|Exam Name||:||Sun Certified Developer for Java Web Services (SCDJWS)|
|Questions and Answers||:||183 Q & A|
|Updated On||:||February 23, 2018|
|PDF Download Mirror||:||310-230 Dump|
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Two companies communicate using Web services in a business transaction.Which mechanism is designed to ensure that business data CANNOT be renounced, or a transaction denied, by either one?
integrity provided by public key certificates and digital signatures
confidentiality provided by asymmetric or symmetric cryptography
identity management provided by private keys and certificate authorities
non-repudiation provided though public key cryptography by digital signing
A Web service developer notes the vast array of Web services specifications has changed since the last large project. The developer begins research for a new project. The clients are banks and exchanges with detailed security needs. The new service must be able to describe its security needs, help other systems determine algorithms, exchange identities and integrate well with SOAP.Which two does the research uncover? (Choose two.)
SOAP Message Security builds on a base expressed in WS-SecurityPolicy.
WS-Trust describes security enhancements for SOAP anonymous key exchange. C. WS-SecurityPolicy can express what needs to be protected via a WSDL.
WS-SecurityPolicy is a next generation specification that can replace WS-Security in many cases.
WS-SecurityPolicy can use an advanced set of reference types when implemented in the context of JAX-WS.
Which two statements are true about Web services specifications (WS-*)? (Choose two.)
WS-Policy is a foundation for describing any WS-* features that are discoverable.
WS-SecureConversation most commonly uses WS-Message with HTTPS.
HTTP is to HTTPS as WS-Policy is to WS-SecurityPolicy.
WS-Policy is to WS-* as WSDL is to standard SOAP messaging.
A customer who desires to use a secure Web service sends a user name token to a Security Token Service (STS). The STS authenticates the client and returns a digitally signed SAML token. The customer system presents the SAML token to the service and is accepted as an authorized user for a request.Which statement is true?
This is an example of the Web Services specifications for SAML 1.0.
This authentication is handled by WS-Trust.
This scenario is the base case of WS-Security.
This is WS-ReliableMessaging viewed from the perspective of the secure system.
This is a case of the WS-SecurityPolicy being attached to a WSDL.
A developer is creating a session bean EJB endpoint for a new application.Which three statements are true about the service? (Choose three.)
It needs to be packaged as a WAR file.
It needs to be packaged as a JAR file.
It needs to be packaged as a .lib file.
The class must not be final or abstract.
It must implement the javax.ejb.SessionBean or javax.ejb.EntityBean interface.
It can be declared an EJB via the @Stateless annotation.
A developer is migrating all of a company's JAX-RPC services into JAX-WS.Which two statements are true? (Choose two.)
JAX-WS is essentially JAX-RPC 2.0, and is backwards compatible with JAX-RPC services.
Deployment descriptors from JAX-RPC can be used under JAX-WS.
Deployment descriptors are no longer required.
javax.xml.ws.Service is a server-side API.
javax.xml.ws.Service is a client API.
sun-jaxws.xml is the standard Java EE deployment descriptor.
Which three statements are true about JAXP APIs? (Choose three.)
They are a part of Java SE 6.
They allow Java developers to access and process XML data without having to know XML or XML processing.
They do NOT support validation of XML documents against schemas.
It supports the Streaming API for XML. E. It requires the Streaming API for XML.
F. They provide developers with a vendor and parser-implementation independent API to process XML.
Which code fragment correctly opens an input stream for processing using StAX?
StAxInputFactory foo = StAxInputFactory.newInstance(); FileInputStream bar = newFileInputStream("readit/readit.xml"); StAxStreamReader reader = foo.createStAxStreamReader(bar);
XMLInputFactory foo = XMLInputFactory.newInstance(); FileInputStream bar = newFileInputStream("readit/readit.xml"); XMLStreamReader reader = foo.createXMLStreamReader(bar);
StreamInputFactory foo = StreamInputFactory.newInstance(); InputStream bar = newInputStream("readit/readit.xml"); StreamReader reader = foo.createStreamReader(bar);
StAxInputFactory foo = StAxInputFactory.newInstance(); InputStream bar = newInputStream("readit/readit.xml"); XMLStreamReader reader = foo.createXMLStreamReader(bar);
Which three statements about parsers are true? (Choose three.)
SAX and StAX are bi-directional.
DOM and StAX are bi-directional.
StAX is a push API, whereas SAX is pull.
SAX is a push API, whereas StAX is pull.
SAX and StAX are read-only.
SAX and DOM can write XML documents.
StAX and DOM can write XML documents.
A company is creating an XML schema that describes various training materials available for purchase by students.Given the namespace aliases and schema that appear in the WSDL file for a document-style Web service: xmlns:xsd=" schemas.xmlsoap.org/wsdl/" xmlns:book=" www.sun.com/books"
<xsd:element name="author" type="xsd:string"/>
<xsd:element name="title" type="xsd:string"/>
Which is a valid message element for this WSDL file?
<part name="bookInfo" type="xsd:bookInfo"/>
<part name="bookInfo" type="book:bookInfo"/>
<part name="bookInfo" element="xsd:bookInfo"/>
<part name="bookInfo" element="book:bookInfo"/>
<part element="bookInfo" type="xsd:bookInfo"/>
<part element="bookInfo" type="book:bookInfo"/>
Since before history began, we have tried to understand our world and our place in it. To the earliest hunter-gatherer tribes, this meant little more than knowing the tribe's territory. But as people began to settle and trade, knowing the wider world became more important, and people became interested in the actual size of it. Aristarchus of Samos (">310-230 BC) made the earliest surviving measurements of the distance between objects in space. By carefully measuring the apparent size of the Sun and Moon and carefully observing the terminator of the Moon when half full, he concluded that the Sun was 18-20 times farther away than the Moon. The actual value is 400, but he was on the right track; he just didn't have precise enough measurements.
A diagram from Aristarchus' work, "On Size and Distances," describing how to work out the relative distances.
Meanwhile, Eratosthenes of Cyrene (276-195 BC) was working on the size of the Earth. He came upon a letter stating that at noon in Syene (modern-day Aswan) on the summer solstice, one could look down a well and see all the way to the bottom because the Sun was precisely overhead. Eratosthenes already knew the distance between Alexandria and Syene, so all he had to do was observe the angle of the Sun on the summer solstice there and then do a little math. Assuming a spherical Earth, he computed the circumference to be 252,000 stadia, which works out to 39,690 km -- which is less than a 2% error compared to the real value. A directly measured size now existed for the world. But what of the heavens? The work of Aristarchus wasn't accurate enough. After figuring out how to reliably predict eclipses, Hipparchus (190-120 BC) used them to get a better estimate of the ratio of distance between Moon and Sun. He concluded that the Moon was 60.5 Earth radii away, and the Sun was 2,550 Earth radii away. His lunar distance was pretty accurate -- that works out to 385,445 km to the Moon, which is pretty close to the actual distance, an average of 384,400 km -- but for the Sun it worked out to 16 million km, about 136 million km short of the actual distance.
Above left: A dioptra, a predecessor to both the astrolabe and the theodolite, of a type similar to the one Hipparchus used to make his measurements.
When Ptolemy (AD 90-168) came along, the Universe shrank for a while.
Using the epicycles he assumed must exist within his geocentric universe, he estimated the distance to the Sun to be 1,210 Earth radii, and the distance to the fixed stars to be 20,000 Earth radii away; using modern values for the Earth's average radius, that gives us 7,708,910 km to the Sun and 127,420,000 km to the fixed stars. Both of those are woefully small (Ptolemy's universe would fit within the orbit of Earth), but they get even smaller if we use his smaller estimate for the Earth's circumference -- he estimated the Earth to be about 16 the size it actually is. (And therein hangs a tale, for Christopher Columbus would try to use Ptolemy's figure when plotting his journey west to the Orient, rather than the more accurate ones that had been developed in Persia since then.)
Ptolemy's world; at the time, the best map that existed of the known world.
By the end of the 16th Century, the size of the Earth was pretty well defined, but the size of the Universe remained challenging. Johannes Kepler solved the puzzle of orbital motion and calculated the ratio of the distance between Sun and various planets, enabling accurate predictions of transits. In 1639, Jeremiah Horrocks made the first known observation of a transit of Venus. He estimated the distance between Earth and the Sun at 95.6 million km, the most accurate estimate to date (and about 23 the actual distance). In 1676, Edmund Halley attempted to measure solar parallax during a transit of Mercury, but was unsatisfied with the only other observation made. He proposed that further observations be made during the next transit of Venus, in 1761. Unfortunately, he did not live that long.
Jeremiah Horrocks, observing the transit of Venus by the telescopic projection method.
In 1761, acting on the recommendations of the late Edmund Halley, scientific expeditions set out to observe the Transit of Venus from as many places as possible. More expeditions set out in 1769 for the second transit of the pair, including a famous journey by Captain James Cook to Tahiti, and in 1771, Jerome Lalande used the data to calculate the Sun's average distance as 153 million km, far larger than previously estimated, and the first time the measurement was close to right. Further transits in 1874 and 1882 refined the distance to 149.59 million km. In the 20th Century, it has been refined further using radio telemetry and radar observations of the inner planets, but it has not strayed much from that value. The size of the solar system was now known.
Above left: Sketch depicting the transit circumstances, as reported by James Ferguson, a Scottish self-taught scientist and inventor who participated in the transit observations.
But the universe is bigger than the solar system. In the 1780s, William Herschel mapped the visible stars in an effort to find binary stars. He found quite a few, but he also worked out that the solar system was actually moving through space, and that the Milky Way was disk shaped. The galaxy, which was at that time synonymous with Universe, was eventually estimated to be about 30,000 light years across -- an inconceivably large distance, but still far too small.
Hershel's map of the galaxy could not tell how far away any of the stars were; stars get dimmer as they move away, but you can only use this to calculate their distance if you know how bright they are to begin with, and how can you know that? In 1908, Henrietta Leavitt found the answer: she noticed that Cepheid variable stars had a direct relationship between their luminosity and the period of their variation, allowing astronomers to deduce exactly how bright they are to start with. Harlow Shapley immediately applied this discovery and found three amazing things when he mapped all the visible Cepheids: the Sun is actually nowhere near the center of the galaxy, the center of the galaxy is obscured by vast amounts of dust, and the galaxy is at least ten times larger than anyone had ever suspected -- so vast that it would take light 300,000 years to cross it. (Shapley was overestimating a bit; it's actually more like 100,000 light years or so.)
Above left: Henrietta Leavitt, one of the few women in astronomy and the only one on this list; she got little recognition for her discovery at the time.
In 1924, Edwin Hubble produced the next major revolution. Using the new 100-inch telescope at Mount Wilson Observatory, he located Cepheids in the Andromeda Nebula, a spiral nebula in which no stars had previously been resolved. He calculated these Cepheids were 1.2 million light years away, putting them far beyond Shapley's wildest estimate for the size of the galaxy. Therefore, Andromeda was not a part of our galaxy at all; it was an entirely separate "island universe," and most likely the same was true of other spiral nebulae. This meant the Universe was very likely far larger than anyone could hope to measure. It could even be infinite.
At left: The 100-inch telescope at Mount Wilson Observatory, where Hubble did his work. It was the world's largest telescope until 1948.
And then Hubble found something even more astonishing. In 1929, Hubble compared the spectra of near and far galaxies, based on distances already known by observations of Cepheid variables. The spectra of more distant ones were consistently redder, and for nearly all of them, there was a linear relationship between redshift and distance. Due to the Doppler Effect, this meant they were receding. He wasn't sure what to make of this observation at the time, but in 1930, Georges Lemaître pointed out a possible solution: he suggested that the universe was expanding, carrying galaxies along with it, and that at one time it had all be compacted down impossibly tight. Hubble went with this and calibrated the apparent expansion against the distance to known standard candles, calculating the age of the most distant objects to be 1.8 billion light years.
At left: Georges Lemaître, who happened to also be a Catholic priest. He died in 1966, shortly after learning about the Cosmic Microwave Background radiation, which further reinforced his theory of the Big Bang.
This was much too small, and in 1952, Walter Baade figured out why: there are actually two kinds of Cepheids, and Hubble had been observing the ones that Leavitt had not baselined. After characterizing this new population of Cepheids, he recalculated from Hubble's observations and brought the Universe's minimum age up to 3.6 billion years. In 1958, Allan Sandage improved it more, to an estimated 5.5 billion years.
Astronomers started to ratchet up their observations of ever more distant objects. In 1998, studies of very distant Type 1A supernovae revealed a new surprise: not only is the universe expanding, but the rate of the expansion is increasing. Today, the Universe is usually estimated to be 13.7 billion years old -- or, more accurately, the most distant things we can observe appear to be that far away. The catch, of course, is that we're observing them in the past. They're actually further away now -- assuming, of course, that they even still exist. A lot can happen in 13.75 billion years. And now that we know the universe's expansion is accelerating, they are even farther away by now. The current estimate for the actual size of the observable universe is 93 billion light-years in diameter, a tremendous size that the human brain cannot begin to fathom on its own, vastly overwhelming the tiny universe of the ancient Greeks.
NASA artist's concept of the progenitor of a Type 1a supernova -- a neutron star stealing matter from a supergiant companion until eventually enough matter is collected to trigger a supernova.
The understanding of the size of the Universe has gone from being impressed by the distance to the Sun, to the size of the solar system, to the vastness of the galaxy, to the staggering distance to neighboring galaxies, to the mindbendingly complicated distances to things that we can only see as they were an impossibly long period of time ago. What will we discover as we measure the Universe tomorrow?
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Stories and extras: The entire section is a story, of how Aristarchus was (probably) led to his heliocentric theory.
Start the class by discussing what is a scientific discovery? Historians of science often argue about "who was first"--but what does it mean?
The Greek philosopher Democritus argued that all matter consisted of "atoms", a Greek word meaning "undividable. " He pointed out that a collection of very small particles--e.g. sand or poppyseeds--can be poured like a continuous fluid, so maybe water, too, consists of many tiny "atoms" of water. Does this qualify as a prediction of the atomic theory of matter?
In the early 1700s, the Irish writer Jonathan Swift wrote "Gulliver's Travels, " a satire of the politics and society of his times, in the form of voyages to distant fantastic countries (today it might have been called "science fiction.") In his third voyage he visits an island floating in the air, which is ruled by an academy of scientists (a spoof on the "Royal Society", an association of Britain's top scientists which still exists). He reports that by using improved telescopes, members of the academy had discovered that two small moons orbiting Mars at a close range.
A century and a half later, an astronomer discovered that Mars indeed had two such satellites, quite similar to what Swift had described. Does it mean that Swift had predicted those moons?
By our standards, these are just lucky guesses. To qualify as a prediction, a claim needs not only to be stated, but also justified, it needs a logical reason. In this lesson we discuss a proposal by Aristarchus, around 270 BC, that the Earth went around the Sun, rather than vice versa. It took 1800 years before this claim was made again, and another century before it was generally accepted.
However, this was not guesswork. Aristarchus, who also estimated the distance of the Moon, had a serious reason for his claim: the Sun, he showed, was much larger than the Earth, making it likely that the Sun, not Earth, was at the center.
Let us go through his arguments.
Give the material of section 9a of "Stargazers. Start by assuming that the shadow of the Earth had the same width as the Earth, and that the Earth had twice the width of the Moon. Later, if time and the level of the class allow it, the teacher may continue with a discussion of the actual shadow of the Earth, which is cone-shaped [Section 9b].)
Guiding questions and additional tidbits
-- Who was Aristarchus of Samos?
Aristarchus was an early Greek Astronomer, living between ">310-230 BC. Samos is a Greek island. [The teacher may point out that dates BC seem to proceed in the opposite direction to what we are used to--e.g. born -310, died -230.]
-- What did Aristarchus establish about the Moon?
He was the first to estimate its distance, about 60 Earth radii, 380,000 km or 240,000 miles.
-- What was the revolutionary proposal Aristarchus made about the Sun?
Two correct answers exist here: That the Sun was much bigger than the Earth That the Earth went around the Sun, not vice versa
-- On what observation did Aristarchus base his claims about the Earth?
Aristarchus tried to see where the Moon was, relative to the Sun, when it appeared to be exactly half-full.
-- What is the Moon's relation to the Earth and Sun, when it is half-full?
When the Moon is half full, the angle Sun-Moon-Earth (corner at the Moon) must be exactly 90°.
-- What does the Sun-Earth-Moon angle (corner at Earth) at such times tell about the Sun's distance?You can measure that angle, for instance, if the half-moon is visible in the daytime, as often happens. It allows one to construct the full Earth-Sun-Moon triangle.
[Draw diagram of the triangle on the blackboard.]If the Sun is very, very far away, the Sun-Earth-Moon angle is also be very close to 90°. In fact, that is the case: the amount by which that angle differs from 90° is too small to be reliably measured. The only thing one could conclude from it is that the Sun was very distant.
As it happened, the measurement made by Aristarchus was inaccurate. It is hard to tell when the Moon is exactly half full! He believed the Sun-Earth-Moon angle was 87°, short of 90° by 3°. The Earth-Sun-Moon triangle then has a sharp corner of 3 degrees, and its proportions were such, that the Sun was about 20 times further than the Moon.
-- If the Sun is 20 times more distant than the Moon, what does it say about the Sun's size?
Since the Sun's size in the sky is about the same as that of the Moon, it must also be 20 times bigger in diameter.
-- What did Aristarchus believe about the relative size of the Earth, compared to the Moon and Sun?
From observation of the Earth's shadow during an eclipse of the Moon, he concluded that the Moon had half the diameter of the Earth (Actually, it is less than 13 that diameter). By his estimate, therefore, the Sun's diameter was 10 times that of Earth (in reality, it is more than 100 times larger).
-- How did Aristarchus view the Sun-Earth system?
He guessed the Earth went around the Sun--probably, because the Sun was very much bgger. Others at the time claimed the Sun went around the Earth. After all, astronomers (including Aristarchus) had shown that the Moon went around Earth, so why not the Sun, too?
--Did other astronomers agree with Aristarchus?
No, they continued to disagree.
--What was their argument? (teacher may help fill details) Aristarchus determined that the Sun's distance was 20 times that of the Moon, or about 1200 Earth radii. If the Earth went in a circle around the Sun, at that distance, its positions half a year apart would be a full diameter apart, 2400 Earth radii, about 10 million miles.
To the ancient Greeks, that was an enormous distance. The stars were clearly more distant than the Sun, but it was hard to imagine that from two positions so far apart, there should not exist some difference in the apparent positions of stars in the constellations of the sky. Yet none could be seen.
That was the argument of Hipparchus and Ptolemy, leading Greek astronomers. Their view prevailed for about 1800 years.
--Was there a flaw in that argument? The argument was valid. It would have been even stronger, had it been realized that the diameter of the Earth's orbit was about 20 times larger than in the estimate by Aristarchus.
However, the stars were much more distant than anyone had held possible. Any shift in their positions was too small to be observed by the eye.
(Such a shift was first observed in 1838, using some of the best telescopes of the era, and even then, only for the stars nearest to us. See the section on parallax.)
(9b) The Earth's Shadow [optional]
This is a detail that may be skipped in the classroom, only perhaps assigned as a project to advanced students. One should start it by making clear that the Sun covers a 0.5° disk of the sky. If we select some point P on Earth and trace all the sun's rays that reach it from that disk, those rays form a narrow cone. That cone contains all the directions in which the Sun's rays arrive at the Earth's vicinity, and the full shadow of the Earth only extends over the region where all those directions are blocked by the Earth. It will only extend a certain distance behind Earth. At greater distances, the Earth will cover less than 0.5° of the sky and will appear smaller than the Sun. At those distances, one can never be in the full shadow of the Earth.
The Lagrangian L2 point, 236 Earth radii from Earth on the side opposite from the Sun, is a good location for a distant observatory. Being more distant from the Sun, it should orbit it more slowly than Earth, but because of the added pull of the Earth, it can move a little faster and thus keep up with Earth (more about Lagrangian points is in the last part of "From Stargazers to Starships").
NASA plans to place its next large infrared telescope at this position. It would be just outside the shadow cone. The Earth would be a little too small to cover all the Sun, which would shine in a bright ring around the dark Earth. In full shadow, the telescope would get very cold--a desirable feature for detecting infra-red light. As it is, it will need a light shield to protect it from the remaining sunlight.
How do astronomers calculate the distance of the Sun from the Earth, or the actual size of the Sun, or the speed of travel of Earth in its orbit around the Sun? Clearly, from an answer to one of these questions one can find out the answers to the others. But how do we find the first answer?
Short version: What we actually measure is the distance from the Earth to some other body, such as Venus. Then we use what we know about the relations between interplanetary distances to scale that to the Earth-Sun distance. Since 1961, we have been able to use radar to measure interplanetary distances - we transmit a radar signal at another planet (or moon or asteroid) and measure how long it takes for the radar echo to return. Before radar, astronomers had to rely on other (less direct) geometric methods.
In more detail:
The first step in measuring the distance between the Earth and the Sun is to find the relative distances between Earth and other planets. (For instance, what is the ratio of the Jupiter-Sun distance to the Earth-Sun distance?) So, let us say that the distance between Earth and the Sun is "a". Now, consider the orbit of Venus. To a first approximation, the orbits of Earth and Venus are perfect circles around the Sun, and the orbits are in the same plane.
Take a look at the diagram below (not to scale). From the representation of the orbit of Venus, it is clear that there are two places where the Sun-Venus-Earth angle is 90 degrees. At these points, the line joining Earth and Venus will be a tangent to the orbit of Venus. These two points indicate the greatest elongation of Venus and are the farthest from the Sun that Venus can appear in the sky. (More formally, these are the two points at which the angular separation between Venus and the Sun, as seen from Earth, reaches its maximum possible value.)
Another way to understand this is to look at the motion of Venus in the sky relative to the Sun: as Venus orbits the Sun, it gets further away from the Sun in the sky, reaches a maximum apparent separation from the Sun (corresponding to the greatest elongation), and then starts going towards the Sun again. This, by the way, is the reason why Venus is never visible in the evening sky for more than about three hours after sunset or in the morning sky more than three hours before sunrise.
Now, by making a series of observations of Venus in the sky, one can determine the point of greatest elongation. One can also measure the angle between the Sun and Venus in the sky at the point of greatest elongation. In the diagram, this angle will be the Sun-Earth-Venus angle marked as "e" in the right angled triangle. Now, using trigonometry, one can determine the distance between Earth and Venus in terms of the Earth-Sun distance:
(distance between Earth and Venus) = a × cos(e)
Similarly, with a little more trigonometry:
(distance between Venus and the Sun) = a × sin(e)
The greatest elongation of Venus is about 46 degrees, so by this reasoning, the Sun-Venus distance is about 72% of the Sun-Earth distance. Similar observations and calculations yield the relative distance between the Sun and Mercury. (However, Mars and the outer planets are more complicated.)
Historically, the first known person to use geometry to estimate the Earth-Sun distance was Aristarchus (c. ">310-230 BC), in ancient Greece. He measured the angular separation of the Sun and the Moon when the Moon was half-illuminated to derive the distance between Earth and Sun in terms of the distance between the Earth and the Moon. His reasoning was correct, but his measurements were not. Aristarchus calculated that the Sun is about nineteen times farther than the Moon; it is actually about 390 times farther than the Moon.
Another ancient Greek astronomer, Eratosthenes (276-194 BC), estimated the distance between Earth and Sun to be either 4,080,000 stadia or 804,000,000 stadia. There is disagreement regarding the correct translation of Eratosthenes' value, and further disagreement over which length of a stadium was used by Eratosthenes. Various sources estimate that the length of a stadium (also called a stadion or stade), converted to modern units, is between 157 meters and 209 meters. Then 4,080,000 stades is less than 1% of the actual Earth-Sun distance, no matter which definition of a stade one chooses. However, 804,000,000 stadia is between 126 million and 168 million kilometers - a range which includes the actual Earth-Sun distance of (approximately) 150 million kilometers. So Eratosthenes may have found a fairly accurate value for the Earth-Sun distance (possibly with some luck), but we can't say for sure.
The first rigorous and accurate scientific measurement of the Earth-Sun distance was made by Cassini in 1672 by parallax measurements of Mars. He and another astronomer observed Mars from two places simultaneously. A century later, a series of observations of transits of Venus provided an even better estimate.
Since 1961, the distance to Venus can be determined directly, by radar measurements, where a series of radio waves is transmitted from Earth and is received after it bounces off Venus and comes back to Earth. By measuring the time taken for the radar echo to come back, the distance can be calculated, since radio waves travel at the speed of light. Once this Earth-Venus distance is known, the distance between Earth and the Sun can be calculated.
As you have indicated, once the distance between Earth and Sun is known, one can calculate all the other parameters. We know that the Sun, as seen from Earth, has an angular diameter of about 0.5 degrees. Again, using trigonometry, the radius or diameter of the Sun can be calculated from the distance between Earth and Sun, a, as 2×Rsun = tan(0.5 degrees) × a. Also, since we know the time taken by the Earth to go once around the Sun (P = 1 year), and the distance traveled by the Earth in this process (approximately 2πa, since Earth's orbit is nearly circular), we can calculate the average orbital speed of Earth as v = (2πa)P.
Anyway, the relevant numbers are:
Earth-Sun distance, a = roughly 150 million km, defined as one Astronomical Unit (AU)Radius of the Sun, Rsun = roughly 700,000 kmOrbital speed of Earth, v = roughly 30 kms
And here are some links with answers to similar questions on other "Ask an Astronomer" sites:
This page was last updated by Sean Marshall on January 30, 2016.
In this day of supercomputers, robotic probes and sophisticated laboratories, it is easy to forget how clever and resourceful scientists of centuries gone by had to be. In their quest to understand nature, they had to rely on direct observations and great ingenuity to find answers.
Let's consider one such journey.
Nicolaus Copernicus (1473-1543) is often credited with being the first to propose a heliocentric ("sun-centered") cosmology, although the idea dates back to the Greek astronomer Aristarchus (">310-230 BCE). In developing his model, Copernicus was able to estimate the scale of the known solar system in terms of the Earth-sun distance, called an Astronomical Unit, or AU.
For example, it could be shown that Mars orbited at about 1.6 AU and Jupiter at 5.2 AU.
But neither Copernicus nor generations of astronomers that followed could determine the missing piece of this trigonometric puzzle — the numerical definition of an AU.
In 1663, mathematician James Gregory realized a solution to this mystery: By measuring the parallax angle during an event called the transit of Venus, the math could be worked to determine the value of an Astronomical Unit.
A very clever proposal, to be sure.Unfortunately, the next transit of Venus would not take place until 1761. When the year finally did arrive, many countries sent expeditions to make measurements of the Venus transit to confirm the Earth-sun distance.
(Now before I get a bunch of hate mail from historians, let me mention that in 1672 mathematician and astronomer Giovanni Cassini calculated the AU using other methods. He derived a value of 87 million miles, just short of its accepted value of 93 million miles.)
The term "transit" in astronomy refers to one celestial body moving across the face of another. So, in our discussion, the silhouette of Venus is moving across the disk of sun; in other words, Venus' orbital motion is carrying it directly between Earth and the sun.
Since Venus has an orbital inclination of 3.4 degrees with respect to the plane of Earth's orbit, it usually is above or below the disk of the sun.
Indeed, transits of Venus are rare and occur in a pattern that repeats every 243 years, with a pair that takes place only eight years apart separated by long gaps of 121.5 and 105.5 years. Only seven Venus transits have occurred since the invention of the telescope; there were none in the 20th century.
The last transit of Venus occurred on June 8, 2004. Now, the celestial waltz of worlds is bringing this extraordinary alignment together for the last time this century.
On June 5-6, we will have the opportunity to experience the transit of Venus for the last time in our lifetimes.
Unfortunately, even though the event lasts almost six hours, Jacksonville will only be able to experience a small part of it. The transit will begin at 6:04 p.m. local time on June 5, with the sun setting around 8:30.
The next transits of Venus will not happen until 2117 and 2125.
This is an opportunity to learn, discover and wonder. It is also a time to reflect upon the explorers who came before us who were able to use their intellect and senses to study nature in a more direct and honest way.
This spectacle is worthy of an audience; do not let this day pass without experiencing it in at least some small way.
In the words of 19th-century astronomer Sir Robert Ball, "Still, to have seen even a part of the transit of Venus is an event to remember for a lifetime. ..."