Venus

The second planet of our Solar System is after Moon the nearest cosmical body. It has a very thick atmosphere, which creates a massive greenhouse effect. Because of that, there is a real hell on the surface of the planet (the temperature is around 480°C), but it reflects a large amount of light as well - after Moon, it is the brightest body on the night sky (-3,3 to -4,2 mag). It has no natural satellite.

A great speciality of the planet is the way it rotates on its axis. The speed of the rotation is much lower than by the other planets of our Solar System (243 days and 4 hours). Moreover, it rotates to the other side than other planets (except Uranus). The average distance from Venus to Sun is 108,200,000 km.

The surface of Venus cannot be watched in visible light, because the planet is veiled by clouds. That's why it has been observed by radar. Radar maps from Earth differ very much, because different frequencies of the radiotelescopes sometimes pick up not the surface, but the subsurface details. The first detailed radar map was obtained by the American probe Pioneer Venus Orbiter. The Venus's surface is generally evener than the Earth's surface. Great influence on its shape had indisputably the volcanic activity (cca 85% of the suface has volcanic origin). An important role in moulding the surface of Venus plays the erosion.

On Venus, there are only two continents, similar to those on Earth (mainly in size): Afrodite Terra, which is like South America, and Ishtar Terra, alike Australia. The continents were probably created by the tectonical activity. There are only three mountainous areas on the Venus. The highest are the Maxwell Montes (10.6 km).

The Venus's atmosphere is very thick, and so the atmospherical pressure on the surface is ninety times higher than on Earth. It mostly consists of CO₂ (it makes 96,6% of the atmosphere, which is cca a thousand times more than on Earth), then of N₂ (3,2%), steam (0,01%) and other substances. The atmosphere is also the cause of the high albedo of Venus (it reflects 59% of light). And because of that and the greenhouse effect it creates, the temperature on the surface can rise up to 480 °C.

The clouds are formed cca 63 - 67 km over the planet surface. They consist of H₂SO₄, HF and HCl. Unclear is the composition of solid articles, present in the atmosphere, because theit identification is difficult.

The only thing we can observe from Earth are the Venus's phases. They depend on mutual position of Venus, Sun and Earth. The maximum points of mutual positions of these bodies are named aspects. Because Venus is an vnitřní planet, it has four aspects: upper and lower conjunction and maximum west and east elongation. In upper conjunction the planet is invisible from Earth (it is hiden by the Sun glare). An exceptional situation arrives by the eclipse of the Sun. In this case Moon overshades the Sun disc and the Sun glare cannot overshine Venus. In lower conjunction Venus is invisible again, this time it is in front of the Sun and it disappears in it's glare. But an exceptional situation can arrive, when Earth, Venus and Sun are exactly in one line. This phenomenon is called the Venus Transit across the Sun disc. So can be Venus seen as a black spot, which moves across the Sun disc. This phenomenon is very rare. We can observe it first after at least 122 years and next one after 8 years. The last transits were on 9th Dec 1874 and 6th Dec 1882, the next one will be onh 6th Jun 2012 and then on 11th Dec 2117 and 8th Dec 2125. Aspects, when Venus is normally visible, are maximum west elongation ("Morning Star" in the east) and maximum east elongation ("Evening Star" in the west). In upper conjunction Venus is in "full moon" (but invisible), in lower conjunction in "new moon", in max. east elongation in first quarter and in max. west elongation in last quarter.

Roman Hájek, Jan Marek