\begin{table}%t2
%\centering
\par
\caption{Photographic positions of Venus in the system of the Tycho-2 catalogue.}\label{tab2}
\begin{tabular}{rlccccc}
\hline \hline
Nn & Date & UTC & $ \alpha $ & $ \delta $ &
(O--C)$_{\alpha}$ & (O--C)$ _{\delta}$\\ \hline
1&2&3&4&5&6&7\\
\hline\noalign{\smallskip}
1&1991, Febr. 21 & 15h59m15.0s & 23h58m30.024s &$-1^{o} 20^{'} 16.17^{''}$ & $-0.019 ^{s}$ & $ 0.05^{''}$\\
2&1991, Febr. 21 & 15 59 55.9 & 23 58 30.157 & --1 20 15.44 & --0.013& --0.11 \\
3&1991, Febr. 21 & 16 00 37.0 & 23 58 30.299 & --1 20 14.24 & 0.001 & 0.20 \\
4&1991, Febr. 22 & 15 48 29.5 & 00 02 56.484 & --0 49 12.41& 0.015&  0.19 \\
5&1991, Febr. 22& 15 49 29.4 & 00 02 56.631 &  --0 49 11.14 &--0.024 & 0.16 \\
6&1991, Febr. 22 & 15 50 29.5 &00 02 56.856 &  --0 49 10.03 & 0.014 & --0.03 \\
7&1991, Febr. 22 & 16   03 39.5 &00 02 59.268 & --0 48 53.03 & --0.027 & 0.21 \\
8&1991, Febr. 23 & 15   55 25.0 & 00 07 25.906 &--0 17 43.87 & --0.003 & 0.32 \\
9&        1991, Febr.   23& 15 56 34.7&00 07 26.098 &--0 17 42.92 & --0.027 & --0.25\\
10&       1991, Febr.   23& 15 57 44.5&00 07 26.311&--0 17 41.21&--0.031 & --0.06 \\
11&       1991, Febr.   23& 16 02 45.0&00 07 27.264 &--0 17 34.41 & --0.010 & 0.20 \\
12&       1991, Febr.   25& 16 07 42.0&00 16 23.811 &0 45 12.94 & 0.013 & 0.09 \\
13&       1991, Febr.   25& 16 11 13.1&00 16 24.445 &0 45 17.34 & --0.007 & --0.10 \\
14&       1991, Febr.   25& 16 14 32.0 &00 16 25.053 &0 45 21.89 & --0.015 & 0.12 \\
15&       1991, Mar. 1 &15 58 52.0 &00  34 11.745 &2 50 11.62   & 0.003 & --0.15 \\
16&       1991, Mar. 1& 16 01 53.6 &00  34 12.279 &2 50 15.94   & --0.025 & 0.23 \\
17  & 1991, Mar.    1 & 16 02 02.0 & 00 34  12.317 & 2 50 16.22 &   --0.013& 0.33 \\
18  & 1991, Mar.    1 & 16 04 47.0 & 00 34  12.811 & 2 50 19.33 &   --0.029 & --0.14 \\
19  & 1991, Mar.    8 & 16 15 43.0 & 01 05  28.669 & 6 27 01.57 &   --0.018 & 0.33 \\
20  & 1991, Mar.    8 & 16 22 53.0 & 01 05  30.027 & 6 27 10.41 &   0.004 & 0.05 \\
21  & 1991, Mar.    8 & 16 23 37.9 & 01 05  30.173 & 6 27 11.66 &   0.011 & 0.34 \\
22  & 1991, Mar.    8 & 16 24 23.0 & 01 05  30.284 & 6 27 12.55 &   --0.018 & 0.28 \\
23&       1991, Mar. 10& 16 21 22.0&01  14 27.206 &7 27 54.64   &-0.013&--0.22\\
24&       1991, Mar. 10& 16 25 11.5&01  14 27.945 &7 27 59.90   & 0.011&0.22\\
25&       1991, Mar. 10& 16 25 24.9&01  14 27.978 &7 27 59.70   & 0.002&--0.26\\
26  & 1991, Mar.    10& 16 29 41.5 & 01 14  28.776 &7 28 05.30  &   0.001 & --0.05 \\
27  & 1991, Mar. 11& 16 22 12.0 &01 18 56.656  & 7 58 05.08 & 0.001 & 0.31 \\
28  &1991,  Mar. 11& 16 26 06.9 &01 18 57.355 & 7 58 09.65 & --0.033 & --0.03 \\
29  &1991,  Mar. 11& 16 30 01.5 &01 18 58.122 &7 58 14.38& 0.002 & --0.19 \\
30  &1991,  Oct. 10& 15 59 43.5 &01 18 59.782 &7 58 25.77  & 0.008 & 0.12 \\
\hline
\end{tabular}
\end{table}