WASHINGTON (AP) — If you haven't seen the mysteriously large comet prominent in the sky in recent weeks, better look soon, astronomers say.

The erupting body and its expanding cosmic dust cloud will soon be overshadowed by a commonplace full moon.

Comet 17P/Holmes, once so faint that it was lost by astronomers for half a century, began shooting out gas and dust in such volume three weeks ago that it can be seen with the naked eye. It's increased in brightness by as much as 1 million times.

But the comet's unusual and not quite understood outburst, which happened twice in 1892 but not since, is decreasing ever so slightly. And the moon is getting fuller and brighter, making the comet harder to see, said Johns Hopkins University planetary astronomer Hal Weaver