T he inferior planets, Venus and Mercury, will be anything but the second week of January.
Venus had a crummy 2014, but it’s now returning with a vengeance. Look west 40 minutes after sunset for two “stars” near the horizon. The brighter is Venus, on the left. The other is Mercury. From the 8th to the 13th, they’ll hover strikingly close to each other.
They’re the only planets with no moons. The only ones that barely spin — needing months to rotate. (Every other planet’s day is less than 25 hours.) The only planets that can appear as crescents. The only ones with high densities similar to our world. These are intriguing resemblances.
But here’s what’s weird. In most other areas, they’re not merely dissimilar, but oddly
opposite.
Venus’ surface sits under more air pressure than any other terrestrial body in the solar system. Its surface matches what you’d experience 3,000 feet (1,000 meters) below sea level — enough to crush a submarine’s hull. It’s like a defective pressure cooker about to explode. By contrast, Mercury is the only planet with virtually no air at all.
Venus is the shiniest planet by far. By contrast, Mercury is the least reflective. It’s as dark as an asphalt parking lot. Venus has the most circular path around the Sun of any planet. This gives it a steady cruise-control orbital velocity of 22 miles (35 kilometers) per second. It doesn’t vary. By contrast, Mercury has the most oval, squashed orbit, so it dramatically slows down and speeds up like a drunk driver. Someone on its surface would sometimes see the enormous Sun rise, then drop below the horizon, then rise a second time.
Next consider axial tilt. Venus’ is off the charts at 177°. That world is actually upside down. By contrast, Mercury has no tilt at all. Not even one degree. Not even half a degree. If you want to be picky, it’s 1/30°. The plane of its equator is its path around the Sun.
Mercury makes three spins while circling the Sun twice, the only planet with a resonance between its day and year. As a consequence, the period from one sunrise to the next is exactly two Mercury years. Nearly six months. Crazy slow. The Sun crosses as much of Mercury’s sky in a month as it traverses ours in two hours. During that long, grueling day, the Sun grows 1.5 times larger and more than doubles its intensity as the planet sizzles through its brutal perihelion. Then the Sun thankfully shrinks again.
