% Load astro symbols at 10pt, 7pt, 5pt
\font\tenastro=cmastro10
\font\sevenastro=cmastro7
\font\fiveastro=cmastro5

% Define a new family; but since we need its family number to be < 8
% (in order to use \mathchardef -- see p. 155 of The TeXbook), we'll need
% to do some rearranging.  So put \sl into family 8 (note that this may mean
% that we can't use it in subscripts any more) and \astro where \sl was
% (family 5).  Plain TeX's definitions from p. 351.
%  Family    Plain TeX:    Now:
%    0         \rm         \rm
%    1         \mit        \mit (math italic)
%    2         \sy         \sy  (math symbols)
%    3         \ex         \ex  (math extension)
%    4         \it         \it
%    5         \sl         \astro
%    6         \bf         \bf
%    7         \tt         \tt
%    8        (none)       \sl
\newfam\astrofam
\let\astrofam=\slfam
\let\slfam=\allocationnumber % last number allocated (pp. 346-7)

% Set up astro's math family now.
\textfont\astrofam=\tenastro
\scriptfont\astrofam=\sevenastro
\scriptscriptfont\astrofam=\fiveastro
\def\astro{\fam\astrofam\tenastro}

% And make sure that \sl still works
\textfont\slfam=\tensl
%\scriptfont\slfam=\sevensl       % we don't have this font
%\scriptscriptfont\slfam=\fivesl  % or this one
\def\sl{\fam\slfam\tensl}

% Finally, define symbols in the astro fonts.  First hex digit = 0 for
% "ordinary" symbol; second = 5 for family 5; third and fourth = ASCII
% code for location of symbol in the font.  But \circ is family 2.
\mathchardef\Sun="0553      % Sun symbol, "S"                 
\mathchardef\Mercury="0531  % Mercury symbol, "1"             
\mathchardef\Venus="0532    % Venus symbol, "2"               
\mathchardef\Earth="0533    % Earth symbol, "3"               
\mathchardef\Mars="0534     % Mars symbol, "4"                
\mathchardef\Jupiter="0535  % Jupiter symbol, "5"             
\mathchardef\Saturn="0536   % Saturn symbol, "6"              
\mathchardef\Uranus="0537   % Uranus symbol, "7"              
\mathchardef\Neptune="0538  % Neptune symbol, "8"             
\mathchardef\Pluto="0539    % Pluo symbol, "9"                
\mathchardef\Moon="054D     % Moon symbol, "M"                
\mathchardef\ascnode="054E  % ascending node (dragon's head), "N"
\mathchardef\descnode="0544 % descending node (dragon's tail), "D"
\mathchardef\VE="0556       % vernal equinox (Aries), "V"     
\mathchardef\AE="054C       % autumnal equinox (Libra), "L"   
\mathchardef\circ="250E     % circle, replacing the one in \sy

$$
\astro	    
	    \Mercury    
	    \Venus 	    
  	    \Earth    
	    \Mars
	    \Jupiter
	    \Saturn
	    \Uranus
	    \Neptune
	    \Pluto
	    \Moon
	    \ascnode
	    \descnode
	    \VE
	    \AE
	    \circ
$$
\bye