Gravity and light |

Gravity and light

CREDIT: Cassini Imaging Team, SSI, JPL, ESA, NASA

After sunset on Christmas, the nearly-full moon will light up the sky.

The moon moves eastward at some 2,000 miles an hour – why does it not fly off into space?

Isaac Newton, in the late 1600s, realized that – just as gravity pulls an apple to the ground – gravity, extending outward a quarter-million miles, keeps the moon bound to the Earth.

How strong is gravity?

Henry Cavendish, in the late 1700s, suspended two small lead spheres by a thread. When he moved much larger spheres, each a foot across, close to the small ones, gravity pulled the small spheres toward the larger, twisting the thread. The stronger the pull, the greater the twist.

(The measurement was so delicate that the slightest breeze – a breath – would have interfered, so Cavendish assembled the experiment in an enclosed room, observing the deflection of the spheres through a window, with a telescope).

Gravity doesn’t stop at a quarter-million miles. Close to the moon, Christmas night, shines the planet Jupiter, half-a-billion miles from the sun, bound to the sun by gravity.

Turning his telescope to Jupiter, Galileo Galilei discovered that the giant planet is accompanied by a retinue of moons. Bracing your arms on something solid, you can see Jupiter’s Galilean moons in binocs. The moons circle Jupiter like clockwork, allowing astronomers to predict, years ahead, when each moon would move in or out of Jupiter’s shadow.

Light moves fast. But is it instantaneous?

In the late 1600s, Ole Roemer noticed that when Jupiter and Earth are close – both on the same side of the sun (as we are this month) – Jupiter’s moons move in and out of the shadow earlier than predicted. But when Earth and Jupiter are distant – on opposite sides of the sun – the moons run late.

Roemer attributed the delay to the time it took light to travel the extra distance. Light is fast, but not instantaneous.

Albert Einstein, in the early 20th century, realized that gravity doesn’t pull only on objects like apples and moons … gravity distorts the very fabric of the universe: it warps space-time.

Since light must travel through space-time to reach us, warping space-time bends the path of light.

Einstein’s equations predicted that the warping of space-time – and the bending of light – would be a very subtle effect. To confirm the prediction – to see light bend — would require something more massive than a mere foot-wide, 350-pound sphere of lead.

The sun, if you could weigh it, would tip the scales at something over two billion billion billion tons. Just about enough, the equations foretold, to put a measurable kink into a beam of light.

In May of 1919, a few years after Einstein published his equations, sun and moon would be in the same part of the sky – in Taurus – at the same time: A total eclipse.

This Tuesday night, the moon will again be in Taurus. Just below the moon (and below Jupiter, for a number of nights), is the face of the bull – a sideways “V” of stars: the Hyades (HIGH-uh-deez; the bright reddish-star is the bull’s eye).

On that spring day in 1919, the moon covered the sun, the sky darkened, the stars came out, and astronomers photographed the Hyades, whose starlight would pass close to the sun on its way to Earth.

When they analyzed the photographs, the Hyades stars were slightly out-of-place. The sun’s gravity had warped the space-time around it, bending the paths of the stars’ light.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages, and may be reached at

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