Why does the xmlns attribute affect XPath Node searches?

Question

The following code works fine. See the XML file below.

XPathDocument xPathDoc = new XPathDocument(@"C:\Authors.xml");
XPathNavigator navigator = xPathDoc.CreateNavigator();
XPathNodeIterator iterator = navigator.Select("/Contacts/Author/FirstName");
iterator.MoveNext();
string firstName = iterator.Current.InnerXml;
Console.WriteLine(firstName);

The value of "firstName" returns "Joe", which is ideal. However, when I add this attribute xmlns = "http://www.w3.org/1999/xhtml" to the '' tag so that it looks like this:

<Author xmlns="http://www.w3.org/1999/xhtml">

then the code does not return the correct value ('Joe') Why then does the xmlns = "http://www.w3.org/1999/xhtml" attribute affect the code above and what am I missing to return the correct value?

Any help would be greatly appreciated.

Here is the xml file:

<?xml version="1.0" encoding="UTF-8" ?> 
<Contacts>
<Author>
<FirstName>Joe</FirstName>
</Author>
<Teacher>
<FirstName>Larry</FirstName>
</Teacher>

<Painter>
<FirstName>Mary</FirstName>
</Painter>
</Contacts>

Answer 1

xmlns - , xml. , xml , xml. , .

Answer 2

XPath "", "" " " . , Author ( -, ) , XPath.

, XmlNamespaceManager, XPath. , .

, LINQ to XML? XML . LINQ to XML, .

EDIT: LINQ to XML:

XDocument doc = XDocument.Load("authors.xml");
XNamespace ns = "http://www.w3.org/1999/xhtml";
var query = doc.Root
               .Elements(ns + "Author")
               .Elements(ns + "FirstName");
foreach (var element in query)
{
    Console.WriteLine((string) element);
}