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Second planet of the Solar System in order of distance from the Sun. After the Sun and the Moon, is the brightest object in the sky. For size, Venus is in sixth place among the planets: its average diameter (12,104 kilometers) is only slightly lower than that of the Earth (12,756 kilometers). Venus describes around the Sun orbit slightly elliptical revolution, having a semi-major axis of 108.2 million kilometers. Its distance from the Sun varies from a minimum of 107.4 million kilometers to a maximum of 109 million kilometers. The inclination of the orbit to the ecliptic plane is 3 ° 23 ‘. The orbital period is 224.7 days. The distance of Venus from Earth varies from a minimum of about 38 million kilometers to a maximum of 261 million kilometers. The most favorable periods for observation of the planet are those in which the elongation from the Sun (ie the difference between its longitude and that of the Sun) reaches the maximum values of 47 ° to 47 ° East or West: in the first case, Venus appears just after sunset, in the second just before dawn. Being a planet ‘lower’ (that is closer to the Earth’s sun), has similar steps to those of the Moon. The planet’s disk appears smaller when it is fully illuminated by the Sun (full step), because it occurs in the superior conjunction position, in which it is located at the maximum distance from Earth. The planet becomes, instead, invisible (minimum phase) in the position inferior conjunction, ie when it is at the minimum distance from Earth, because in this situation the hemisphere illuminated by the Sun is opposite the Earth. The phases of Venus were discovered by Galileo in 1611, with the newly invented telescope. Among the inner planets, Venus has the most massive atmosphere: it includes about one ten-thousandth of the mass of the planet, while the atmospheres of Earth and Mars contain fractions of 10-6 and 10-7, respectively. The surface pressure is about 95 bars.