Today (March 10) at 4 hours UT (or GMT) comet C/2011 L4 (PANSTARRS) reached perihelion, or the point in its orbit when it is closest to the Sun. The time of perihelion passage corresponds with 9 pm on the night of March 9 for Tucson (MST -7 hours). At that time PANSTARRS was located at a distance of 0.302 AU from the Sun (28.0 million miles or 45.1 million km) which is ~3.3 times closer to the Sun than the Earth’s average distance and about 1 million km closer to the Sun than Mercury gets. On March 5 it also came as close to the Earth as it will get, a rather distant 1.097 AU (102.0 million miles or 164.1 million km). This is 10% further than the distance between the Earth and Sun.
As the comet rounds the Sun it is quickly heading north. After months of being only visible from south of the Equator, those of us up north will have the comet to ourselves starting this week. Already observers as far north as New Jersey (latitude 40°) have reported seeing the comet. Here in Tucson I tried to find the comet last week but it was still too deep in the bright twilight to see. Now that a recent bout of rain and clouds has moved through I will try again this evening.
The video below by Jay Lawson from Sparks, Nevada (latitude 39.5°) shows what the comet looks like in a small telescope or pair of binoculars (note the video is in black and white so it doesn’t show the true color of the comet and sky).
Another video from Brisbane, Australia back on Feb. 23 gives a good idea of what the comet will look like to naked eye observers. Please note that even though the comet is a very bright magnitude 1.5 and this does place it among some of the brighter comets of the past, it is not an easy object to see. If it were located high up in the night sky it would be unmistakable with a tail many degrees long to the naked eye. Such a well placed comet would even be visible under bright city lights. But Comet PANSTARRS is not located up high in a dark sky but rather close to the horizon against a bright twilight sky. By the time the sky is dark the comet will either have set or will be only a few degrees above the horizon.
Bob King has a great post about PANSTARRS on his blog Astro Bob. He highlights one of the problems of spotting a comet so close to the horizon, especially one in the western sky near the just set Sun. Many reports of PANSTARRS have actually been of distant aircraft contrails. His posting shows the different appearances of the comet versus plane contrails.
Bob also has a nice post with finder charts for locating the comet over the next few weeks.