For the past few weeks, the brightest comet for nearly 6 years has been visible in the western sky after sunset.
On the nights of April 4th and 5th, it passes through the constellation Andromeda, appearing, by chance, in the sky close to a giant spiral galaxy, the Andromeda Nebula, similar to our own Milky Way Galaxy.
Should the skies be clear, we will have a chance of seeing both in the same binocular field of view.
Under ideal conditions, both the comet, called Ikeya-Zhang after its Japanese and Chinese co-discoverers, and the Andromeda Galaxy would be visible to the unaided eye but, as they are low in the sky, binoculars will be needed.
The best time to see them is about 8:30 to 9pm as the Sun's light is finally fading from the western sky.
Ian Morison, an astronomer at the Jodrell Bank Observatory, described how to find the comet and galaxy:
"First find the planet Mars. This will be almost due west about 15 degrees above the horizon and, reddish in color, is the brightest object in the low western sky. (An outstretched hand at arms length is about 15 degrees.)
"Now sweep round to the north-west (45 degrees or three hand spans), keeping about the same height above the horizon. You should find two fuzzy looking objects close in the sky. The upper is the Comet and you should see its tail arcing up and to the right -- away from the Sun.
"The bright central region is called the coma -- a great cloud of gas evaporated off the comet's nucleus as it passed near to the Sun.
A little lower in the sky, you should see a second smudge of light. This is light from the bright nucleus at the center of the Andromeda Galaxy. It lies over 2 million light years away -- that is, it has taken its light over 2 million years to reach us. It is the most distant object in the Universe that we can see with our unaided eyes or small binoculars.
"In contrast, the comet is a member of our solar system and its light only takes a few minutes to reach us. It is thus pure coincidence that they appear in the same direction."
There is a link between comets and the other name of the Andromeda Galaxy, M31. The Andromeda Galaxy is the 31st entry in a catalogue of "nebulous" objects published by the French astronomer Charles Messier in 1774 -- hence M31.
Messier was one of the great cometary discoverers of his day and found many fuzzy objects that initially appeared to be comets but remained in fixed locations in the sky -- as opposed to the relatively rapid motion of comets. In order for these not to confuse other astronomers, he produced a catalogue, the Messier Catalogue, which finally contained over 100 objects. We now know that many of these objects are galaxies like the Andromeda Nebula.
The comet Ikeya-Zhang (pronounced ee-KAY-uh JONG) was discovered on February 1st by the Japanese astronomer Kaoru Ikeya, also famous for the discovery of the bright comet Ikeya-Seki in 1965, and the Chinese astronomer Daqing Zhang.
It passed closest to the Sun on March 18th at a distance of 47 million miles and is now moving back out into deep space.
It will pass within 38 million miles of the Earth on April 30th. The comet has a period of 341 years and is believed to be the same comet as that seen in 1661. It might then have been seen by the young boy, Edmond Halley, whose name is given to the most famous periodic comet, Halley`s comet, last seen in 1985 and which returns every 76 years. At 341 years, Ikeya-Zhang has the longest period of any periodic comet yet discovered.
M31 is a spiral galaxy very similar in size to our own Milky Way galaxy. Along with a third spiral galaxy, M33, about half their size, they are the three major galaxies in our own "Local Group" of galaxies.
This contains a total of about 30 galaxies spread throughout a volume of space about 10 million light years across. The other galaxies are all relatively small and include the Large and Small Magellanic Clouds, companions to our Milky Way. M31 is moving towards us through space and in about 5,000 million years, M31 and our own galaxy may well merge into one.
Comet Ikeya-Zhang (1023k jpeg image)