Now that the bright Moon has left the evening sky, it's agood time to turn our attention to one of the most amazing sky objects which ispassing almost directly over our heads this week between 7:30 and 8:00 p.m.
This object was known as the "little cloud" to thePersian astronomer Abd-al-Rahman Al-Sufi, who described and depicted it in his Bookof Fixed Stars in 964 A.D. But it may have been commonly known to Persianastronomers at Isfahan as far back as 905 AD, or even earlier. An expert onstar nomenclature, Richard Hinckley Allen, once reported that it also appearedon a Dutch star map from the year 1500.
To see this "little cloud" requires good eyesightand a dark and crystal-clear night with no street or house lighting nearby.With the unaided eye it appears as nothing more than an indefinite, mysteriousglow: a diffuse elongated smear perhaps two or three times the apparent widthof the Moon.
To find it, locate the Great Square of Pegasus. Then, focusbinoculars on the bright star Alpheratz, which is at the upper left corner ofthe Square. Then move straight across to the east (left) and get the starMirach in Andromeda) in your field of view. Then move slowly up to a fairlybright star above Mirach and continue to run up in the same direction until youfind the "little cloud." That will be your stopping place.
Today we know it as the great AndromedaGalaxy.
Galileo's rival, Simon Marius, is usually credited with thefirst telescopic observation of this object in December of 1612. He describedthe nebula as an indefinite glow "like a candle shining through the hornwindow of a lanthorn (lantern)."
Even today, binoculars and telescopes reveal this "cloud" aslittle more than a smooth oval blur, which gradually brightens in the center toa star-like nucleus. While it will certainly look larger and brighter than withyour eyes alone, there is little to suggest the grandeur of this object as itis often shown in long exposure observatory photographs. It's oval because fromour vantage point we're viewing it not far from edgewise, but in fact, it's anearly circular, flat spiral assemblage of star clouds.
The light from that "little cloud" is actually the totalaccumulation of light from more than 400 billion stars. It is listed as Messier("M") 31, in Charles Messier?s famous catalogue: hazy objectsresembling comets, but later proved to be galaxies, nebulae and star clusters.
Here is the most distant object that can be seen with theunaided eye.
M31 has been estimated to be nearly 200,000 light-years indiameter or one and a half times as wide as our own Milky Way galaxy. Itsbright nucleus is the hazy patch that is visible to the unaided eye. Like ourown galaxy, M31 has several attendant satellite galaxies. Two of these: M32 andM110 can be picked out with low magnification in a small-to-medium sizedtelescope, in the same field of view as M31. There are yet two other smallercompanions (NGC 147 and 185) which are much fainter and placed much fartheraway, close to the border of nearby Cassiopeia.
As you look at the Andromeda Galaxy tonight you'll be doingsomething that no one else in the world except a stargazer can do; you willactually be lookingback into the distant past.
There is a very good reason that this patch of light appearsso very faint to the naked eye. When you see it tonight, consider that thislight has been traveling some 2.5 million years to reach you, traveling allthat time at the tremendous velocity of 671 million mph.
The light you are seeing is around 25,000 centuries old andbegan its journey around the time of the dawn of human consciousness. The lightyou are now getting is at least 480 times older than thePyramids; the distance it has traveled is so inconceivable that even towrite the number of miles is all but meaningless.
When it began its nearly 15-quintillion (15, followed byeighteen zeros!)-mile journey earthward, mastodons and saber-toothedtigers roamed over much of pre-ice-age North America and prehistoric man wasstruggling for existence in what is now the Olduvai Gorge of East Africa.
For a very long time, M31 was popularly referred to as theAndromeda "Nebula." But although big reflecting telescopes such asLord Rosse's 72-inch at Birr Castle in Ireland were in operation during themid-19th century, it was not until astronomer Edwin P. Hubble finally resolvedM31 into individual stars with the 100-inch telescope at Mount WilsonObservatory in 1923.
Yet there were those who many decades earlier suspected thatM31 was much more than just a luminous cloud. Read this prophetic comment outof W.H. Smyth's A Cycle of Celestial Objects written back in 1844:
"Sir John Herschel . . . concludes that it is a flatring, of enormous dimensions, seen very obliquely. It consists probably, ofmyriads of solar systems at a most astounding distance from ours, and affords adistinct lesson that we must not limit the bounds of the universe by the limitsof our senses."
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and otherpublications, and he is also an on-camera meteorologist for News 12 Westchester, New York.