Thank goodness for the renegades of the world. Science is full of them. They are the ones with the crazy ideas and theories that seem to go against the conventional wisdom of the day. One such renegade was Galileo, who in the year 1610 used his crude telescopes to check out sunspots on the Sun, discovered the four largest moons of Jupiter, and found the planet Venus going through phases similar to our moon.

The bright white dot would appear as a crescent, first quarter, gibbous, full and then back again. This was stunning news considering the widely held belief of the time was that the Sun and Venus both orbited the Earth. Venus could only show phases if Venus and the Earth both orbited the Sun. Now that was a radical idea!

Galileo spread the word of his discovery quietly at first. He figured there was no use getting thrown in jail as a lunatic until he had definitive proof. But soon enough he grew confident of his discovery and began to publicly spread the word. Soon he would be considered a crazy man and be put under house arrest for spreading the word of a Sun-centered, rather than Earth-centered, planetary system. What a renegade.

Today we can see Venus in all its glory in the early morning hours before dawn. It is the bright beacon that welcomes us to a new day, the brightest object in the sky other than the Sun and Moon. It is an "inferior" planet, meaning it orbits the Sun inside the Earth's orbit. The two inferior planets, Venus and Mercury, only appear as "morning stars" or "evening stars", i.e. before dawn or after sunset.

Mercury is quite small and close to the Sun so it never rises too far above the horizon. It is also dim. Venus is bigger and brighter; much brighter. It is nearly the same size as Earth and completely shrouded in clouds. The clouds reflect the sunlight and give Venus such a bright appearance.

The planets Jupiter and Saturn also appear in the night sky, although these planets orbit outside the Earth's orbit. They appear as bright stars in the evening sky this month, although not as bright as Venus. Jupiter and Saturn are gas giants, their atmospheres consisting primarily of hydrogen gas and their huge size allowing them to reflect a lot of sunlight. For a complete listing of planetary appearances, check out www.skyandtelescope.com.

While the gas giants are interesting in their own right, Venus has always captured the imagination of scientists because of its similarity to Earth. It is slightly smaller than Earth with a diameter at the equator of 7,521 miles. The Earth's equatorial diameter is 7,926 miles. Venus's average distance to the Sun is 67 million miles while Earth's is 93 million miles.

As you might expect, it is hotter on Venus because it is closer to the Sun. Hotter is a relative term here. Venus is really sizzling at an estimated 900 degrees Fahrenheit. Its atmosphere is primarily carbon dioxide and scientists look on this planet as having a runaway greenhouse effect. Similar to the glass in a greenhouse, carbon dioxide allows shortwave, high-energy sunlight to enter and warm the planet. The planet then emits cooler, longwave energy that is blocked by the carbon dioxide. With enough heat trapped over time, the planet warmed to its current state.

Our planet may have evolved the same way, but our oceans absorbed carbon dioxide and prevented it from building up in the atmosphere. Venus may have had oceans at one time, too, but they may have boiled away before absorbing the carbon dioxide. Venus has the addition of sulfuric acid clouds. There is a small amount of water vapor in the sulfuric acid and it is this water that helps to trap outgoing longwave radiation from Venus' surface. Add to the equation a few spewing volcanoes and you have a hellish existence.

If astronauts were to visit Venus, they would have to arrive in a spacecraft capable of withstanding a soaking of sulfuric acid, temperatures hot enough to melt metal, and pressures equivalent to a half-mile deep in the ocean. Even if the spacecraft could survive and reach the surface, the astronauts would look out on a ruddy landscape that would appear distorted due to the high atmospheric pressure bending the light rays. Lightning would be seen from distant volcanoes, and the ground would glow at night from the excessive heat.

Lucky for us we can simply enjoy Venus as a beacon of brightness in the pre-dawn sky and marvel at how two "similar" planets can evolve so differently.