Now is the best time to observe Comet Lulin. Two nights ago, Comet Lulin was easy to find as it passed close to Saturn, last night the comet came closest to Earth (0.411 AU or 38.2 million miles or 61.2 million km), and either tonight the comet will be at its brightest.
Don’t worry if you are not able to see the comet tonight. Just because the comet is at its brightest doesn’t mean it won’t be bright enough to see for the next few nights. And if you have a nice pair of binoculars or a telescope, the comet will be visible for the next 2 months.
Now why is the comet at its brightest tonight rather than last night when it was closer to Earth? First off, most observers will have a very hard time telling that the comet is brighter than last night. Chances are the comet will only be a few percent brighter, but there is a chance it could be brighter by 10-20%. Why? It all is related to how the dust particles in the atmosphere (or coma) and the tail of the comet scatter light.
Tonight, the comet will be located directly opposite in the sky from the Sun. In fact, the angle between the observer (on Earth), the comet and the Sun (Earth-comet-sun angle or phase angle) will only be 0.1 degrees. At such small phase angles, objects often appear much brighter than at larger phase angles. The reason is relatively simple, most of the time when you observe something, whether a comet, the Moon or blades of grass, you are looking at the object from the side a bit. So some fraction of the object is in shadow. When you are looking at the object with the Sun directly behind your line-of-sight, the shadows are no longer visible because they are behind the object. In effect, you are only seeing the sunlight side of the object.
A great explanation of the opposition effect (with pictures) can be found at the Atmospheric Optics site.This is also a great site for learning about a whole bunch of neat things that can be seen in the atmosphere.
More on Comet Lulin can be found here at the “In The Sky This Month – February 2009” post.
A higher resolution finder chart for observers with binoculars or telescopes can be found at Heavens Above.
A great series of images can be found at Spaceweather.com.