This week the Moon is essentially absent. Skies are as dark as can be. We can see lots of stars – very tranquil, and adding to the calm, we certainly don’t seem to be moving at all. But we are indeed moving. Astronomers now can do more than merely pin down Earth’s trajectory through space; they’ve used that motion to guide their hunt for the mysterious WIMPS: the leading Dark Matter candidates.
The idea is that Earth may plow through a stationary cloud of microscopic WIMPs as the solar system revolves around the center of the Milky Way galaxy. And although such Weakly Interacting Massive Particles do not normally mingle with the atoms that comprise our bodies and our planet, some secondary effects should be detectable.
Ten years ago, a Chinese/Italian collaboration detected more WIMP events each June than each December, and attributed this variability to Earth’s motion. Specifically, our planet travels 18 miles per second around the Sun while also speeding at 144 miles per second around the center of the galaxy. The two motions lie roughly in the same direction in June, boosting Earth’s forward speed at that time. So we plow through more WIMPS just as school lets out. In December, however, our revolution around the Sun takes us the other way – against the flow of our galactic motion – and we encounter fewer particles.
These controversial results notwithstanding, it is exciting to find a practical, scientific use for our motion through space, a velocity that has perennially fascinated the scientifically curious. If you’ve wondered how fast we move on Spaceship Earth, here’s today’s best answer. Your motion on the rotating Earth has everything to do with your latitude: zero at the Poles and 1,038 miles per hour at the Equator. Here in the mid-Hudson Valley, we spin at the speed of sound, around 768 miles per hour.
Next is Earth’s 18.5-mile-per-second revolution around the Sun. To get a feel for this movement, simply face the midday Sun and picture yourself orbiting to the right. When watching a sunrise, our planet’s spin carries you forward (toward the Sun) while Earth’s revolution simultaneously hurls you upward, toward a spot almost overhead.
Visualizing the much-faster around-the-galaxy motion (144 miles per second) is also easy this month. We – and most of the stars of the night sky, our neighbors in the carousel ride around the galactic nucleus – rush toward the constellation Cygnus, marked by the bright star Deneb nearly overhead at nightfall. We’re rushing straight up, at dusk. It’s part of a huge circular orbit around the galaxy’s center, which hovers lowish in the south, marked by a brightening in the Milky Way’s glow. Looking in its direction at nightfall, picture Earth, the Sun and nearly all the night’s stars speeding toward the zenith as we revolve around that spot every 240 million years.
Peering even farther, beyond all the night’s naked-eye stars to the Milky Way’s overall motion relative to the nearest 5,000 galaxies, is a movement some 180 miles per second toward the Little Dipper in the north.
So, definitely and forever, we are on the move. And this month all those forward, around and sideways motions are particularly easy to pin down, as we continue to go nowhere – fast.