The Moon’s companions
HAVEYOU EVER HEARD of the Lagrange Points? If you’re a regular reader of Australian Geographic, you may have (see AG 91). But it’s usually only astronomers and mathematicians who are familiar with Joseph-Louis Lagrange (1736–1813) and the imaginary dots in space that bear his name.
The Lagrange Points are five positions around an orbiting body where gravity and centrifugal acceleration cancel each other out to leave no gravitational pull. Astronomers number them L1–5.T wo of them, L4 and L5, are of particular interest. They lie in the same path as the orbiting body, but are always respectively 60 degrees ahead of and 60 degrees behind it. And they’re stable in the sense that smaller objects can orbit them. Wait a minute: small objects orbiting… imaginary points? Yes, that’s right.
Perhaps the best way to visualise this is to think of the planet Jupiter orbiting the Sun, the L4 and L5 points of which track along Jupiter’s orbit, ahead of and behind the planet. Each has asteroids orbiting it, known as Trojan a steroids. Together, they number more than 7000.
Now, imagine the Earth and Moon. Are there any Trojan asteroids in the Moon’s orbit? The answer is no, but there is something else. Back in 1961, astronomer Kazimierz Kordylewski claimed to have photographed a cloud of interplanetary dust near the Moon’s trailing Lagrange Point – good old L5.
While Kordylewski’s photographs were difficult to verify, his name became synonymous with the Moon’s Trojan dust clouds. And now, he’s been vindicated by modern imagery from Hungarian astronomers .They have convincingly photographed the faint L5 Kordylewski Dust Cloud and are already setting their sights on its counterpart ahead of the Moon.
If nothing else, it’s great to see the dust being blown off old ideas.
FRED WATSON is Australia’s Astronomer-at-Large.
The Lagrange Points associated with the Sun–Earth system.