Spot the Planet Uranus with the Naked Eye

The “Great Planet Race” in the western evening sky this summer is beginning to wrap up; Venus has caught up with Mars as they both leave Saturn behind. As these planets set in the west, though, another rises in the east at about 9:30, and will have cleared most buildings and trees by 10 p.m. And this one is involved in a conjunction of its own.

This is none other than Jupiter, king of the planets. Once Jupiter rises, it is easy to find because it outshines everything in the sky except the sun, the moon, and Venus.  Just look east for the brightest thing in the night sky.  Last I checked, Jupiter is still missing one of its belts.  For the rest of this year, Jupiter remains well placed for observing in convenient evening hours.  If you have a telescope, watch for yourself and see if the belt returns!

Although Jupiter seems to be by itself among the much, much dimmer stars of Pisces, it in fact has a close companion that few of us ever get to see without a telescope, the planet Uranus. We typically identify Mercury, Venus, Mars, Jupiter, and Saturn as the five naked eye planets, and for practical purposes, that’s true.  However, Uranus is actually visible to the unaided eye under perfect conditions.  In the time before man made light dimmed the skies, many people could see Uranus.  However, they were unable to recognize it as a planet because it is dim and changes position very slowly. (It takes 84 years for Uranus to reappear near the same stars).  Thus Uranus, although plainly visible, went undiscovered for centuries.  For example, in 1690, John Flamsteed was cataloguing stars and constellations, numbering stars in each constellation from west to east.  However, the ‘star’ he catalogued as ’34 Tauri’ (#34 in Taurus) was in fact the planet Uranus. 

 Replica of Herschel’s telescope

In March 1781, William Herschel became the first to identify Uranus as a planet when he observed it in his telescope.  As Uranus is about twice as far from the sun as Saturn, Herschel’s discovery doubled the size of the known solar system.  Herschel wanted to call the new planet ‘George’ (actually Georgium Sidus in Latin) after his patron, King George III.  German astronomer Johann Elert Bode, who had calculated an orbit for the new planet, suggested calling it ‘Uranus’ because in Graeco-Roman myth, Saturn had been the father of Jupiter and Uranus the father of Saturn. 

Here are charts showing the relative positions of Jupiter and Uranus from now into the new year.  The event depicted is a triple conjunction, in which two outer planets align on three separate occasions only a few months apart.  This occurs when distant planets align while Earth is on the same side of the sun as they are.  As Earth passes the slower outer planets, we see them slow down, stop, and reverse direction for while.  We see the planets resume direct motion once Earth has pulled far enough ahead on its much faster inner orbit.  As a result, we see three conjunctions instead of just one.  The three closest alignments of Jupiter and Uranus occur on June 8, 2010, September 18, 2010, and January 3, 2011.  At all three, Jupiter is less than one degree (about the width of an adult’s pinkie held at arm’s length) under Uranus.  The conjunction of June 8 occurred in the morning sky, but the two yet to come will be visible in convenient evening hours.  On the night of September 20-21, Earth is directly in line with the pair, causing them to rise at dusk at set at dawn–Jupiter and Uranus will be up all night long.  By winter, Jupiter and Uranus will be high in the south southwest at nightfall. 

Late 2010 is a good time to get a glimpse of a world we don’t usually notice in the sky.  A small telescope or even binoculars will reveal Uranus.  And if you find yourself away from the city on a moonless night, see if you can pick out which of the dim points of light just above Jupiter is a little more than meets the eye. 

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.

Neptune
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.