What Russ said - are your referring to the masthead (steaming) light, which is white, on the front of the mast, and sweeps 112.5 degrees horizontally each way from the boat's centerline, same as the sidelights (probably a single bi-color unit on the bow) other than the color?
If it's the masthead (steaming) light, it should be on it's own switch, as you only use it when under engine power, not while sailing. It identifies the boat as under power to other boats, which makes the rules of the road a little different than when it's not lit, meaning you're under sail. The stern light is also 2-mile, white, but only sweeps 135 degrees, and is the only light that shows when observing from more than 22.5 degrees abaft the beam on either side (where the stern light takes over, and the side and masthead lights fade out).
And no, it's couldn't be used as an anchor light if it's the masthead light. Either put one on top of the mast, or use a portable (white, 2-mile or better range) all-round light that you can plug into a power source and hang from some rigging. I prefer mine lower than the top of the mast, as I usually only anchor in crowded areas, and on top of the mast wouldn't be seen by other boats weighing anchor and racing back to the river after the fireworks show. For instance.
