(This is written for the world's mid-northern latitudes, including the U.S., Canada, Europe except the north, China, Korea, and Japan.)
Look due west around the middle of twilight on a nice clear evening from about March 12–18, and with a little luck you can spot a one-time-only visitor newly arrived from very deep space. But it's tiny and faint, and you should bring binoculars if you have them.
Skywatchers have been anticipating Comet PanSTARRS for nearly two years. It has just passed its closest by Earth and is now being lit its most brilliantly by the Sun. In the past couple weeks it decorated the twilight sky for folks in the Southern Hemisphere. Now people in the world's mid-northern latitudes have their turn.
"Our good views begin around March 12th and 13th, when the crescent Moon is there to point the way," says Alan MacRobert, a senior editor at Sky & Telescope magazine. "Before that the comet is too near the horizon. It will start fading later this week, so if the sky is clear, don't miss your chance." By March 20th it may be only half as bright as it was seven days before.
The best time to look is now roughly 40 or 50 minutes after your local sunset. Your window of viewing time comes when twilight fades enough for the comet to show through at least a bit, but before it sinks too low and sets.
On Tuesday March 12th, look for the very thin crescent Moon very low due west. The comet will be just to its left, by two or three finger-widths at arm's length.
On Wednesday March 13th, you'll see a less-thin crescent Moon higher up. Look below it by about the width of your fist at arm's length.
On Thursday the 14th, look two fists below the Moon and perhaps a little to the right.
After that, the comet will gradually move to the right from one evening to the next as it begins to fade.
Look for a tiny, slightly fuzzy "star" with a short, faint upward tail. Binoculars will give a much better view. And if you have a telescope, now's the time to bring it out!
See our PanSTARRS Updates page for the latest.
Have you seen Comet PanSTARRS? Let us know in the comments below — and share your photos with us in our photo gallery!
A Once-Only Visitor
The comet is known to astronomers as C/2011 L4 to distinguish it from others named PanSTARRS. The automated Pan-STARRS sky survey in Hawaii discovered it in June 2011 as a tiny, distant speck heading in from the far reaches of the solar system. Even though it's passing closest to us around now, it's still a distant 105 million miles or so from Earth, rather far even by comet standards. That's why it looks small.
"What we're seeing," says Sky & Telescope editor in chief Robert Naeye, "is mostly a plume of dust, lit by sunlight, that's spraying from the comet's tiny little nucleus. The nucleus is an icy frozen dirtball just a few miles across. As it comes near the Sun, its surface heats up and some of the ice evaporates, letting loose dust and debris."
Comet PanSTARRS's orbit is bringing it by the Sun for the first time, after it has spent billions of years in the cold of deep space. In the coming months it will fly back out again, never to return.
This is the first of two noteworthy comets expected in 2013. The other is Comet ISON, which may put on a brighter display in the dawn sky of early December; see more about it at SkyandTelescope.com/ison.