The planets Venus and Mercury will be truly noteworthy the next ten days. They’ll do a photobomb.
Venus had a crummy 2014, but now returns with a vengeance. Look low in the west at 5:15 p.m. You need a really unobstructed view, like from all the Kingston malls’ parking lots. Between 5:15 and 5:30 you’ll see two stars down low. The brighter is Venus, on the left. The other is Mercury. From now through the 15th, 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 be crescents; the only ones with high densities, similar to our world. These are intriguing resemblances.
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 body in the known universe. Its surface matches what you’d experience 3,000 feet under the sea: enough to crush a submarine’s hull. By contrast, Mercury is the only planet with no air at all: zero surface pressure.
Venus is the shiniest planet by far. By contrast, Mercury is the least reflective. It’s darker than an asphalt parking lot. Venus has the most circular path around the Sun of any planet. By contrast, Mercury has the most oval, squashed orbit, so it dramatically slows down and speeds up like a drunk driver.
Next, consider axial tilt: Venus’ is off-the-charts at 177 degrees. That world is actually upside-down. By contrast, Mercury has no tilt at all – not even one degree. Not even 1/20th of a degree.
Mercury makes three spins while circling the Sun twice, the only planet with a resonance between its day and its year. As a consequence, the period from sunrise to its next sunrise 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.
Mercury’s bizarre three-to-two day/year ratio means that we telescopically observe the same Mercury features every other time it circles the Sun. No wonder we believed until the 1960s that its spin rate matched its orbit, the way our Moon does. Observers seemed to keep sketching the same dusky markings. Turned out, they saw repetitions only half the time. They’d simply tossed out the ones that didn’t fit: human nature.
Could the contrasts of those worlds get any odder? Actually, yes. Venus has a nonexistent day/night temperature range. Its ultra-thick air keeps both its day and night hemispheres baking at an even thermostat setting of 870 degrees Fahrenheit. You don’t need an oven there; the Cytherean gourmet cookbook calls for just two seconds for pot roast. Same for rice, since Venus’ high pressure only lets water boil above 500 degrees. But Mercury, always the contrarian, has the greatest day/night range of any object in the universe. There, the mercury (ha!) plunges 1,000 degrees after sunset. Its rocks go from hot enough to melt lead to cold enough to liquefy oxygen.
Venus gets brightest when it’s on our side of the Sun and nearly at its closest to us. But Mercury is brightest when it’s about as far away as it can get. Right now, both have recently emerged from behind the Sun, on the far side of their orbits. Thus, Mercury is now nearly at its brightest, while Venus is dimmest (but even a “dim” Venus always outshines Mercury). Venus alters its brightness threefold during its full cycle, but Mercury varies its brilliance by an astonishing factor of 1,000: more than any other planet in the universe.
They’re both fascinating and bizarre – worth a look, any evening between now and mid-month.
Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.