closer to the Earth at the moment than it has been since the 1960's. This past Friday I found myself peering up through the canyons of Manhattan, and there it was, brilliant enough to outshine the up-lit urban canopy, muscling aside the landing lights of jetliners and helicopters, second only to the post-harvest Moon. There was something enthralling about it. This tiny disk was so distinctly alien. In my mind's eye I superimposed the great gas giant, king of worlds, two and half times as massive as all the other planets in our system put together. Its vast jet streams and storms, its incredible family of 63 moons, the potent magnetic field, the great plasma torus as Io burrows through a tight orbit. There it was, a place in the sky, a tiny condensed blob of mass. It struck me just how vivid this perspective was on the yawning gulf of interplanetary terrain. Gravity does an incredible job at packing matter down to a small volume, our solar system is indeed mostly empty void, but for these extraordinary cusps of curved space.
The scientist's curse is that any semblance of poetry often gives way to curiosity about numbers. As far and as small as Jupiter appeared in the sky it was surely doing more than just bouncing photons into my eyes. All that mass, now a mere 592 million kilometers away, shouldn't I be able to feel the Jovian lure?
It turns out not by much. If Jupiter was at the zenith then it pulls at us with a gravitational acceleration a few hundred millionths that of the Earth. At first I was disappointed, it had felt so much more out there on the street. It's all a matter of relating though. The Empire State Building is a 365 thousand ton chunk of rock and steel, that's about 730 million pounds. When Jupiter sweeps close, and rises into the night, it reaches out and makes this iconic tower weigh about 26 pounds less than normal. That's not much, but 26 pounds is something I can envisage - it's a chunk of cornerstone, maybe a small floodlight. Not so much that anyone would notice, but there nonetheless.
Of course, the lunar and solar tides sweep across us all the time and do far more - but they lack the charisma of King Jove, our planetary gravity lord. If some distant race were to be monitoring our Sun, seeking the tell-tale dips and wobbles as it is pulled at by the planets, then they too would most likely first see the presence of this gas giant. In their catalog of exoplanets Sol b would be the entry. If anyone bothered with this nondescript system it might eventually gain a Sol c, another gas giant orbiting a little further out. Would anything compel them to keep looking, to seek those small inner worlds that might, or might not, be there? Looking in from the outside is very different from looking out from the inside at a bright object in the September sky.