Venus - Malcom Scrimger

VENUS

Malcom Scrimger

Venus can be the brightest object in the sky, beside the sun and moon. Since its orbit is inside that of the earth, we see Venus only when we are looking in the general direction of the sun. This bright planet is visible only during the first few hours after sunset, when it is known as "the evening star," or before sunrise, when it is known as "the morning star." Venus can be brighter than magnitude -4, and can even cast shadows.

Venus is covered with thick layers of clouds through which we cannot see. From earth, we see no structure, though we do see Venus go through phases as it orbits the sun. (Only planets that have orbits smaller than the earth's - Venus and Mercury - can go through a crescent phase. The fact that Venus goes through a complete cycle of phases, including the crescent phase, was discovered by Galileo and was a major proof of the validity of Copernicus' idea that the sun rather than the earth is at the center of the solar system.)

When Venus is just about to pass between the sun and us, it appears as a crescent and is at its largest. We may even see sunlight bent around toward us through its thick atmosphere. When Venus appears at the same longitude along the ecliptic as the sun, it is at conjunction. When Venus lies nearly on a line of sight between the sun, and us it is at inferior conjunction. When we can see all of Venus' lighted side (its "full" phase), the planet is on the far side of the sun, at its farthest point from us and therefore at its smallest. It is then at superior conjunction.

From spacecraft looking through ultraviolet filters from the vicinity of Venus, we have been able to study the circulation of the planet's clouds. The Pioneer Venus Orbiter in 1979 even compiled a map of Venus' surface, using radar.

A series of Soviet spacecraft have landed on Venus and sent back photographs of its surface (Another link.) From studies of the composition of the surface, we know that the same kind of geological processes that formed the earth's surface also were at work on Venus. The earth, though, has several continents and is mostly covered with deep ocean basins. Venus, on the other hand, has only a few small continents and few deep basins; it is mostly covered with broad rolling plains.

Venus' clouds consist primarily of sulfuric acid droplets. Its atmosphere is mostly carbon dioxide, and the surface pressure is 90 times that on earth. The carbon dioxide in Venus' atmosphere traps sunlight, which enters mostly as visible light but is changed to infrared radiation when it heats Venus' surface. The infrared can't escape, mostly because of the carbon dioxide but also somewhat because of other gases and particles, so the atmosphere heats up to a temperature of 500'C (900'F) on Venus' surface. This effect is known as the green house effect

Transits of Venus

Only rarely does Venus transit - go directly in front of - the sun as viewed from Earth. It is then visible as a black dot projected on the solar disk; light bent forward to us by Venus' atmosphere makes a bright ring around the planet. Historically, transits of Venus have been important for setting the distance scale in the solar system; we can now find the distance scale more accurately with radar and by tracking spacecraft, so transits of Venus are now merely an observational curiosity.

Transits of Venus come in pairs separated by 8 years; the interval between successive pairs is more than 100 years. The last transits were in 1874 and 1882. The next will be on June 8, 2004, and June 5-6, 2012.