\begin{table}%t1
%\centering
\par
\caption{Precessional quantities
determined with different sources. Units: coefficients
in arcsecond and time in century.\label{tab1}}
\begin{tabular}{lcrrrr}
\hline\hline
Source &        & $t\quad$\ \ \ \  & $t^2\quad$\ \ \ \  & $t^3\quad$\ \ \ \  & $t^4\quad$\ \ \ \  \\
\hline
L77             &$P_{\rm A}$&  4.1976~~  &0.19447~~~~&--0.000179~~~~&  \\
(Newcomb)          &$Q_{\rm A}$&--46.8150~~  &0.05059~~~~& 0.000334~~~~&  \\
\hline
from S94 &$P_{\rm A}$&  4.199706  &0.1939713~&--0.00022351~&--0.000001036 \\
(VSOP87)           &$Q_{\rm A}$&--46.809269  &0.0510429& 0.00052233~&--0.000000567 \\
\hline
W94           &$P_{\rm A}$&  4.199610  &0.193971~~&--0.000223~~~~&--0.000001~~~~  \\
(VSOP87+IERS1992 masses)  &$Q_{\rm A}$&--46.809560  &0.051043~~& 0.000522~~~~&--0.000001~~~~  \\
\hline
P03   &$P_{\rm A}$&  4.199094  &0.1939873&--0.00022466&--0.000000912  \\
(VSOP87+Fit to DE406) &$Q_{\rm A}$&--46.811015  &0.0510283& 0.00052413&--0.000000646  \\
\hline
\end{tabular}
\end{table}