Is the transit useful to the scientist in any way?

Certainly, it is. For example, we can use it for studying the effect of the Bailye’s drop, which is created when the planet’s little disk comes near the edge of the solar disk. However, current importance of the transit is far lesser then it used to be in the past. Before the refractor was invented at the beginnig of the 17th century, nobody has probably observed this phenomenon. Some scientists indicate, that the Arabs knew about the transits even in the 8th and the 9th century, but we have no evidence about their observations. The Venus’s little disk cannot be distinguished on the background of brightly shining Sun just with by the eyes without a sufficient filter. A new possibility of how to establish the size of one astronomical unit (i.e. central distance between the Earth and the Sun) with the aid of the observation of the transits, came out after the refractor was invented. Since 1610, when Galileo Galilei turned his refractor to the sky for the first time, six transit have taken place, whence one of them was not observable very well in the Europe. During these centuries, the value of the solar parallax was finally established quite exactly.

 What is then the value of the solar parallax and the astronomical unit?

The transit observation did bring some solid results, but the value of these entries had not been measured with sufficient accuracy until the second half of the 20th century with the aid of cosmic sond and the reflections of the radio pulses transmitted from the Earth to the surface of Venus. The modern value of solar parallax is 8.79414“, which corresponds to the value of the astronomical unit, which equals to 149,597,870 km.