You probably learned in school all about the tides: how the Moon’s gravity tugs on the oceans, pulling along a great bulge of water that manifests `at the seashore as the twice-daily rising and falling tide. But what they probably didn’t tell you is something even more astonishing: the Moon’s gravity actually causes the solid Earth under your feet to rise and fall, too. Taking about 12 hours to complete a single cycle, this is one of Earth’s slowest natural vibrations.
This short sound clip is a continuous recording of the motion of the ground at a seismic observatory in Australia over a span of 18 months. I’ve sped it up a whopping 4,000,000 times to make the sound of this “solid Earth tide” audible. At this time scale, an entire year of Earth time flies by in only 8 seconds. It’s equivalent to transposing the sound upward by 22 octaves. At first hearing, it may sound like little more than a scratchy old vinyl LP recording. But don’t be deceived: that scratchiness is actually the sound of thousands of earthquakes, large and small, around the world. If you listen carefully, you’ll hear a faint low-pitched hum hiding beneath that “noise”. This is what we’re after: it is the sound of planet Earth itself vibrating under the gravitational influence of Sun and Moon.
You can hear it better in this next clip, in which I’ve filtered out most of the earthquakes.
We don’t ordinarily perceive these vibrations, of course, because they unfold so very slowly. And although it’s a common sight to see the liquid oceans sloshing up and down at the seashore under the influence of these great tidal forces, we don’t actually see the mountains rising up to greet the Moon because we are likewise being lifted right alongside them. Nevertheless, the effect is very real. The Moon’s gravity physically deforms the solid Earth, creating a broad bulge in the Earth’s crust about a foot high that follows along beneath the Moon. This bulge remains aimed at the Moon all day long, while the Earth slowly turns beneath it. Continents, oceans, mountains, lakes, rivers, cities, you and me — we all take a turn slowly riding up one edge of the bulge, then down the other, twice each day. 
But it’s not only the Moon that’s involved. The Sun’s tidal pull, although only half as strong as the Moon’s, has a similar effect, raising its own bulge on Earth.
From our vantage point on the Earth’s surface, the Moon and Sun thus move across the heavens, dragging their heavy burdens around the planet day after day. But they do so at slightly different rates, which leads these two systems of bulges to slip in and out of phase over the course of a month, mirroring the phases of the Moon.
So here we are, riding atop a one-foot-high undulating roller-coaster of crisscrossing tidal bulges. It’s a very bumpy ride! You can hear that bumpiness in the fast pulsating beat of these sounds. When the hum briefly pulses louder, Sun and Moon are in phase (spring tide); when it briefly fades, Sun and Moon are out of phase (neap tide). And so on.
The final sound clip is an extended version of this pulsating tone, here simulated by computer to reflect the tidal pulls of Sun and Moon over a 25-year period. (If you’re so inclined, you can count off the months as you listen: one cycle of two loud pulses = 1 lunar month.) Although the actual vibrations of the solar and lunar tides are far lower in pitch, this sound bears the same mathematical form as the natural tides.
I find this a very soothing sound. When left playing softly in the background, it makes for a comforting sonic “wallpaper”. I wonder if its pleasing familiarity stems from our long history of living here on planet Earth. Over countless eons we have all been gently lifted and lowered by the invisible hand of gravity, rocked by the reliable rhythms and hum of our nearby celestial neighbors. Had we evolved on another world, with no moons, or perhaps with 3 or 10, we might have grown accustomed to a different kind of hum. But this one belongs to us; it’s in our bones.
- Recorded December 2002-June 2003 at IDA station WRAB (Tennant Creek, Australia). Broadband seismometer, vertical component, sampled at 20Hz.
- Ground displacements due to the solar and lunar tides have been measured at 40cm (16 inches) at one seismic observatory. See Lay and Wallace, Modern Global Seismology (San Diego: Academic Press, 1995), p. 13.
- Why twice daily? Because a similar, symmetric bulge rises up on the opposite side of the Earth, forever pointed away from the Moon. Contrary to popular misconception, the cause of this opposing bulge has nothing at all to do with “centrifugal forces” from the Earth’s rotation. Even if the Earth were non-rotating, there would be two such bulges. The twin bulges arise from a symmetry inherent in the laws of gravity. For a good analysis of the shape of Earth’s tidal potential field, see Stacey, Physics of the Earth (New York: Wiley & Sons, 1977).
- The tidal effects of the Sun are about 45% those of the Moon. See, for example, Stacey, Physics of the Earth (New York: Wiley & Sons, 1977), p. 94. The tidal effect from our other planetary neighbors are tens of thousands of times smaller still and are inaudible here. For a fascinating discussion of the possible gravitational effects on Earth of an alignment of the planets, see “Interplanetary Low Tide”, by Science@NASA, May 4, 2000.
- The solar bulge takes exactly one day (24 hours) to complete its circuit, whereas the lunar bulge lags behind by about 48 minutes, owing to the Moon’s orbital motion around Earth. Every two weeks — at full Moon and at new Moon, when Sun, Earth, and Moon are aligned — the bulges coincide, causing the tidal forces to combine and create the unusually high (and low) “spring” tides. When the Moon is at first and last quarter, the bulges counteract each other, leading to the relatively mild “neap” tides.