[jQuery] accessing offsetHeight

[jQuery] accessing offsetHeight


I'm new to Javascript and jQuery, so I'm sorry if this is basic
stuff. How do I access offsetHeight for an element? This is what I
have, but offsetHeight comes up undefined:
    var headerElem = $("#header");
    var headerHeight = headerElem.offsetHeight;
whereas this works:
var docHeader = document.getElementById("header");
headerHeight = docHeader.offsetHeight;
Thanks --MERC