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The Telescope GuideBook

Vol. 2:  Night Sky Navigation

Simple explanations of the night sky coordinate system,  

what sidereal time means,  

how to polar align your telescope in 10 minutes, and

how to find any sky target in minutes using setting circles. 

This chapter reviews the earth coordinates of  Longitude & Latitude lines.  This is a good starting point because these coordinate lines are almost identical to the sky coordinate lines and are used in Chapter 4 below to explain the sky coordinates.

Includes a list of coordinates for over 1200 U.S. locations, and procedures to determine the precise longitude and latitude for any observing site

Click to enlarge

This chapter reviews the key time systems used on earth, the world time zones, local time (key for sky navigation), standard time, universal time (UT, GMT), daylight time, and time conversions.

The coordinate lines are imagined to be drawn on a giant celestial sphere (infinitely larger than the earth)  which has the earth at its center.  The structure of the lines is almost identical to that of the earth coordinates.  The longitude lines have been relabeled as "Right Ascension" lines, and renumbered from 0 to 24 "hours" (don't  let the term "hours" confuse you here: just read it as a line label, without any connection to time).  The Latitude Lines are relabeled as "Declination" lines, but are numbered exactly the same as the earth Latitude lines (see Chapter 2).                         

Sidereal time is simply the Right Ascension line that is directly overhead a given observing site, at a given moment.  Example,  if the 11hr Right Ascension line is directly overhead a given observer, then the local sidereal time for that observer is 11:00 at that moment.  Due to the earth's rotation, the 12hr line will rotate into the overhead position 1hr later (refer to chapter 4), and the sidereal time will then be 12:00 .

The lookup tables provided in this chapter lists the sidereal time as a function of the day of the month and local clock (standard & daylight) time.

The first operational step is the initial calibration using a calibration star which is close to the target object.  3 calibration stars are provided for each sky object in the Target List of Chapter 8.  These stars are the brightest (usually Mag 1 -3) stars nearest the target.   The second step is to navigate to the desired sky object and get it in the field of view of the eyepiece.  The third step is to correct for the decalibration effects of the earth's rotation.

Several different alignment procedures are described.  They include both the northern and southern hemispheres.  All procedures are very simple and can be executed in 10 minutes. 

The Daylight Alignment procedure is done in daylight using a simple compass, level, and protractor, and has an accuracy of about 1 degree.  

There are  several different Star Alignment procedures.  They are performed at night using the telescope finder scope, with accuracies ranging from 0.7 to 0.0 degrees.

A unique feature of the list is that the objects are listed in order of sky position rather than ID number.  This simplifies choosing an object for the visible sky region described in Step 2 of the Navigation Procedures below.  An additional Cross Reference Target List is provided in Appendix A,  in which all the targets are listed by ID number.

Another unique feature is that up to 3 different Calibration Stars, along with their coordinates and brightness,  are listed for each target.  These stars are  the brightest (usually Mag 1 -3) stars nearest the target, and are used to calibrate the setting circles as explained in Chapters 6 & 8.

8 full page Star Maps are provided to help the observer find the Calibration Stars.  These stars are  the brightest (usually Mag 1 -3) stars nearest the target, and are used to calibrate the setting circles as explained in Chapters 6 & 8.

Chapter 8:  Navigation Procedures: 

This chapter brings all the pieces together in the form of simple user procedures.  The following are a brief summaries of the steps.