MIRA's Field Trips to the Stars Internet Education Program 
Venus
Moons - Orbital and Rotational Detail - Physical Properties - Atmosphere - Surface and Geology - Interior - Magnetic Field - Phases of Venus
The planet Venus as imaged
by
NASA's Galileo spacecraft.
Venus is the second planet closest to the Sun. It is named after the ancient Italian goddess of gardens and spring, identified by the Romans with Aphrodite, the Greek goddess of love and beauty. It is the brightest object in the sky (except for the Sun!). One of the reasons why Venus is so bright is that its cloud layer reflects back into space 75% of the incident sunlight. This makes Venus very easy to detect, along with the fact that it is never very high up in the sky: it shines as the "evening star" in the western horizon after sunset, or as the "morning star" in the eastern horizon before sunrise. Because Venus is close to the Sun, it is never in the sky in the middle of the night, or high in the sky at night.
Venus is often mistaken for a UFO because when it's close to the horizon its light is refracted by the Earth's atmosphere, making its color and position appear to change rapidly. If at the same time Venus is seen through moving clouds, it is hard to tell if it is Venus or the clouds that are moving!
MoonsVenus has no moons
Orbital and Rotational DetailsVenus's orbit is the most circular of all the planets' orbits, and the orbital plane is tilted slightly more than 3 degrees with respect to the plane of Earth's orbit around the Sun. As seen from Earth, Venus is never more than 48 degrees away from the Sun, which is why we always see it in the early morning or early evening.
| Average distance from the Sun | 108.2x106 km (67.2 million miles, 0.723 astronomical units) |
| Orbital eccentricity | 0.0068 (most circular orbit of all planets) |
| Mean orbital speed | 35.0 km/s (21.8 miles/s) |
| Sidereal period | 224.70 Earth days (0.6152 years) |
| Synodic period | 583.92 days |
| Inclination of orbit to the ecliptic | 3.39 degrees |
| Maximum or greatest elongation | 46 to 48 degrees |
| Inclination of equator to orbital plane | 177.2 degrees |
| Rotation period (retrograde) | 243.08 Earth days |
| Solar rotation period (noontime to noontime, or one Venusian day) | 116.67 Earth days |
The rotation period of Venus could not be measured until the 1960's because its thick clouds prevent us from seeing any feature on the surface. In 1962 radar waves were bounced off the surface of Venus and the rotation period was finally determined. It was also discovered that Venus rotates with retrograde motion, that is, in the opposite sense most of the other planets.
Venus has no seasons because its rotation axis is tilted only 3 degrees from the perpendicular to its orbital plane around the Sun. If you compare Venus's period of revolution around the Sun (sidereal period) and its rotation period (day and night) you can see that they are almost the same. This is due to the tidal forces of the Sun on Venus.
Physical DataVenus is also referred to as "Earth's sister" because both planets have similar diameters, masses and average densities, as can be seen in the table of planetary properties. Of the terrestrial planets, these two planets have major atmospheres and are the most geologically active. Venus's shape is a perfect sphere, as opposed to Earth, whose diameter at the equator is 26 miles larger than the diameter at the poles. Venus's atmosphere is so thick that the pressure on the surface is 90 times the pressure on Earth's surface! The surface temperature stays very hot and constant all the time, as explained below in the section Atmosphere. Scientists in the 1960s were shocked to learn about the high temperature and pressure.
| Mass | 4.869×1024 kg (0.815 Earth masses) |
| Equatorial radius | 6051 km |
| Polar radius | 6051 km |
| Mean density | 5.25 g/cm3 |
| Equatorial acceleration of gravity | 887 cm/s2 |
| Escape velocity | 10.36 km/s |
| Surface temperature | 730 ± 5 K |
| Surface pressure | 90 ± 2 bars |
| Albedo | 0.76 |
Atmosphere
Venus's atmosphere is so thick and massive that it only allows a small amount of visible sunlight to penetrate. This visible light is reflected by the surface as infrared radiation, which is then trapped by the CO2 in the atmosphere. Just like the "greenhouse effect" that you experience in your everyday life when you leave your car sitting in the sun with the windows closed and come back to find it much hotter than the outside, Venus gets very hot and the temperature stays uniform near the surface, both at daytime and nighttime. Winds don't form near the surface because of the constant temperature there, coupled with the slow rotation of the planet. On the other hand, strong winds with speeds of 220 miles/hr (360 km/hr) are observed in ultraviolet light at altitudes higher than 50 km (30 mi). The temperatures and pressures in the atmosphere allow the formation of clouds. Their composition is mostly sulfuric acid (H2SO4), which forms in the presence of ultraviolet light (i.e. photochemically) from sulfur dioxide (SO2) and water. Some of the SO2 in the atmosphere probably comes from volcanic eruptions.
