NavList:

The most popular two-body fix is a Sun-Moon (daytime, no flashlight needed, warmer, easy bodies, etc.)  If you are planning a Sun-Moon fix, Celestial Tools makes planning easy.  Just use the Sight Planner tool.  Enter the date and the latitude and longitude of the place where you hope to take the sight.  Click on the "Get Twilight Times..." button.  (This needs to be done once even though you will not be using the information.)  Then enter ANY time in the "Enter desired ZT..." box.  Enter the range of available azimuths for your location.  If you will accept the Sun or Moon being below 15º altitude, lower the value in the "Enter minimum altitude..." box.  (If you want a higher minimum altitude, raise the value).  Then click on "List Visible Bodies/Sun-Moon Fix" button.  The information about the availability of a Sun-Moon for the date, location, azimuth range, and minimum altitude will then appear in the lower right corner of the window.  It gives the range of times, if any, and the phase (percent illumination) of the Moon when the "cut" is at least 45º.

If you are interested in something other than a Sun-Moon fix, you can go through the same process, but put a more proper time, like the time of the end of PM civil twilight, in the "Enter desired ZT..." box.  Then you can do the math, using the listed azimuths of the visible bodies.  But, even easier, is to click on the "View Visible Bodies" button instead of the "List Visible Bodies/Sun-Moon Fix" button.  Then select "Two-body fix" in the "Best Bodies Aid" box in the lower left corner.  Use the "Rotate CCW" and "Rotate CW" buttons to move the red lines that appeared.  The ideal cut of 90º is shown by the solid red lines, but, in order to meet the minimum 45º requirement, just rotate the lines until you can pick one body between the dash-dot pair of red lines and one body between the dash-dash pair of red lines.  If you want to use the limited azimuth range previously entered, be sure to check the "Limit azimuth range" box.  You can reduce clutter by un-checking the "Show 2nd..." and "Show 3rd..." magnitude star boxes.  To see all the body names at once, check the "Show Names" box, or just hover the cursor over the body of interest and its name, azimuth, and altitude will be shown on the right.

Stan