Millihertz to Degrees per hour
mHz
°/h
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Quick Reference Table (Millihertz to Degrees per hour)
| Millihertz (mHz) | Degrees per hour (°/h) |
|---|---|
| 0.1 | 129.6 |
| 0.5 | 648 |
| 1 | 1,296 |
| 5 | 6,480 |
| 10 | 12,960 |
| 100 | 129,600 |
| 500 | 648,000 |
About Millihertz (mHz)
A millihertz (mHz) is one thousandth of a hertz, corresponding to periods of minutes to hours. Millihertz frequencies appear in oceanography (tidal oscillations, slow wave action), geophysics (free oscillations of the Earth after major earthquakes), and physiology (very slow biological rhythms). The Earth's fundamental free oscillation modes — the lowest-frequency seismic normal modes — ring at a few millihertz in the aftermath of great earthquakes. Infrasound below 20 Hz also has a millihertz region for its slowest components.
Earth's gravest free oscillation mode rings at about 0.3 mHz (period ~54 minutes) after large earthquakes. A 1 mHz signal completes one cycle every 16.7 minutes.
About Degrees per hour (°/h)
Degrees per hour (°/h) is used for very slow angular motions, particularly in navigation, geophysics, and astronomy. High-precision gyroscopes are rated by their drift in °/h — a navigation-grade ring-laser gyro may drift less than 0.01°/h, while a consumer MEMS gyro drifts hundreds of degrees per hour. Earth's rotation corresponds to 15°/h (360° ÷ 24 h), which is why the Sun appears to move 15° per hour across the sky. Telescope drive motors use this rate to compensate for Earth's rotation during long exposures.
Earth rotates at exactly 15°/h, so astronomical telescope drives track stars at 15°/h. Navigation-grade laser gyroscopes achieve drift below 0.01°/h. The Moon moves about 0.55°/h against the background stars.
Millihertz – Frequently Asked Questions
What does Earth sound like when it rings at millihertz frequencies after an earthquake?
After a magnitude-9 earthquake the entire planet vibrates like a struck gong, with its deepest mode at about 0.3 mHz — one oscillation every 54 minutes. The surface rises and falls by fractions of a millimeter. You cannot hear it (human hearing starts at 20 Hz), but gravimeters and seismometers worldwide pick it up. The 2004 Sumatra quake kept Earth ringing measurably for weeks.
Why do ocean scientists care about millihertz frequencies?
Ocean swells, tidal constituents, and seiches (standing waves in harbours or lakes) all oscillate in the millihertz band. A 10-second ocean swell is 100 mHz; a harbour seiche with a 10-minute period is about 1.7 mHz. Monitoring these frequencies helps coastal engineers predict resonance in ports and design breakwaters that don't amplify destructive wave energy.
Can humans sense anything at millihertz frequencies?
Not directly — our senses are far too fast. But some physiological rhythms operate here: the Mayer wave, a ~0.1 Hz oscillation in blood pressure, sits at the high end of the millihertz scale, and slower vasomotion (tiny blood vessel contractions) can dip below 10 mHz. You don't feel them as vibrations, but they show up clearly on a continuous blood-pressure monitor.
What is infrasound and does it overlap with millihertz?
Infrasound is sound below the ~20 Hz threshold of human hearing. The lowest infrasound blends into the millihertz range — the International Monitoring System for nuclear-test detection listens down to about 20 mHz. Sources include volcanic eruptions, meteor airbursts, severe storms, and ocean microbaroms (standing pressure waves between ocean swells and the atmosphere).
How are millihertz signals detected if they are too slow to hear?
Instruments record a time series (pressure, acceleration, displacement) over hours or days, then apply a Fourier transform to extract frequency content. Superconducting gravimeters can resolve Earth's free oscillations below 1 mHz by measuring gravity changes of 10⁻¹² g. The trick is not a fast sensor but a patient, ultra-stable one and enough data to separate signal from drift.
Degrees per hour – Frequently Asked Questions
How fast does the International Space Station orbit in degrees per hour?
The ISS completes one orbit (360°) in about 92 minutes, giving roughly 235°/hr — almost 16 times faster than Earth's rotation. That is why astronauts see 16 sunrises every 24 hours. At an altitude of ~408 km, the station covers about 7.66 km/s of ground track. If you could watch it from a fixed point in space, it would visibly sweep through the sky at a rate where one degree takes only about 15 seconds.
Why are gyroscope drift rates measured in degrees per hour?
Because even tiny drift accumulates into serious navigation errors over a flight or voyage. A navigation-grade ring-laser gyroscope drifts less than 0.01°/hr; over a 10-hour flight that is only 0.1° of heading error. A cheap MEMS gyro drifting 10°/hr would accumulate 100° of error in the same time — useless for navigation. Expressing drift in °/hr makes the operational impact immediately obvious to a pilot or engineer.
How do telescope mounts use the 15°/hr rate for star tracking?
Equatorial telescope mounts have a motorised right-ascension axis aligned with Earth's rotation axis. By driving that axis at exactly 15°/hr (one sidereal rate), the telescope counter-rotates against Earth's spin, keeping a star fixed in the eyepiece. Without this drive, stars would drift out of view in seconds at high magnification. Astrophotographers rely on it for long exposures without star trails.
How fast does the Moon move across the sky in degrees per hour?
The Moon's apparent motion has two components. It shares the sky's overall 15°/hr westward motion due to Earth's rotation. But it also orbits Earth, moving about 0.55°/hr eastward relative to the stars (360° ÷ 27.32 days ÷ 24 hr). The net effect: the Moon moves westward across the sky at roughly 14.5°/hr, which is why moonrise occurs about 50 minutes later each day.
Why does a Foucault pendulum appear to rotate at fewer than 15°/hr at most latitudes?
A Foucault pendulum's swing plane rotates relative to the floor at 15° × sin(latitude) per hour. At the North Pole (90°) that is the full 15°/hr; at 45° latitude it is about 10.6°/hr; at the equator it is zero. The pendulum always swings in a fixed plane in inertial space — it is the Earth rotating underneath it. The sine factor comes from the fact that only the vertical component of Earth's angular velocity vector projects into the pendulum's swing plane. Paris (48.9°N) sees about 11.3°/hr, which is why Foucault's original 1851 demonstration took most of a day to complete a visible rotation.