ASTR-401 Lessons

4.1: A Planet by Any Other Name 

Welcome, students, to a new year in Astronomy 401!  I trust that you have rested your brains well over the holiday and that you are ready once again to explore the heavens.  Last year we focused exclusively on planet Earth, but this year we will branch out further and explore our fellow wanderers around the Sun.  Until further notice, the course requirements for this and later years will be the same as they were for the previous three years.

Syllabus

Week	Lesson	Assignments
1	A Planet by Any Other Name	Quiz and Essay
2	Mercury	Quiz
3	Venus	Quiz and Observation
4	Mars	Quiz and Essay
5	Jupiter	Quiz and Midterm Exam
6	Saturn	Quiz and Essay
7	Uranus and Neptune	Quiz
8	Beyond Neptune	Quiz
9	Mystery Lesson	Quiz, Essay, and Final Exam

 

What’s in a Name?

Now that we have discussed the general outline for the year, we can turn to the interesting part of the lesson - the planets.  The word “planet” comes from Ancient Greek word πλᾰνήτης (planḗtēs), which means “wanderer.”  Even before the first telescopes were invented, the ancients looked to the heavens and made observations.  They saw that stars moved across the sky with the seasons, and the positions of certain stars were used to determine when to plant crops.  They also noticed that, amongst these predictable stars, several other bright lights would wander across the sky, moving through constellations and appearing in different places from year to year.  In time, early astronomers were able to track the movements of these wanderers, give them names, and identify their most obvious characteristics.

 

The ancient Roman symbols for the planets (inner square) and the zodiac signs (outer circle).

Source: here 

There were seven known wanderers in the ancient world.  We know them today as the five planets visible with the naked eye - Mercury, Venus, Mars, Jupiter, and Saturn - as well as the Sun and Moon. Those five are the only planets that can be seen without the aid of a telescope.  Mercury, Venus, and Mars are close to Earth, which makes it easier for us to see them even though these planets are relatively small.  Jupiter and Saturn are much further away, separated from us by the asteroid belt, but they are also much bigger than the other planets in the solar system. Their size allows a greater amount of light to be reflected off of them, making them viewable from Earth.  Another interesting tidbit to note is that Mars is the only planet that appears to be a different color from the stars around it - the rest of the planets just appear brighter.  This is because the surface of Mars has a great amount of a red material called “iron oxide,” something that you will be exploring later this year. 

 

Asteroid belt (white) and the planets of the solar system.

Source: here

During the Renaissance in Europe, the word “planet” took on a different definition; instead of defining any celestial body that wandered between the fixed stars, it came to mean any celestial body that orbited the Earth.  It wasn’t until Galileo proved that the Earth and the other planets orbited the Sun that Earth officially became known as a planet in its own right.  By the 19th century, a planet was instead defined as any large celestial body orbiting the Sun.

Since no one was able to say how large a planet should be, there were a lot of planetary discoveries made in the 19th century.  Neptune was discovered in 1846, at the time thought to be the furthest planet from the Sun; other discoveries were made closer to home.  Ceres, Pallas, and Vesta were all discovered within about a year of each other.  These “planets” orbited the Sun in the asteroid belt between Mars and Jupiter.

  

The four largest asteroids are much smaller than the Moon.

Source: here  

Ceres, Pallas, and Vesta also posed a problem for astronomers of the time.  Although they were similar to planets in that they reflected the Sun’s light and were large enough to be observed by the telescopes of the day, these celestial bodies didn’t have typical size or (except for Ceres) the spherical shape of a planet; furthermore, their orbits were confused and overlapping.  Because of these differences, Ceres, Pallas, and Vesta were given a new classification as asteroids, literally meaning “star-like,” while planets were redefined as being objects orbiting the Sun with enough mass to pull themselves into a spherical shape and distinct orbit.

Pluto is the most recent “planet” to be discovered (it is now considered a dwarf planet instead of a planet, for reasons that are discussed below).  Notably, magical people were strongly involved in this dwarf planet’s discovery.  One man in particular, Percival Lowell, dedicated his life to the discovery of “Planet X.”  Upon his death, the burden of discovery was placed on Clyde Tombaugh, a Hufflepuff Hogwarts graduate who had been working in the field of Muggle astronomy for six years while he awaited acceptance into a magical astronomical institute.  He applied to several but, upon his discovery of a ninth “planet,” abandoned his interest in further education to pursue his own experiments.  His discovery was officially announced to the world in March 1930.

Pluto is a bit of an odd duck.  It is much further away from Neptune, its closest neighbour, than any of the other planets are from their neighbours. It is smaller than all the planets, even Mercury, and its orbit is much more irregular, even crossing over Neptune’s.  Pluto and Neptune have not collided, however, because whereas Neptune’s orbit is close to the same plane as the orbits of the other planets, Pluto’s is tilted so that it is sometimes above or below the planets in the solar system.

Video of Pluto’s orbit compared with those of Uranus and Neptune

Source: here

 

It was another 70 years before more “planets” were discovered; Eris, Haumea, and Makemake were all discovered by Professor Mike Brown in the early 2000s. Eris especially was very close in size to Pluto, though further away from Earth and the Sun.  Eris had all the makings of being the tenth planet, but it also posed a question: how many more objects like Pluto exist?  The Kuiper Belt is a second asteroid cloud located beyond Neptune.  Like the asteroid belt, it contains a lot of space debris left over from the formation of the solar system. Based on the discoveries of Mike Brown and others, we can also conclude that the objects in the Kuiper Belt are sometimes larger than the average asteroid.

 

Location of the Kuiper belt

Source: here 

This left the International Astronomical Union with a problem as well: is there such a thing as too many planets?  Ultimately, the IAU decided that, since so many new planets were being discovered so quickly, they needed to redefine what it was that makes a planet a planet.  They developed three criteria that every celestial object must meet to be considered a planet of the solar system:

1. The object must orbit the Sun.

2. The object must have enough gravity to assume a round or nearly round shape.

3. The object must have cleared the area around its orbit of other space debris.

Pluto meets only the first two of the three requirements of planethood, as it orbits the Sun and has enough gravity to create and maintain a round shape, but it has not managed to become the gravitationally dominant figure in its orbit.  Unlike the planets, which have either flung away space debris or pulled all other objects into their gravitational field to create moons, Pluto’s orbit is full of debris, some as big as Pluto itself.  As Pluto has not managed to push away or pull in all of these objects, it has been relegated to dwarf planet status along with Eris, Haumea, and Makemake, while Ceres has been elevated from asteroid to that position.

 

Three of the five confirmed dwarf planets (aside from Pluto and Ceres).

Source: here 

 

As Pluto had been called a planet for so long - the icing on our solar system’s cake - many people took it to heart when the IAU took Pluto off the list of planets.  New maps of the solar system had to be drawn and orreries recalled to remove the extra non-planet, not to mention that many opposed (and sometimes still oppose) the controversial demotion.

Naming Planets

Astronomy is perhaps the most democratic science: anyone can look up at the sky and study the movement of the stars and planets.  Every ancient civilization studied the stars and every culture named them differently.  Nowadays, we use a standard naming system put forth by the International Astronomical Union, drawing often on Greek and Roman mythology as well as more modern classic literature. These influences make the naming system strongly reliant on Western culture.

While the idea of what makes a planet has changed greatly over time, the names of the planets have changed very little in the last one hundred years. The IAU’s guidelines were set in 1919 and have needed very few changes since then.  Their idea was to be sure that astronomers all over the world could communicate effectively, so they standardised the naming conventions for celestial objects.  Most objects don’t get a proper name - they only get an alphanumeric denomination such as PSRB1257+12C.  Objects that are important to us get a de facto name like Venus.

The list is long; only significantly large objects receive names. Two objects cannot receive the same name, and names cannot be translated, but can be written in any alphabet.  Although there is no rule that says that planets are named for gods, every planet except for Earth is named after some Greek or Roman god. Features on those planets can also be named for people or places in modern literature.  The rules are numerous but fascinating, and often provide insight into what astronomers like. 

The naming conventions of the IAU are, in some ways, a way to inject some fun and whimsy into long hours spent peering through a telescope’s lens.  As telescopes become more powerful, more features can be distinguished on the surfaces of planets, satellites, and other celestial bodies.  This means that, with each new advancement made in telescopes, more astronomers get to make their “mark” on a planet’s surface.

The name of any celestial object cannot be bought or sold.  The astronomer who discovers an object is given the chance to name it, although the IAU also reserves the right to reject the name and request another.  Unfortunately, there are a lot of websites that will try to sell you the right to name a star for a friend or loved one.  While they may keep their own databases of named stars, the IAU does not recognize these names.  You would be better off spending your Galleons on flowers, chocolates, or a good book!

Remembering the names of the planets and their order in the solar system has been the subject of many mnemonic devices.  My parents taught me the phrase Muggles Validly Expect Magic Just Starts Utter Nonsense.  Another option would be Many Valiant Educators Must Juggle Several Unique Names.  The first letter of each word corresponds to that of the eight planets, including Earth.  This way, so long as you don’t confuse Mercury and Mars, you can remember the order of all the planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune!

The Eight Planets of the Solar System

Note: Venus looks white, not orange, from above the clouds.

Source: here

 

Apparent Magnitude and the A.M.E. of Planets

We turn now to the more scientific aspects of the planets.  Muggles have measured the brightness of celestial bodies very precisely; this means you can get the most accurate information on the subject by consulting Muggle sources.  The brightness of a body from our viewpoint on Earth is called its apparent magnitude.  Unfortunately, calculating it may be a bit complicated, so bear with me while I walk you through the process.

The ancient Greek astronomer Hipparchus divided the stars he could see into six groups according to how bright they looked.  The brightest stars were called stars of the first magnitude, the next brightest were said to be of the second magnitude, and so on; the faintest stars he could see were of the sixth magnitude.  More recently, astronomers determined that the average first-magnitude stars were about a hundred times as bright as the average sixth-magnitude stars.  With that, they found that in order to go from one magnitude to the next, you need to multiply the brightness of your celestial body by 2.5.

This is an example of the Weber-Fechner law, which states that multiplying the strength of a stimulus by the same amount adds the same amount to one’s perception of it. Take a crowd of people, for example. Adding two more people to a group of four makes a noticeable difference. Conversely, you would need to add 50 people to a crowd of 100 to make a similarly noticeable difference in the size of the crowd. Adding only two more people would not be perceived as a change.  The law holds for all of our senses. The sound of a note is raised an octave when the frequency is doubled, then an octave again when doubled again. You would notice if someone deposited some hippogriff droppings in the middle of the Great Hall, but you wouldn’t notice the same standing out in the fields where there are droppings galore.

Muggle astronomers refined Hipparchus’s scale by including magnitudes between whole numbers and magnitudes less than one (brighter than the average first-magnitude star) and more than six (fainter than the average sixth-magnitude star).  On that scale, the Sun’s average magnitude is -26.74 and the full Moon’s average magnitude is -12.74.  That’s a difference of 14 magnitudes.  Do you recall that first-magnitude stars are 100 times as bright as sixth-magnitude stars?  It follows that a difference of five magnitudes means that one object is 100 times as bright as the other.  Therefore, a difference of 15 magnitudes (5+5+5) means that one object is one million (100*100*100) times as bright as the other one.  Subtracting one magnitude divides that million by about 2.5, so the Sun is about 400,000 times as bright as the full Moon.  The brightest star (except for the Sun), Sirius, has an apparent magnitude of -1.47.  The faintest stars an average observer can see with the naked eye from a place with no light pollution have a magnitude of about 6.0.

The apparent magnitude of a planet varies depending upon its distance from the Sun and its distance from the Earth.  With each planet we discuss in the following lessons, we’ll include its highest and lowest brightness values in terms of apparent magnitude.  If you would like a reference for the apparent magnitude of various celestial bodies, I’ll hand some out at the end of the class.

Now, the base A.M.E. Quotient of a planet or moon (that is, the amount of magic it reflects to the Earth before interference is taken into consideration) may differ from the amount of light it reflects, because its magical albedo may differ from its optical albedo.  Since stone and metal absorb magic, but not perfectly, the magical albedo of a planet or moon whose surface is rocky will be less than its optical albedo.  Since magic passes through water, including liquid water, water vapour, and ice, it will not be reflected by the ice on an icy planet.  Instead, a bit of it gets absorbed on the way through to the rocks underneath as well as by the rocks themselves, so the body’s magical albedo will be even less than if the surface were only rocky.  In particular, ice and stone each divide the magical albedo by two, so that if both are present, the magical albedo will be four times as small as the optical albedo.  Clouds made of other substances rather than water reflect magic as well as they reflect light, so the magical albedo of a planet or moon covered with those sorts of clouds will be the same as its optical albedo.  We’ll discuss the A.M.E. of the various planets in the next few lessons.

The material in this section will not be tested on the quiz, but expect to get questions on it on the midterm and final exams.

In Lessons Two through Eight, we are going to go on an imaginary tour of the solar system, each leg (except the first) starting where the previous one left off.  

That is all for this evening.  As promised,  here is a Muggle-written article which concludes with a table of the apparent magnitude of several celestial bodies. 

  

Original lesson written by Professor Gagarina.

The last section was written by Professor Plumb.