and Aquarius) is specified as having a Declination of zero, shown as 0° 0'
0".
2. Right Ascension: The celestial analog to Earth longitude is called "Right Ascension," or "R.A.," and is
measured in time on the 24 hour "clock" and shown in hours ("hr"), minutes ("min") and seconds ("sec")
from an arbitrarily defined "zero" line of Right Ascension passing through the constellation Pegasus. Right
Ascension coordinates range from 0hr 0min 0sec to 23hr 59min 59sec. Thus there are 24 primary lines of
R.A., located at 15 degree intervals along the celestial equator. Objects located further and further east of
the prime Right Ascension grid line (0hr 0min 0sec) carry increasing R.A. coordinates. All celestial objects
are specified in position by their celestial coordinates of Right Ascension and Declination. The telescope's
R.A. and Dec setting circles (7 and 8, Fig. 7) may be dialed to the coordinates of a specific celestial object,
which may then be located without a visual search. However, before you can make use of the telescope's
setting circles to locate celestial objects, your telescope must first be polar aligned.
b. Polar Alignment
The Meade ETX can be used for astronomical observations in the same way as just described for terrestrial
observations: simply set the telescope on a tabletop, locate objects through the viewfinder, and then observe
through the main telescope. Objects can be tracked, or followed, as they move across the sky by turning one or
both of the R.A. and Dec Slow-Motion Controls (10 and 16, Fig. 1). Such usage of the telescope, however, is
unnecessarily cumbersome, since, by attaching the Table Tripod legs to the ETX and following a procedure called
Polar Alignment, the telescope can be made to track objects completely automatically.
As the Earth rotates once on its axis every 24 hours, astronomical objects appear to move across the sky in an arc.
This apparent motion is not obvious to the unaided eye, but viewed through a serious telescope such as the ETX,
this motion is rapid indeed. If the motor drive has not been engaged, objects centered in the telescope's eyepiece
move entirely out of the field of view in 15 to 60 seconds, depending on the magnification employed.
For easy tracking of astronomical objects the ETX should be polar aligned, as follows:
1. Make sure the viewfinder is aligned with the ETX Astro Telescope, as previously described in this manual.
2. Attach the three tripod legs to the drive base of the telescope. (see Fig. 7) Two identical legs thread into the side
of the telescope's drive base (first remove the threaded cover-buttons). The third leg, which includes a scribed
scale along its length, should be threaded into the center hole of the drive base. The scribed scale reads the
latitude of the observing location. Threading in the three legs to a firm feel is sufficient.
3. A small thumbscrew (4, Fig. 7) is attached to the center tripod leg. Loosening this thumbscrew allows the outer
section of the leg to slide over the inner section, so that the length of the leg can be extended. Extend the center
tripod leg so that the center of the thumbscrew-head is lined up with the latitude of your location on the scale. Then
retighten the thumbscrew to a firm feel. Determine the latitude of your observing location from a road map or atlas,
or call your local airport; determining your latitude within about one degree is sufficient. The latitudes of major cities
R.A.: 11hr; Dec: +50° .
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