50 Fun Facts About Gravity

How long would a drop through a frictionless tunnel bored right through Earth take to reach the far side?

About 42 minutes is the textbook answer for a uniform-density Earth, the same as half the orbital period of a low satellite. A realistic density profile trims the trip to about 38 minutes.

Gravity on the Moon, where Apollo astronauts bounded along in their suits, is roughly what fraction of Earth's?

About one-sixth is right: lunar gravity is 1.62 m/s², 16.6% of Earth's, so 80 kg of suit and backpack felt like 13 kg. It still varies by 1.6% across the surface because the Moon is lumpy inside.

Crew on the International Space Station, where gravity is still 90% of surface strength, float because they are in what?

Free fall is the whole trick: station and crew are falling toward Earth together, but moving sideways at 28,000 km/h so they keep missing it. Calling the condition weightlessness is fine; calling it the absence of gravity is not.

On most coastlines, how many high tides does the Moon's pull raise in the course of a single day?

Two high tides a day is the norm because the Moon raises one bulge on the side facing it and another on the far side, and Earth turns through both. A few places, such as parts of the Gulf of Mexico, get only one.

Which Canadian bay, a funnel-shaped basin that resonates with the tide, has the highest tidal range on Earth?

Bay of Fundy tides average about 16 metres, against a worldwide average of roughly one metre, enough to push rivers that drain into it backwards twice a day.

Which planet, with 318 times Earth's mass, has the strongest surface gravity of any planet in the Solar System?

Jupiter pulls at about 2.5 g, 24.8 m/s², so an 80 kg person would feel like 200 kg at the cloud tops. Its bulk is so spread out that the figure is only 2.5 times Earth's despite 318 times the mass.

Ignoring air resistance, how fast must a rocket leave Earth's surface to coast away without ever falling back?

About 11 km/s, 11.2 km/s to be exact, is Earth's escape velocity, roughly 40,000 km/h or 33 times the speed of sound. The Moon's is just 2.4 km/s, which is why Apollo could leave it on a single small engine.

Gravitational waves, ripples in spacetime first directly detected in 2015, cross the universe how fast?

The speed of light is exactly how fast gravitational waves move, and when GW170817's ripples and its gamma-ray flash arrived within two seconds of each other after 130 million years, the match was confirmed to astonishing precision.

Why does the Moon always show Earth the same face, leaving its far side unseen until a probe flew past in 1959?

Tidal locking means the Moon spins exactly once per orbit, so the same hemisphere always faces us. Earth's tides braked its rotation into step billions of years ago, and the Moon is slowly doing the same to Earth.

What nickname did NASA's reduced-gravity aircraft earn from the parabolic arcs that leave passengers briefly weightless?

Vomit Comet was the passengers' own name for it, since each flight strings together dozens of plunges and many riders are sick; Project Mercury astronauts trained aboard one in 1959.

A belly-down skydiver stops speeding up once air drag balances gravity, settling at a terminal speed of roughly what?

About 55 m/s, near 200 km/h, is the terminal speed for a face-down skydiver, and a head-first dive can nearly double it. A parachute simply moves that balance point down to a survivable 5 m/s or so.

Because Earth bulges at the equator, which Ecuadorian volcano's summit is the farthest point from the planet's centre?

Chimborazo's summit is about 2 km farther from Earth's centre than Everest's, even though it is 2,500 m shorter above sea level, which makes it the peak that reaches farthest into space.

Which Greek philosopher's Physics claimed bodies fall at speeds proportional to their weight, a view Galileo demolished?

Aristotle held that a stone ten times heavier falls ten times faster, a claim that went largely unchallenged for almost two thousand years. Galileo noted that tying a small stone to a heavy one should then both slow it and hurry it along.

Instead of dropping weights from a tower, Galileo mostly studied falling bodies by rolling balls down what?

Inclined planes slowed the motion enough for Galileo to time it with water clocks, and showed that distance grows with the square of the time. The tower drop, if it happened at all, was over too fast to measure.

The extra-high spring tides arrive twice a month when the Sun and Moon line up, at which pair of lunar phases?

New and full moon put Sun, Moon and Earth in a line called syzygy, so the Sun's tidal pull reinforces the Moon's; the name has nothing to do with the season. At the quarter moons the two pulls partly cancel, giving weak neap tides.

Laser pulses bounced off reflectors left by Apollo crews show the Moon receding from Earth by how much each year?

About 3.8 cm a year is the Moon's retreat, paid for by tidal friction that is also slowing Earth's spin. Run the clock back and the Moon once loomed far larger in the sky, with tides to match.

At Earth's exact centre, with equal mass pulling from every direction, the net gravitational force on you would be what?

Exactly zero is the answer: inside a uniform sphere gravity falls off in step with depth and vanishes at the centre by symmetry. You would still be crushed, not by gravity but by the pressure of everything above.

Compared with an identical clock on a mountaintop, a clock at sea level, deeper in Earth's gravity well, runs how?

Slightly slower is right: the closer a clock sits to a mass, the slower it ticks, an effect atomic clocks can now detect across a height difference of 33 cm. Your head is ageing faster than your feet.

Despite 95 times our planet's mass, Saturn's fast spin and equatorial bulge leave its gravity at the equator how?

Lower than Earth's, 8.96 m/s² against our 9.8, is Saturn's equatorial pull: its spin flings the equator outward and its mass is spread through a huge, low-density globe. At the poles the figure is about a third higher.

Gravity on Mars, a planet with 11% of Earth's mass, comes to what percentage of Earth's surface gravity?

About 38%, 3.72 m/s², is the Martian figure, so a person could jump well over twice as high there as on Earth. The feeble pull is one reason Mars lost most of its atmosphere to space.

In which year did Einstein present the field equations of general relativity to the Prussian Academy in Berlin?

1915 was the November when Einstein, after years of false starts, delivered the equations that recast gravity as the curvature of spacetime. Newton's law survives inside them as the weak-field limit.

Tidal friction from the Moon is braking Earth's rotation and lengthening the day by roughly how much per century?

About 2 milliseconds a century adds up: fossil corals suggest a day around 400 million years ago lasted roughly 22 hours. Leap seconds exist partly to keep atomic time in step with the slowing planet.

General relativity's first success explained a stubborn leftover drift in Mercury's perihelion of how much per century?

About 43 arcseconds a century, a hundredth of a degree, was the gap Newton's laws could not close, and Einstein's equations matched it exactly. He later said the result gave him heart palpitations.

Which hypothetical planet did Urbain Le Verrier propose between Mercury and the Sun to explain Mercury's stray orbit?

Vulcan was hunted by astronomers for half a century; it never existed, and general relativity accounted for the orbit instead. Le Verrier had earned the benefit of the doubt by predicting Neptune the same way.

Eddington's 1919 eclipse party went to Príncipe; a second team measured starlight bending from Sobral, in which country?

Brazil's Sobral station actually produced the cleaner plates, with star shifts matching Einstein's prediction and not Newton's; the results made Einstein a global celebrity overnight.

The 2017 neutron-star merger seen in gravitational waves and light forged up to 13 Earth masses of which precious metal?

Gold by the planet-load: a single collision may make between 3 and 13 Earth masses of it, which is why the gold in a wedding ring is best thought of as neutron-star shrapnel.

LIGO's twin L-shaped detectors in Louisiana and Washington measure length changes across mirrors spaced how far apart?

4 kilometres of vacuum pipe in each arm lets LIGO sense a stretch of less than a ten-thousandth of a proton's width, among the tiniest measurements ever made.

Virgo, Europe's gravitational-wave detector and LIGO's partner, sits outside which Italian city, Galileo's birthplace?

Pisa is a short drive from Virgo's 3 km arms at Cascina, and the detector's data helped pin the 2017 neutron-star merger to a single galaxy.

What is the nickname of the first confirmed gravitational lens, found in 1979, which doubles one object's image?

The Twin Quasar, Q0957+561, is one object split by the gravity of a galaxy in front of it; light along the two paths arrives about 14 months apart.

The Bullet Cluster, whose lensing map shows mass separated from hot gas, is the aftermath of a collision between what?

Two galaxy clusters passed through each other, and the lensing mass kept going with the galaxies while the gas lagged behind, which is hard to explain without dark matter.

How many Lagrange points, where a small body can hold station relative to two large orbiting ones, does any pair have?

Five Lagrange points exist, L1 to L5; the James Webb telescope parks at L2, while swarms of trojan asteroids crowd the stable L4 and L5 of the giant planets.

Gravity Probe B, launched in 2004 with ultra-precise gyroscopes, tested the geodetic effect and what other prediction?

Frame-dragging is spacetime being swirled around by Earth's rotation, and the probe's gyroscopes tilted by the predicted whisker of 39 milliarcseconds a year.

What name is given to the hypothetical massless, spin-2 particle that would carry gravity in a quantum theory?

Graviton is the name, though none has ever been detected; it would need spin 2 because gravity's source is the stress–energy tensor, where the photon's is a mere current. Catching one may be impossible even in principle.

What do geodesists call the lumpy surface the oceans would settle into under gravity and rotation alone, with no winds?

The geoid dips about 100 m below the reference ellipsoid south of India, the deepest 'gravity hole' on the planet, and sea level faithfully follows it.

Which twin satellites, nicknamed Tom and Jerry, mapped Earth's gravity by tracking the 220 km gap between them?

GRACE watched Greenland and Antarctica lose ice by weighing them from orbit, as vanishing meltwater changed the pull below; its ranging could sense a gap change of 10 micrometres.

NASA's GRAIL mission mapped lunar gravity in 2012 with twin probes named by Montana fourth-graders as what?

Ebb and Flow were the winning entries from a Bozeman elementary school, and the probes went on to chart the Moon's buried mass concentrations, the 'mascons' that tug satellites off course.

In 1774, Astronomer Royal Nevil Maskelyne estimated Earth's density by measuring a plumb line's deflection toward what?

A Scottish mountain, Schiehallion, was picked for its tidy symmetric shape, and the survey of it incidentally gave the world contour lines.

The 1959 Pound–Rebka experiment measured gravitational redshift over a 22.5-metre tower at which American university?

Harvard's Jefferson Laboratory tower was enough: gamma rays climbing it lost a few parts in a thousand trillion of their energy, just as Einstein predicted, thanks to the newly discovered Mössbauer effect.

Irwin Shapiro's 'fourth test' of relativity predicted that what, bounced off a planet past the Sun, would return late?

Radar signals skimming the Sun take measurably longer, because spacetime near a mass is effectively stretched, and the same delay in pulsar timing now lets astronomers weigh neutron stars.

Surface gravity on a neutron star, which packs a star's mass into a city-sized ball, is roughly how many times Earth's?

About 200 billion g, 2×10¹² m/s², is typical, so an object dropped from one metre would hit the surface at about 1,400 km/s.

Which unit of acceleration, equal to 1 cm/s² and used in precision gravimetry, is named after Galileo?

The gal is tiny: Earth's surface pull is about 981 gals, and field gravimeters hunting ore bodies or magma chambers resolve changes of a few microgals.

On which moon of Saturn would thick air and weak gravity let a person strap on wings and fly by flapping?

Titan's gravity is 14% of Earth's while its surface air is about four times denser than ours, so muscle-powered flight would be easy; NASA's Dragonfly rotorcraft is built to exploit the same physics.

Surface gravity is lowest anywhere on Earth, at 9.764 m/s², atop which Andean peak, the highest mountain in Peru?

Nevado Huascarán combines altitude with a near-equatorial latitude, the two things that weaken the pull, against 9.834 m/s² at the surface of the Arctic Ocean. The spread across the whole planet is only about 0.7%.

Because orbital gravity is weaker and the satellites move fast, GPS clocks drift from ground time by how much a day?

About 38 microseconds a day sounds trivial, but left uncorrected it would push GPS fixes off by about 11 km within a day; the weaker gravity alone makes the clocks gain 45 microseconds, and their speed claws back 7.

In which year did Hulse and Taylor win the Nobel for a pulsar pair whose decaying orbit betrayed gravitational waves?

1993 came 22 years before LIGO caught a wave directly; PSR B1913+16's orbit shrinks by about 3.5 metres a year, exactly as Einstein's theory predicts.

In 2023, CERN's ALPHA experiment dropped antihydrogen atoms to settle whether antimatter falls which way under gravity?

Down, like ordinary matter, was the verdict, ruling out the science-fiction hope of antigravity from antimatter; the atoms had to be trapped and cooled for years before anyone could watch one fall.

Galileo reportedly noticed a swinging cathedral lamp kept the same rhythm however wide it swung, timing it against what?

His own pulse was the only timer Galileo had, and the observation led both to pendulum clocks and to pendulums as the first precise gravity meters.

At 'magnetic hills' like New Brunswick's, where parked cars seem to roll uphill, what is actually at work?

An optical illusion is all it is: the surrounding landscape hides the true horizon, so a slight downhill slope reads as uphill and gravity does exactly what it always does.

Mariner 10 reached Mercury in 1974 via the first planet-to-planet gravity assist, bending its path around which world?

Venus bent Mariner 10's path inward and dropped its perihelion to Mercury's orbit, a slingshot trick Voyager later used to tour the outer planets for free.

Gravity at the Sun's visible surface, which is 109 times wider than Earth's, comes to roughly how many g?

About 28 g, 274 m/s², is the solar surface figure; the Sun's 333,000 Earth masses are offset by a radius so large that the pull at the surface is tamed.