How Satellites Work

How Satellites Work

How is a Satellite Launched into an Orbit?

All satellites today get into orbit by riding on a rocket or by riding in the cargo bay of the Space Shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into orbit on a regular basis.

For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.

After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket's nozzles to

 tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest.

How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth's circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 km). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 km). To travel around that circumference in 24 hours, a point on Earth's surface has to move at 1,038 mph (1,669 kph). A launch from Cape Canaveral, Florida, doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A, one of its launch facilities, is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth's rotational speed there is about 894 mph (1,440 kph). The difference in Earth's surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate -- fatter around the middle -- not a perfect sphere. For that reason, our estimate of Earth's circumference is a little small.)

Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the February 11, 2000 lift-off of the Space Shuttle Endeavor with the Shuttle Radar Topography Mission required launching a total weight of 4,520,415 pounds (2,050,447 kg). It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.

Once the rocket reaches extremely thin air, at about 120 miles (193 km) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.

Orbital Velocity and Altitude

A rocket must accelerate to at least 25,039 mph (40,320 kph) to completely escape Earth's gravity and fly off into space (for more on escape velocity, visit and .

Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit. With satellites, the object is not to escape Earth's gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity's pull on the satellite and the inertia of the satellite's motion -- the satellite's tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 km). Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line

The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity. At an altitude of 124 miles (200 kilometers), the required orbital velocity is just over 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 km) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permits the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called "geostationary."Geostationary orbits are ideal for weather satellites and communications satellites.

The moon has an altitude of about 240,000 miles (384,400 km), a velocity of about 2,300 mph (3,700 kph) and its orbit takes 27.322 days. (Note that the moon's orbital velocity is slower because it is farther from Earth than artificial satellites.)

• To get a better feel for orbital velocities at different altitudes, check out NASA's orbital velocity calculator.
• To learn more about orbits and other topics in space flight, check out this site.

In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite can stay in orbit for centuries (take the moon as an example).

Satellites usually start out in an orbit that is elliptical. The ground control station controls small onboard rocket motors to provide correction. The goal is to get the orbit as circular as possible. By firing a rocket when the orbit is at the apogee of its orbit (its most distant point from Earth), and applying thrust in the direction of the flight path, the perigee (lowest point from Earth) moves further out. The result is a more circular orbit.

What is a Satellite Launch Window?

A launch window is a particular period of time in which it will be easier to place the satellite in the orbit necessary to perform its intended function.

With the Space Shuttle, an extremely important factor in choosing the launch window is the need to bring down the astronauts safely if something goes wrong. The astronauts must be able to reach a safe landing area where rescue personnel can be standing by. For other types of flights, including interplanetary exploration, the launch window must permit the flight to take the most efficient course to its very distant destination. If weather is bad or a malfunction occurs during a launch window, the flight must be postponed until the next launch window appropriate for the flight. If a satellite were launched at the wrong time of the day in perfect weather, the satellite could end up in an orbit that would not pass over any of its intended users. Timing is everything!

What is Inside a Typical Satellite?

Satellites come in all shapes and sizes and play a variety of roles. For example:

• Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.

• Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.

• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).

• Scientific satellites perform a variety of scientific missions. The Hubble Space Telescopeis the most famous scientific satellite, but there are many others looking at everything fromsun spots to gamma rays.

• Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.

• Rescue satellites respond to radio distress signals (read this page for details).

• Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
  
  

• Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include:
  
  • Relaying encrypted communications
  • Nuclear monitoring
  • Observing enemy movements
  • Early warning of missile launches
  • Eavesdropping on terrestrial radio links
  • Radar imaging
  • Photography

Despite the significant differences between all of these satellites, they have several things in common. For example:

• All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch.
• All of them have a source of power and batteries for storage.
  

  Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets . Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.

• All of them have an onboard computer to control and monitor the different systems.
• All of them have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
• All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.
  

  The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.

  
  

Will the world really end in 2012?

Will the world really end in 2012?

There have been countless theories throughout time about how the world will end and how -- or if -- life will cease to exist. At the turn of the 21st century, conspiracy theorists claimed that the Y2K bug was only a small part of the impending devastation: The new century would bring about total destruction, and no one would survive. Others believe that Earth is slated for another ice age, which will kill off all living things. And according to astronomers, billions of years from now, the sun will become a red giant, expanding to a size larger than the Earth's orbit and consuming the Earth in the process. Even if the planet somehow survives, the sun will eventually shrink, becoming a white dwarf and gradually cooling off until it can no longer warm anything in the solar system.

An apocalyptic view of Seattle sinking into the Puget Sound -- will something like this happen in 2012?

In 2006, Mel Gibson released a movie about the Mayan civilization. "Apocalypto" follows one man's journey from slavery back to his family. During the course of the movie, a young woman prophesizes that a man will bring an end to the Mayans and wipe out their civilization. In the real world, some theorists don't believe a man will be the end of the Mayans -- instead, a celestial event will be the cause. The Mayan calendar even gives us a date for this theoretical event: Dec. 21, 2012.

How did the Mayans develop a calendar that could predict the end of the world? How does the Mayan calendar work? Do the Mayans really believe we have only four years left on Earth, and if so, why is Dec. 21, 2012, doomsday? Read on to find out if the end is near.

Devastation Predictions

Nostrad amus

Michel de Nostredame, also known as Nostradamus, was a 16th-century physician who also had a penchant for writing. He penned a series of prophecies, focusing prima rily on wars, disasters and destruction. Using metaphors and mystery, Nostradamus wrote these prophecies as quatrains, or four-line verses. His followers say he predicted the rise of Hitler, the Apollo moon landing and the attacks on Sept. 11, 2001. His critics say his writings are nothing more than ancient horoscopes, written to tell of events that undoubtedly will occur again and again. To find out more about Nostradamus and his prophecies, read our article on How Nostradamus Works.

The Long Count to the End

The Mayans have a complex system of calendars, and each calendar has a different purpose. There are thought to be 20 calendars in use, though only 15 have been revealed to the public. The Mayans keep the other five a secret within their culture. Some of the more commonly known Mayan calendars are:

• The Tzolk'in Calendar - Used primarily in crop rotation, this calendar allows one 260-day period to ready the land and one 260-day period to grow and harvest the corn.
• The Haab Calendar - This calendar lasts 360 days, with a 5-day period called the wayeb. Close to the Gregorian calendar we use today, this calendar follows the cycle of the sun.
• The Calendar Round - The Calendar Round gave the Mayans a way to record history in longer increments. It's a combination of the Tzolk'in and Haab calendars and goes through about 52 years.

Fifty-two years was longer than the Mayans' average life span at the time of the Calendar Round's creation. However, Mayan historians wanted to create a calendar that could be used to record history for centuries. This led to the Long Count calendar. The Long Count incorporates an era called the Great Cycle, which lasts approximately 5,125.36 years [source: Jenkins]. The idea that the world is on its way to an end comes from the Long Count.

Sometime during the , the Mayans stopped using the Long Count -- at least as far as the Spanish were concerned. A Mayan historian began recording events on both the Long Count and Gregorian calendars. Scholars then compared the dates on both calendars and confirmed the beginning of the current Great Cycle as August 13, 3114 B.C., making the end of the fourth Great Cycle -- you guessed it -- December 21, 2012. Theorists believe this is the day that the world will end, and all living things on <>Earth will die.

The end of the world may resemble another ice age.

However, the Mayans themselves don't actually believe that the world is going to end at the end of this cycle. In fact, they believe that it's a time of great celebration and luck when the planet lasts through a full Great Cycle. After all, we've made it safely through three other Great Cycles, and the world is still turning.

What makes this cycle so different, some believe, is that it ends on a winter solstice. On this particular winter solstice, the sun will align with the center of the Milky Way. This particular event happens only every 26,000 years or so [source: USA Today]. But is this celestial occurrence really going to end the world and kill us all? Probably not. Many scholars see this theory as extremists trying to cash in on the fears of others [source: USA Today].

So what will happen on Dec. 21, 2012? It's likely that the day will pass with no major event happening at all. People may not even realize it's the projected doomsday, although that's unlikely considering how much press it's likely to garner. We'll just have to wait and see what happens -- and hopefully update this article on Dec. 22, 2012.

TOP 5 SCARY TECHNOLOGIES OF 2008

TOP 5 SCARY TECHNOLOGIES OF 2008

Technology makes our lives better. You're enjoying your new high-definition television, and have your work and personal appointments organized on your computer. You pay your bills electronically and save time and money. That's fine and dandy, until someone takes out the electronic banking system and the machines take over the planet.

Sounds crazy? Perhaps. None of the technologies we included in our top 5 are likely to rain death and destruction on our little planet, though that doesn't prevent people from being unnerved by them. Read on to find out what weirdness may await in 2009 -- and beyond.

5th

This may not sound so scary, and once you know what's going on, it isn't. But it might be a little unsettling to be walking in a store and hearing voices asking you to buy things. If you look around, you won't see anyone speaking, and none of the other shoppers will seem alarmed -- no one else can hear it but you.

A company called Holosonics developed the Audio Spotlight system, which uses tiny speakers to focus sound into a very narrow beam. Ultrasonic frequencies are too high for the human ear to hear, but as the sound travels from the Audio Spotlight system's speakers, air distorts the sound and makes it audible. It's perfect for in-store advertising, but you'd have to be standing in the right place to hear it. You can learn more about it if you read, Can companies beam advertisements into my brain?

Perhaps hearing voices isn't frightening, but what about having your computer taken away? Keep reading to learn more.

4: Law Enforcement Takes Your Laptop

Paul Sakuma/
Judge Alex Kozinski of the 9th Circuit Court of Appeals ruled that U.S. border agents can search electronic devices without probable cause.

If you travel with your laptop or Black Berry, listen up. According to the 9th Circuit Court, it is perfectly legal for Federal Customs and Border Patrol agents to seize your technological devices when you enter the United States. You might not have anything illegal on your computer's hard drive, but even if you're just carrying your personal computer and you have all your kids' vacation pictures on there, there's no telling when -- or if -- you'll get your machine back.

The idea is that in the interest of national security, U.S. officials have the right to confiscate electronics to search them for evidence of a crime -- even without probable cause. If that sounds scary to you, you can read more in our article,can government take away my latop?

Perhaps the government can use this decision to prevent an attack, but can anyone prevent an all-out cyberwar? On the next page, we'll take a look.

3: Cyberwar

Brian Hendler/
Israeli hacker Avi Buzaglo has been fighting a cyberwar with crackers taking aim at Israeli computers.

Imagine a war fought completely by computer. No, we're not talking about a scene out of the movie WarGames, we're talking an all-out attack on a nation's electronic infrastructure. What's that, you may ask? Those are the systems that control emergency response services, banks and other electronic commerce, the systems that run the electrical grid, water and fuel pipeline controls, communications networks, and oh, yeah: defense weaponry. A well-executed attack could cause serious disruption and open the populace up to physical threats.

Attacks like these have already been launched against some countries. Russians attacked Georgia with denial-of-service attacks. Hackers have taken on the Pentagon, and some suspect terrorist organizations of training their operatives to launch computer assaults. So how do you defend against a cyberattack? Educating people about computer viruses and Trojan horses will help, and using updated antivirus software is also important. You can find out more by reading is cyber war coming?

Cyberattacks might actually be useful tools against machines who have learned to think for themselves and chosen to eliminate humanity. It's the stuff of science fiction, but why do some people believe this could happen? Learn more by reading the next page.

2: The Technological Singularity

Stan Honda/
The team that created IBM's Deep Blue computer poses for a photo. A computer might beat a human at chess, but can it learn how to build more like itself?

Artificial intelligence (AI) has come a long way since computers first made the scene. Yet we're not at the edge of a dystopian society in which the machines run amok and humankind fights for its survival. At least, not yet.

Vernor Vinge, a math professor at San Diego State University, proposed what he calls the singularity -- a time at which computer networks may become self-aware through advanced AI, and interfaces between people and computers help humankind evolve. Biological advancements may become so sophisticated that doctors can even engineer human intelligence. There is a possibility, however, that AI might allow machines to take over the world. There's no guarantee that such a scenario will really happen, and technological limitations may prevent it. Still, the idea that machines might someday decide we're irrelevant and arrange for our destruction is more than a little creepy. You can read more when you take a look at What's the technological singularity?

In 2008 scientists completed a gigantic machine that they believe will help solve some of the secrets of the universe. Some critics of the project were so afraid of it that they filed a lawsuit to prevent it from being turned on. Why? Find out on the next page.

1: The Large Hadron Collider

Fabrice Coffrini/AFP/
The Compact Muon Solenoid in the LHC will help detect subparticles released during collisions.

The Large Hadron Collider is the largest and most powerful particle accelerator ever built. It's designed to smash tiny particles into tinier ones to find out the stuff from which the universe is made. The European Organization for Nuclear Research (CERN), the organization that built the collider, are optimistic that their experiments will help them find the elusive Higgs boson particle or evidence that string theory exists. You can learn more by reading How the Large Hadron Collider Works.

Critics, on the other hand, have filed suit to have the LHC deactivated. They fear that the LHC could create a black hole at the surface of the Earth, or create a weird -- and for now, hypothetical -- material called strangelets that cosmologists theorize could have a highly destructive gravitational field. CERN scientists reject both theories. Even if the collider could produce black holes or strangelets, they would decay too quickly to cause any harm.

How 'The Da Vinci Code' Doesn't Work

How 'The Da Vinci Code' Doesn't Work

Since its 2003 publication, "The Da Vinci Code" has caused quite a stir. Since its debut to glowing reviews, it has sold more than 40 million copies in at least 44 languag es [ref]. In addition to being a bestseller, it's sparked a lot of controversy. It's a work of fiction, but it presents itself as based in fact, and many critics have raised questions about whether those facts are accurate. 

It's no secret that the HowStuffWorks staff likes to take things apart and see what makes them tick. Some of us are also the kind of sticklers who point out science and technology mistakes in TV shows and movies, much to the chagrin of the people listening. But when we heard about the controversy surrounding "The Da Vinci Code," we couldn't resist picking it apart. 

In this article, you'll learn what happened when we took a close, hard look at "The Da Vinci Code" and how it uses science, technology, art and history. 

Trouble at the Louvre 

"The Da Vinci Code" begins with a crime at the Louvre Museum in Paris. At the behest of someone known as "the Teacher," a man named Silas murders curator Jacques Saunière. After reviewing the evidence, French investigators summon Harvard symbologist Robert Langdon for questioning.

Warning: Spoiler Alert This article discusses numerous plot points from "The Da Vinci Code."

Captain Bezu Fache of the Direction Centrale Police Judiciaire (DCPJ) is sure that Langdon is the murderer. Fache has one of his lieutenants plant a GPS dot in Landon's pocket. It's a "metallic, button-shaped disk, about the size of a watch battery." This dot, according to cryptographer Sophie Neveu, is accurate to two feet and lets the DCPJ track Langdon's location, no matter where he is. In other words: 

• It's tiny.
• It's amazingly accurate.
• It works indoors.

However, real global positioning system (GPS) devices: 

• Can be small, but they're usually bigger than a watch battery. The unit described in the book would also have to fit a power source and a second radio transmitter into its tiny shell in order to communicate with police computers.
• Are accurate to somewhere between 13 and 328 feet (4 and 100 meters).
• Don't work well indoors, under dense tree cover or in urban areas with tall buildings.

One fact explains all of these points -- by definition, a GPS receiver uses radio waves to communicate with satellites that are 11,000 miles above the Earth's surface. The receiver has to have an unbroken line of sight to these satellites, something it doesn't have indoors. Even military GPS technology can't typically get a fix on a soldier who is in dense tree cover or otherwise concealed.  

The troublesome GPS dot keeps causing problems as the story moves along. Sophie tells Langdon that if he throws the dot away, the DCPJ officers will see that it is no longer moving and know he's onto them. She comes up with an ingenious plan. She imbeds the receiver in a bar of soap, breaks a restroom window and throws the soap onto the roof of a passing truck. 

That seems like a good plan, and it works. The officers rush to apprehend the truck, believing that Langdon is on the roof. This buys him and Sophie some time. Unfortunately: 

• The restrooms of the Louvre have liquid soap, just like most other public restrooms.
• According to a "Da Vinci Code" tour guide, the restrooms in that part of the Louvre do not have windows [ref].

In spite of its inaccuracies, this move does buy Sophie and Langdon some time. But the mistakes in the Louvre don't stop there.  

A Note on Chronology

"The Da Vinci Code" doesn't specify the date on which its fictional events took place, but it's probably set in 2000 or 2001: 

• Sir Leigh Teabing references the recent passing of the millennium.
• The book references current events, like the success of Harry Potter.
• Critical events take place in the Louvre's Salle des Etats, which was closed for renovation from 2001 to 2004.

Another possibility is that the book takes place sometime between 2001 and its publication date of 2003. If that's the case, the inclusion of the Salle des Etats is a major oversight.

Museums, Medicine and Other Mistakes

Knowing Is Half The Battle Langdon and Sophie both seem sure that lots of evidence incriminates Langdon as Sanière's murderer, and little exonerates him. But anyone who's watched police procedurals on television (or the movie "The Boondock Saints") knows that investigators can test a person's hands for gunshot residue (GSR). Maybe neither of the two heroes watches TV.

Rather than escaping through their newfound window of opportunity, Sophie and Langdon follow another clue to the Salle des Etats. The Salle des Etats, also known as the Salle de la Joconde, is the Mona Lisa's home. Before too long, the DCPJ apprehends them there.

In another burst of quick thinking, Sophie removes Leonardo's "Virgin of the Rocks" (called "Madonna of the Rocks" in the novel) from the wall opposite the "Mona Lisa." She uses the painting as a shield and threatens to destroy it by pressing her knee through the canvas. Naturally, the apprehending officer allows her to escape in order to prevent the destruction of the priceless artwork. 

A few critics have remarked that the scene in the Salle des Etats is impossible because Leonardo painted "Virgin of the Rocks" on wood, not canvas. However, a royal art restorer called Hacquin transferred the painting to canvas in 1806. The scene is impossible as written for other reasons: 

• "Virgin of the Rocks" hangs in the Grand Gallery, not the Salle des Etats. The painting directly across from the "Mona Lisa" is Caliari's "The Wedding Feast at Cana." This painting is an enormous 32 feet (9.9 meters) wide. To be fair, we have not found a source detailing which painting faced the "Mona Lisa" before the 2001 closure of the Salle des Etats. However, in several older photographs, reflections in the "Mona Lisa"'s protective glass indicate that it wasn't "Virgin of the Rocks."
• Even if "Virgin of the Rocks" did hang opposite the "Mona Lisa," it's 6.5 feet (1.99 meters) tall, too tall for Sophie to see over as described. The painting's ornate wooden frame is also too heavy for an average person to lift unassisted.
• Sophie's removal of the painting from the wall does not activate any sort of security system. This contradicts the beginning of the book, in which Saunière removes a painting from the wall to activate a security system that seals off an entire corridor. It also contradicts the Louvre's real security system, which includes proximity and movement detection [ref]. For the record, this security system also uses real security cameras, which staff monitor 24 hours a day [ref].

Dennis Hurley, creator of the satirical film "The Albino Code," which addresses the way that "The Da Vinci Code" presents people with albinism.

The farther the story moves away from the Louvre, the more it begins to focus on events in the distant past and artistic interpretation rather than verifiable details. But it does make several other concrete errors, including its depiction of people with albinism. One of "The Da Vinci Code"'s villains is a man named Silas, who is an albino. He has white skin and hair as well as pink eyes with red pupils. Silas is good with a gun and drives a car at night in pursuit of the heroes.

Albinism is a real medical condition in which a person's body cannot produce the proper amount of the pigment melanin. Most people with albinism have very pale skin and hair and light-colored eyes. Very few people with the condition have pink eyes, though. Most have light blue eyes. 

Albinism prevents a person's retina and ocular nerves from forming properly. For this reason, doctors use eye examinations to diagnose people with the condition. Most people with albinism don't see well because their retinas don't function properly. Although few are blind, many do not see well enough to drive a car or, as seen in "The Da Vinci Code," to shoot people from a distance. In other words, it's extremely unlikely that Silas could perform the tasks described in the novel. You can learn more about albinism at the National Organization of Albinism and Hypopigmentation. 

Other errors aren't as complex as the ones described above: 

• When traveling from the Paris Ritz to the Louvre, Langdon and a DCPJ agent pass the Opera House and cross Place Vendôme. However, the Paris Ritz is on Place Vendôme. In order to pass the Opera House, the officer would have to head in nearly the opposite direction of the Louvre.
• Langdon says that Saunière, a devotee of the ancient "sacred feminine," was interested in Wiccan relics. However, Wicca is a modern religion, not an ancient one.
• The pyramid entrance to the Louvre contains 793 panes of glass, not 666 [ref].
• Tarot decks contain 78 cards, not 22, although a deck does have 22 major arcana cards. 
• The first of the Dead Sea Scrolls was discovered in 1947, not the 1950s.
• The novel implies that the Louvre has one curator. It really has a staff of 60 curators in eight departments. The "Mona Lisa" has its own curator.
• Harvard does not have a professor of "symbology," and symbology is not a real academic discipline 
• There are no metal detectors at Westminster Abbey, and people cannot make charcoal rubbings of the plaques there

Art and History

"The Da Vinci Code" makes a lot of claims about art and the Christian Bible. Of all the disputed statements in the book, these can be the hardest to prove or quantify. Although some people spend their entire lives studying and interpreting art or religious scriptures, both fields are by nature imprecise. It can be impossible to determine an artist's actual intent for a particular piece or the exact meaning behind a particular religious passage.

According to the novel, Leonardo placed hidden symbols and codes in his paintings. For example, the book makes a lot of assertions about the "Mona Lisa," including: 

• Leonardo carried the painting around with him and refused to part with it.
• The painting is a "well-documented collage of double entendres and playful allusions."
• Unevenness in the background makes the painting more majestic from the left than the right, which is a testament for Leonardo's love for the feminine.
• The painting represents an androgynous person or a self-portrait of the artist.
• Leonardo named the painting for Egyptian deities -- fertility god Amon and fertility goddess Isis.

Image courtesy NASA

Which of these points are true? Here's what we found: 

• Leonardo did keep the painting rather than give it to the person who had commissioned it.
• Plenty of art scholars have proposed theories about the painting and who it represents. However, these are all theories -- Leonardo didn't leave behind a step-by-step analysis of his intentions behind the painting.
• Some scholars have pointed out similarities between the "Mona Lisa" and Leonardo's self portrait. However, the widely accepted theory is that the painting depicts Lisa Gherardini, the wife of Francesco del Giocondo. This is why the painting is known as "La Joconde" in France and "La Gioconda" in Italy.
• Although you can blend "Amon" and "Isis" and get "Mona Lisa," there's a much simpler explanation. "Mona" is a title meaning "my lady" in Italian, and the woman who sat for the portrait was named Lisa.

"The Da Vinci Code" also proposes theories about Leonardo's painting of the "Last Supper." According to the book, it shows Mary Magdalene at the right hand of Jesus as well as a disembodied hand bearing a knife. Langdon's explanation for why people don't notice the painting's hidden meaning involves "scotoma" -- the brain blocking knowledge associated with powerful symbols. However, "scotoma" is a medical term that simply means "blind spot." A scotoma typically stems from neurological or ocular dysfunction -- not from exposure to a powerful symbol. 

The figure to the right of Jesus does have a feminine appearance, but most scholars agree that it is the apostle John, who typically has a youthful, delicate appearance in artwork of the period. Careful examination of the painting also reveals that the "disembodied" hand really belongs to Peter, although he is holding the knife in a somewhat awkward position. 

The novel also makes numerous assertions about history and other works of art. Here's a run-down of some of the frequently contested points: 

• Alexander Pope did not deliver a eulogy at Isaac Newton's funeral, although Pope did write a poem about Newton.
• The Gospel of Philip was probably written in Greek, not Aramaic, although the only surviving manuscript is written in Coptic.
• Approximately 50,000 people -- men and women -- died during the years of witch hunts, not 5 million.
• The Priory of Sion is a fictitious organization, founded and publicized by Pierre Plantard in 1956 [ref].
• King Philip of France did arrest and torture members of the Knights Templar on a Friday the 13th in 1306. However, there are many other reasons behind some people's superstition regarding Friday the 13th. 
• Accounts about the history of Leonardo's "Virgin of the Rocks" differ. However, it seems that money, not displeasure about the symbolism of the painting, caused the dispute that led to two versions of the painting [ref].
• Nothing in the Bible or any other existing historical document proves that Jesus was married to Mary Magdalene. At the same time, no existing document proves that he was not.

Finally, most of the theories in "The Da Vinci Code" about Jesus's relationship to Mary Magdalene, whether they had a child, the "real meaning" of the Holy Grail and the history of the Catholic church come from one source. That source is "Holy Blood, Holy Grail" by Michael Baigent and others. Also known as "The Holy Blood and the Holy Grail" in the United Kingdom, this book is marketed as a work of nonfiction. However, many critics have raised serious questions about its accuracy.