Changing paragraph to a width of 80 character

I have the following text file and want to change the paragraphs so that
they fill 80 characters rather than the usual 65 in this case. Is there an easy way to do this?

All three groups were distinguished by the remarkable
manner in which they moved about on the solar surface, not
only in longitude but also in latitude.
The first group moved forward 5�"2 in longitude, and o 0 �6
upward in latitude, in� the four days following its formation.
In the fortnight that it was on the further side of the Sun
it moved forward 4�"0 in longitude, and 1�"1 downward in
latitude. It was no'Y a single circular spot, and remained nearly
stationary till its disappearance.
The second group, that of June 16, the principal group of
the year, showed the most extraordin,ary drift, as the first
diagram will show.
Some of the minor irregularities are probably due to faculous
or photospheric mat,ter drifting above portions of the group
and concealing certain di.stricts of it, revealing them again
by their witharawal later on. But as for the greater part of
its history, tbe group practically consisted of but one welldefined circular 
spot, there can be no doubt that the apparent
change of place from day to day corresponds very closely to
the real movement of the centre of the group upon the
solar surface. In this and in the second diagram the observed
positions for the first and last days at each appearance are not
given, as only a portion of the group was visible on some of
those occasions, and the centre of the part observed was therefore
not the true centre of the group.

The third group of long duration, that of August z, was a very
perfect example, of the type above described. The first faint
spots were seen on .August 2. By August 4 the long stream of
spots with a large spot at either end had fully formed ; and by
August I 1 the leader alone remained. During the next rotation
the group consisted only of one large circular well-defined spots.
At its third appearance this circular spot was preceded by a
number of small spots, but. after September 26 the entire group,
and especially the smaller spots, began to diminish, and the
group had disappeared before reaching the west limb. The most
striking feature in �the history of the group was the rapid backward drift 
shown after its first appearance The forward move ..
ment during the� first: rotation, the r~t.rogression whilst on _the

Try:

tr -d '\n' < file | fold -s -80
All three groups were distinguished by the remarkable manner in which they moved
about  on  the  solar  surface, not only in longitude but also in latitude.  The
first group moved forward 5�"2 in longitude, and o 0 �6 upward in latitude,  in�
the  four  days  following  its  formation.  In the fortnight that it was on the
further side of the Sun it moved forward 4�"0 in longitude, and 1�"1 downward in
latitude.  It  was  no'Y  a single circular spot, and remained nearly stationary
till its disappearance.  The second group, that of June 16, the principal  group
of  the  year,  showed  the most extraordin,ary drift, as the first diagram will
show.  Some of  the  minor  irregularities  are  probably  due  to  faculous  or
photospheric mat,ter drifting above portions of the group and concealing certain
di.stricts of it, revealing them again by their witharawal later on. But as  for
the  greater  part  of  its  history, tbe group practically consisted of but one
welldefined circular spot, there can be no doubt that  the  apparent  change  of
place  from  day  to  day  corresponds  very closely to the real movement of the
centre of the group upon the solar surface. In this and in  the  second  diagram
the  observed  positions  for the first and last days at each appearance are not
given, as only a portion of the group was visible on some  of  those  occasions,
and  the  centre  of  the part observed was therefore not the true centre of the
group.

The third group of long duration, that of August z, was a very perfect  example,
of  the  type  above described. The first faint spots were seen on .August 2. By
August 4 the long stream of spots with a large spot  at  either  end  had  fully
formed  ;  and by August I 1 the leader alone remained. During the next rotation
the group consisted only of one large circular well-defined spots.  At its third
appearance  this  circular  spot  was  preceded by a number of small spots, but.
after September 26 the entire group, and especially the smaller spots, began  to
diminish,  and the group had disappeared before reaching the west limb. The most
striking feature in �the history of the group was the rapid backward drift shown
after  its  first  appearance  The  forward  move  ..   ment  during the� first:
rotation, the r~t.rogression whilst on _the

Im testing...
This was done by adding first line: .ll 80 above the text then using nroff:

nroff -e text

If you're a vim user:

vi file
[SHIFT :]
type - set textwidth=80 [ENTER]
type - gg
type - gqG
[SHIFT :]
type - wq 

Hi.

I usually use:

NAME
       par - filter for reformatting paragraphs
...

DESCRIPTION
       par  is  a  filter  which  copies its input to its output, changing all
       white characters (except newlines) to  spaces,  and  reformatting  each
       paragraph.   Paragraphs are separated by protected, blank, and bodiless
       lines (see the TERMINOLOGY section  for  definitions),  and  optionally
       delimited by indentation (see the d option in the OPTIONS section).

...

If it is not in your system repository, it's available at: Par 1.52 - paragraph reformatter -- the web page contains an example of formatting to 44 characters, and pretty-printing comment lines (historical enclosures for an email reply) at the same time.

Best wishes ... cheers, drl

Many systems provide an fmt utility that is designed for simple stuff like this. It also correctly handles indented paragraphs. Try man fmt to see if your system includes this utility. If it does, the command you want is:

fmt -w80 your_text_file_name

Thank you all. I have done things with fmt, but also looking at the other ways you mentioned as well.

True. fmt is the first choice here.
Solaris needs fmt -w 80