Sky watch: The two hottest planets, side by side

It was back in 1530 that the science of astronomy was turned on its ear.

In that year, the Polish astronomer Nicolaus Copernicus published a book titled "On the Revolutions of the Celestial Spheres" in which he presented his ideas that it was the sun, not the Earth, that was motionless - a concept that severely contradicted the teachings of Aristotle, the second-century astronomer Ptolemy and the all-powerful Catholic Church.

Of course, we know today that Copernicus was correct - the sun is stationary, and the planets revolve around it. But legend has it that Copernicus, for all his excellent mathematical work, never actually saw the closest planet to the sun: Mercury. We, on the other hand, have a great opportunity to do this at dusk during the middle of April.

To find Mercury, look low in the western sky shortly after sunset. You'll spot Venus because it's the most brilliant object in that direction. But Mercury lies just to its right and appears as a much fainter "star."

If you have difficulty finding it, the thin crescent moon will lie just to its upper right on the evening of April 15.

Now, while Mercury is the closest planet to the sun, it isn't the hottest of our solar system. Oh, it's hot, that's for sure; Mercury's daylight-side temperature soars to about 800 degrees Fahrenheit.

But the hottest planet is Venus, even though it lies twice as far from the sun as Mercury. Its thick cloud cover holds the temperature at a toasty 900 degrees Fahrenheit - hot enough to melt lead.

If the western sky is clear at dusk on the week of April 5 and you aim a small telescope toward Mercury, you'll notice that it doesn't appear completely round, but rather like a tiny crescent moon.

Even with a telescope, however, don't expect to see much. This planet is relatively small, some 2 1/2 times smaller than the Earth. Its appearance close to the horizon also means its light must pass through a tremendous amount of distorting atmosphere before reaching our eyes.

Whether Copernicus ever saw Mercury is debatable. One thing, however, is not. Since the telescope didn't exist at that time, he never saw Mercury like this.

Wouldn't Copernicus be thrilled!