The Discovery of Neptune

Neptune was observed before its "official" discovery, starting with Galileo in the 1610s. However, for every observation of Neptune, the astronomer thought he was seeing a star, so would record it as such. After almost two centuries of observation, Neptune's position did not seem to fit what scientists felt should be a planetary orbit.

John Couch Adams

In 1643, Englishman John Couch Adams was a new astronomer who wanted to reconcile the observational data on Neptune with gravitational perturbations in the orbit of Uranus. Using the relatively new invention of calculus (Isaac Newton "discovered" calculus in the 1700s), Adams was able to show that there should be an object at the relative location of Neptune's orbit causing these variations of Uranus' orbit. Showing his calculations to the Astronomer Royal, George Airy, Adams felt that there could be a planet out there. George Airy did not take the calculations seriously, and so did not actively look for Neptune.

Urbain Le Verrier

However, at the same time Adams was performing his calculations, Frenchman Urbain Le Verrier also made the same calculations as Adams. Unlike Adams, his calculations were taken seriously, and Johann Galle at the Berlin Observatory used the predictions given him by Le Verrier to look for Neptune. On September 23, 1846, within half an hour of looking, Galle was the first to see Neptune and realize that it was a planet and not a star. The position of Neptune compared to Le Verrier's calculations was within 1° and 12° of Adams.

Johann Galle

When it was discovered by George Airy that a Frenchman had done the same calculation as Adams and a German had found Neptune, Airy wanted to make sure that an Englishman had made the same calculations. At first, Le Verrier thought that Adams had plagarised his work, but soon it was realized that Adams and Le Verrier had never interacted and that both work independently coming up with the same prediction. As a result, Adams and Le Verrier were considered co-discoverers of Neptune, until the 1990s, when the Royal Observatory decided that since Adams calculations were not used to find Neptune, he would no longer be considered a discoverer.

Neptune was named for the Roman god of the sea because of its blue appearance. It also stayed in line with the naming of the planets after Roman and Greek gods.

The Discovery of Uranus

Uranus is the first planet that was not known to ancient astronomers and was not discovered until more than 180 years after Galileo first turned his telescope to the sky and discovered the Galilean satellites. Uranus is bright enough to have been seen by Galileo with his telescope, but if he had seen it (and there is evidence that he might have), he would have only assumed it was a star as his telescope did not have enough resolution to see Uranus as a disk rather than a pinpoint of light. Also, that pinpoint of light would not have moved fast enough to distinguish itself as a planet or some other object rather than a star.

In 1690, John Flamsteed actually saw Uranus with his telescope. But much like Galileo, he did not see it moving fast enough that in the six times that he observed it, he thought it was a star in Taurus and even labeled it as 34 Tauri. The 34 means that to him, Uranus was the 34th star discovered in Taurus. Since he did not observe it long enough, he was not able to see it moving faster compared to the actual stars in Taurus.

Pierre Lemonnier also spotted Uranus between 1750 and 1769. However, he also was not able to distinguish it as a planet as he did not see it moving among the stars.

Finally, in 1781, William Herschel made careful observations of Uranus and was able to tell that it moved faster than the farther stars. Not only that, his telescope had good enough resolution to see Uranus was a fuzzy disk rather than a pinpoint like a star would appear in a telescope. By watching Uranus over time, he was able to tell whether or not it was a comet or a planet. A comet would have been moving much faster than a planet as a comet would start to come in towards the inner solar system, but Uranus did not. By following the trajectory of Uranus, he was able to determine how far away it was.

Herschel wanted to name the planet after the King George III of his adopted country, England. But the name was argued back and forth, British astronomers decided to name it Uranus, who was Zeus' grandfather in Greek mythology. It became the first planet, other than Earth, not be named after a Roman god.

Uranus

Image Credit:

NASA/Voyager 2

 

No planet or celestial object causes as much snickering as the seventh planet from the Sun. Depending on whom you ask, it can be pronounced one of two ways. The first way is the one that causes the snickering, especially among high school and college students (I know, I taught college astronomy). The other way, the way that I prefer to use, does not make it sound so humorous. Of course, I am talking about the planet, Uranus.

I pronounce Uranus as if it sounds like "You're A Nus" or "You're a Nis". Trust me, it helps people from giggling when you say its name. The name Uranus itself comes from Greek mythology as Uranus was the father of the first Titans (including Cronus) and the grandfather of the Olympians, the Greek gods. It is the first planet (other than Earth) not named for a Roman god. If the tradition had held using Roman names, Uranus should have been called Caelus, the father of Saturn and in turn, the grandfather of Jupiter.

Uranus is also the first planet discovered with a telescope. Up to its discovery in 1781 by William Herschel, only the six planets were known (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Of course, before the heliocentric model of the Solar System, Earth was not considered a planet. There is evidence that Galileo saw Uranus, but mistook it for a star. Two other astronomers also observed Uranus, but did not identify it as a planet.

Like Jupiter and Saturn, Uranus is a Jovian planet, i.e. a gas giant planet. It does not have a solid surface, but contains hydrogen, helium, methane, ammonia, and water in the outer layers. Its "mantle" is a mixture of ice and rock and has a heavy element core. The density of Uranus is 1.29 g/cm³, making it the seventh densest planet, only ahead of Saturn.

Uranus is the third largest planet in terms of diameter, but the fourth largest in mass (Neptune is larger in mass, but has a smaller diameter). It orbits 19.18 AU from the Sun, taking just over 84 years to orbit the Sun. Since its discovery in 1781, it has only completed two orbits, its third orbit won't be complete until 2033.

Since it is a Jovian planet, it also exhibits two properties that Jupiter and Saturn display: a ring system and multiple moons. The rings are more similar to Jupiter's rings than Saturn's rings as they are very faint and were not confirmed until Voyager 2 imaged them directly. Uranus has 27 confirmed moons which are all icy bodies. Five are considered to have planetary mass which means that they are spherical. Titania and Oberon were actually first discovered by William Herschel in 1787, after his discovery of Uranus.

Image Credit:

W. M. Keck Observatory

Lastly, the most amazing thing about Uranus is its day. Recall that Venus has an inclination of almost 180°. Uranus' inclination is not as severe, but may considered stranger. Its inclination is 98° which means that its axis of rotation is almost parallel to Uranus' orbital plane.

Telescopes

Telescopes come in two basic types: refractors and reflectors. Refractors, or refracting telescopes, use lenses to focus electromagnetic radiation to a focal point to be detected by some sort of detector: eye, CCD computer, camera. Reflectors use mirrors to reflect light to a focal point to be captured by the detector.

Refracting telescopes are basically used to observe one particular type of electromagnetic radiation, visible light. The telescope has two basic pieces; the aperature or objective lens, and the eyepiece or detector. The eyepiece was used by up until the invention of photgraphic astronomy, and now can be replaced by a camera or a computer to take images. Lenses have a property called focal length which is the distance parallel light coming into a lens must travel after passing through the lens to come to a point. The focal length depends on the curvature of the lens.

Knowing the focal length, we can define a couple of terms.

• telescopic magnification power is the ratio between the focal lenght of the objective lens and the focal length of the detector. The larger this ratio, the stronger the telescopic power
• light-gathering area is only dependent on the area of the objective lens. With more area, more light can be gathered and therefore a distant object will be brighter than with a smaller lens. Compare images of Jupiter with a telescope to just your eye. The aperature of a telescope is much bigger than that of the human eye.
• Resolution is how small a detail a telescope can make out. Depends on both the diameter of the objective and the wavelength of light. The longer the wavelength, the bigger the diameter of the telescope's objective to have good resolution. This is why visible light telescopes have smaller diameters than radio telescopes.

Refracting telescopes can only get so big before the lenses cannot support their own weight and will sag in the center. To resolve this problem, reflecting telescopes were invented. There are typically two types: a Newtonian reflector and a Cassegrain reflector. Newtonian reflecting reflect light to a secondary mirror which then reflects the light off to the side of the telescope. Cassegrains reflect light to a secondary mirror which then reflects the light back down through a hole in the primary mirror (or objective mirror).

The Three Types of Telescopes mention in This Blog

Image Credit:

Me

  

Arecibo Observatory in Puerto Rico. This is a radio telescope which is basically a giant reflecting telescope. The detector is at the top, at the focal point of the mirror.

Image Credit

NAIC - Arecibo Observatory, a facility of the NSF

Galileo Galilei

Galileo Galilei was a famous Italian mathematician and astronomer.  He is well-known for being charged with heresy by the Catholic Church in Italy for teaching and promoting the heliocentric model of the solar system.  He was never officially declared a heretic, but he spent the rest of his life under house arrest.  He was not forgiven for this teaching until 1992 by Pope John Paul II.

Why did Galileo believe that the heliocentric model was correct and not the geocentric model, which had been the official Church canon for centuries?  In one word: telescopes.  He used his telescope to look to the skies and observe some things that he saw.  Based on his observations, he concluded that Copernicus and Kepler were right and that Ptolemy was wrong.

Some of his observations were:

1. Jupiter had moons going around it.  It had always been argued that the only object that things could orbit around was the Earth.  Jupiter had four moons that countered this.  I'll discuss the Galilean moons later.
2. The Moon had a bumpy surface.  The Church believed, as did scientists up to this point, that all celestial objects were perfect spheres.  By observing craters and mountains on the Moon, Galileo showed that this was incorrect.
3. He found sunspots on the Sun.  As mentioned in #2, the Sun was thought to be perfect and had no blemishes.  This was incorrect.
4. The milky strip across the night sky was found to contain many stars.  For that matter, the stars on the "fixed sphere" were found to not be fixed, but moved with respect to other stars.  This will be addressed in a future post.
5. Venus experienced phases.  Based on the geocentric model, Venus always had its darkened face towards the Earth, but when Galileo looked at Venus, he discovered that this was not true.  The next post will go into more detail about this and help explain why the phases of Venus could explain why Venus goes around the Sun and not the Earth.