```

varies by, and its period of variation.

This amount of detail may be much more than you really want to know,
and some of the terms used will be somewhat baffling. The links to the help
system will help you, though; if terms such as "proper motion" are new to
you, a mouse-click on them will bring up a definition.

When you click for more info on a planet,  Guide will give you some
data as to its position,  distance from you,  apparent size, and what
percentage of its surface is illuminated.  In the case of the Sun,  it
will also give you times when the three different kinds of twilight
(civil, nautical,  and astronomical) begin and end.

and librations. The Moon rotates at a constant rate, but its orbital speed
is not exactly constant; this makes it appear to rock back and forth.
Also,  its poles alternately tip toward and away from us slightly. The end
result is that we see a little more than 50% (about 59%) of its surface.
The libration data will tell you what part of the moon is currently tipped
toward you and therefore more easily observed than it might usually be.

etc.) over the next week.

6a:  Measuring angular distances on the screen

Angular distances between pairs of points on the screen may be easily
measured.  Click with the RIGHT mouse button on the first point; holding
the mouse button down,  drag the mouse over to the second point.  A line
will "rubber band" between where you clicked and where the mouse is.

When you release the right mouse button,  the distance and position
angle between the two points will show up in a box in the center of the
screen. Click any mouse button or hit any key to get rid of the box.  The
"position angle" is the angle formed by the line you drew and a line
running straight north from the first point. The term is commonly used to
describe how a pair of double stars is oriented, how the long axis of a
galaxy is oriented, and so forth.

The exact distance and position angle between two _objects_ may be
obtained in a different way:  Right-click on the first object; the
information box described in the preceding chapter will appear. Click OK,
and repeat the process with the second object. Now hit Insert.  Guide will
now compute exact data between the two objects,  and measuring errors are
avoided.  In general,  just dragging a line between two points will work
well enough;  but there are cases where you may want a truly precise value.

6b:  Quick Info

For some general-purpose information,  one wants easy access.  The
"Quick Info" option in the Help menu lists much of this sort of data,
like planetary positions, times of lunar phases,  current local and