On This Day: The planet discovered with mathematics

Imagine finding a planet billions of kilometres away because your calculations told you where to point the telescope.

That’s essentially what happened on September 23, 1846, when Neptune was identified in one of astronomy’s great detective stories.

The planet they knew was there

Astronomers had noticed that Uranus wasn’t moving quite as expected. The possibility emerged that the gravity of another large, unseen planet was affecting its orbit.

French mathematician Urbain Le Verrier calculated where this mystery world should be and sent his prediction to Johann Gottfried Galle at the Berlin Observatory.

On the night of September 23–24, Galle went looking and he found Neptune within about one degree of Le Verrier’s predicted position.

It was an extraordinary triumph of mathematics and astronomy. Neptune sits an average 4.5 billion kilometres from the Sun and takes about 165 Earth years to complete a single orbit.

HMAS Australia goes into battle

On September 23, 1940, HMAS Australia went into action against Vichy French forces during the Battle of Dakar in West Africa.

The Australian heavy cruiser was part of Operation Menace, an Allied attempt to bring the strategically important port of Dakar under Free French control. The operation lasted three days, with HMAS Australia exchanging fire with French ships and shore positions before the Allied force eventually withdrew.

The costly measurement mistake

September 23 delivered another remarkable space story in 1999 — for all the wrong reasons.

NASA lost contact with its Mars Climate Orbiter as the spacecraft attempted to enter orbit around Mars. The cause was simple, ground software produced data using English units, while the navigation system expected metric units.

The error put the spacecraft on the wrong trajectory, sending it far too close to Mars. Contact was never regained.

Born on this day

Bruce Springsteen was born on September 23, 1949, meaning The Boss turns 77 today.

And he shares the date with another giant of music: Ray Charles, born on September 23, 1930.