The MousePosition control displays geographic coordinates of the mouse pointer, as it is moved about the map.
You can use the prefix- or suffix-properties to provide more information about the displayed coordinates to the user:
var mousePositionCtrl = new OpenLayers.Control.MousePosition({
    prefix: '<a target="_blank" ' +
        'href="http://spatialreference.org/ref/epsg/4326/">' +
        'EPSG:4326</a> coordinates: '
    }
);| OpenLayers. | The MousePosition control displays geographic coordinates of the mouse pointer, as it is moved about the map. | 
| Properties | |
| autoActivate | {Boolean} Activate the control when it is added to a map. | 
| element | {DOMElement} | 
| prefix | {String} A string to be prepended to the current pointers coordinates when it is rendered. | 
| separator | {String} A string to be used to seperate the two coordinates from each other. | 
| suffix | {String} A string to be appended to the current pointers coordinates when it is rendered. | 
| numDigits | {Integer} The number of digits each coordinate shall have when being rendered, Defaults to 5. | 
| granularity | {Integer} | 
| emptyString | {String} Set this to some value to set when the mouse is outside the map. | 
| lastXy | {OpenLayers.Pixel} | 
| displayProjection | {OpenLayers.Projection} The projection in which the mouse position is displayed. | 
| Constructor | |
| OpenLayers. | |
| Functions | |
| destroy | |
| activate | |
| deactivate | |
| draw | {DOMElement} | 
| redraw | |
| reset | |
| formatOutput | Override to provide custom display output | 
{OpenLayers.Projection} The projection in which the mouse position is displayed.
formatOutput: function( lonLat ) 
Override to provide custom display output
| lonLat | {OpenLayers.LonLat} Location to display | 
destroy: function() 
activate: function() 
deactivate: function() 
{DOMElement}
draw: function() 
redraw: function( evt ) 
reset: function( evt ) 
Override to provide custom display output
formatOutput: function( lonLat )