What Galileo Almost Saw

Throughout this International Year of Astronomy, 2009, we have been thinking back on Galileo Galilei and the historic discoveries he made with is telescope back in 1610. However, it’s also interesting to reflect on a discovery that Galileo almost made–the planet Neptune.

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Galileo Galilei
Creative Commons License photo credit: dizarillo

Astronomers did not become aware of  Neptune until 1846.  On September 23 of that year, Johann Galle of the Berlin Observatory received a letter from Urbain Le Verrier in Paris.  Le Verrier had been trying to understand why Uranus was not quite where people expected it to be.

When William Herschel announced the discovery of Uranus in 1781, astronomers went to work calculating its orbit around the Sun.  In 1821, Alexis Bouvard noticed that his tabulated positions of Uranus, based on Newton’s laws, did not quite match up with Uranus’ real positions.  He suggested that an eighth planet beyond Uranus was perturbing Uranus’ orbit.  Urbain Le Verrier painstakingly calculated where in the sky this planet might be in order to affect Uranus’s orbit in just the observed way and mailed his predictions to Galle.  Galle, assisted by a student, Heinrich d’Arrest, found Neptune in his telescope the same day he received Le Verrier’s data.  (John Couch Adams of England made similar observations and calculations over the same period.)

Galle and d’Arrest were the first to recognize Neptune, but not the first to see it.  At magnitude 7.9, Neptune is too dim to be seen with the unaided eye, but it does show up as a point of light in simple telescopes and even in binoculars.  From the moment of its discovery, astronomers wondered if earlier telescope users might have seen Neptune without realizing it.

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Creative Commons License photo credit: Image Editor

In the winter of 1612-1613, Jupiter began to align with Neptune from Earth’s point of view.  The alignment was so complete that on January 4, 1613, Jupiter’s disk actually blocked (occulted) Neptune’s.  Galileo, having discovered four moons around Jupiter in January 1610, was still observing Jupiter three years later.  He made careful drawings of Jupiter, its moons, and any background stars in his telescope’s field of view.  Upon comparing the background stars in Galileo’s drawings to the positions Neptune would have had that winter, astronomers have concluded that Galileo drew Neptune as a background ‘star’ in drawings he made on December 28, 1612, and on January 27 and 28, 1613.

Galileo’s simple telescope was not powerful enough to resolve Neptune into a disk.  (You need a telescope at least 10-12 inches in diameter to do this).  In order to recognize it as a planet, Galileo would have needed to see Neptune change position against background stars. Since it orbits about 30 times as far from the Sun as Earth does, Neptune takes 146 years to go around the Sun once.  As a result, its motion against the background stars is harder to notice.  Once a year, Earth comes around to Neptune’s side of the Sun.  This makes Neptune seem to slow down, stop, and reverse direction against the background stars.  (This is called ‘retrograde’ motion.)  As it turns out, in December 1612, Earth was just coming around to Neptune’s side of the Sun, and Neptune was virtually stationary and about to begin retrograde motion.  Neptune’s motion against the background stars would have been all but unobservable in December 1612.

The Roman god Neptune, for whom
the planet is named.
Creative Commons License photo credit: OliBac

By January 1613, however, Neptune was in full retrograde motion.  On January 27 and 28, Galileo did notice that one of his background stars had slightly changed position compared to another.  According to University of Melbourne physicist David Jamieson, this indicates that Galileo knew he had found a new planet.  However, we see no sign that he attempted a second observation of that mysterious star, or that he reported the finding of a new planet. Thus Galileo, first to see Neptune, does not get credit for discovering it.

Others who saw Neptune in their telescopes and mistook it for a star include Jerome Lalande of the Paris Observatory, whose staff conducted a detailed survey of the sky in 1795, and William Herschel’s son John, who happened to see it in 1830.

Uranus is another planet seen before its formal discovery.  In fact, at visual magnitude 5.6, Uranus is right at the threshold of visibility to the naked eye.  This means that if you’ve been out on a clear night with no clouds or light pollution, and Uranus happened to be up, you’ve probably seen it.  And so have countless observers across the globe throughout history who looked up in pristine skies.  Uranus moves so slowly (taking 84 years to orbit the Sun once) and blends in so well with the stars in its general direction, that our eyes pass right over it.  That’s why it took William Herschel’s telescope in 1781 to recognize Uranus for what it is.  When John Flamsteed, the very first Astronomer Royal of the United Kingdom, prepared a catalog of visible stars, he misidentified Uranus as a star, designating it ’34 Tauri’ (the 34th star of the constellation Taurus).

As 2009 ends, Jupiter is once again approaching Neptune in our sky.  As I write this (late November 2009), Jupiter is by far the brightest thing in the south-southwest at dusk (unless the Moon is out).  Neptune is just under 4 degrees to Jupiter’s upper left (three fingers held together at arm’s length block about 5 degrees).  Since Jupiter is orbiting much faster than Neptune, we see Jupiter gain on Neptune’s position  during the next few weeks.  Unlike in 1613, Jupiter will not align with Neptune exactly; the two planets are just over half a degree apart at closest approach on December 21.  (One half of one degree is about the apparent size of the Moon’s disk.)  Jupiter then pulls ‘ahead’ of Neptune and is just over two degrees away by New Year’s.  Here is a  finder chart to help you identify which point of light among the stars is Neptune.  This holiday season, then, you have the chance to repeat Galileo’s observations from the winter of 1612-1613.  But you, unlike Galileo, will know exactly what you’re seeing.