Now that the bright Moon has left the evening sky, it's a good time to turn our attention to one of the most amazing sky objects which is passing almost directly over our heads this week between 7:30 and 8:00 p.m.

This object was known as the "little cloud" to the Persian astronomer Abd-al-Rahman Al-Sufi, who described and depicted it in his Book of Fixed Stars in 964 A.D. But it may have been commonly known to Persian astronomers at Isfahan as far back as 905 AD, or even earlier. An expert on star nomenclature, Richard Hinckley Allen, once reported that it also appeared on a Dutch star map from the year 1500.

To see this "little cloud" requires good eyesight and 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, mysterious glow: a diffuse elongated smear perhaps two or three times the apparent width of the Moon.

To find it, locate the Great Square of Pegasus. Then, focus binoculars on the bright star Alpheratz, which is at the upper left corner of the Square. Then move straight across to the east (left) and get the star Mirach in Andromeda) in your field of view. Then move slowly up to a fairly bright star above Mirach and continue to run up in the same direction until you find the "little cloud." That will be your stopping place.

Today we know it as the great Andromeda Galaxy.

Galileo's rival, Simon Marius, is usually credited with the first telescopic observation of this object in December of 1612. He described the nebula as an indefinite glow "like a candle shining through the horn window of a lanthorn (lantern)."

Even today, binoculars and telescopes reveal this "cloud" as little more than a smooth oval blur, which gradually brightens in the center to a star-like nucleus. While it will certainly look larger and brighter than with your eyes alone, there is little to suggest the grandeur of this object as it is often shown in long exposure observatory photographs. It's oval because from our vantage point we're viewing it not far from edgewise, but in fact, it's a nearly circular, flat spiral assemblage of star clouds.

The light from that "little cloud" is actually the total accumulation of light from more than 400 billion stars. It is listed as Messier ("M") 31, in Charles Messier?s famous catalogue: hazy objects resembling comets, but later proved to be galaxies, nebulae and star clusters.

Here is the most distant object that can be seen with the unaided eye.

M31 has been estimated to be nearly 200,000 light-years in diameter or one and a half times as wide as our own Milky Way galaxy. Its bright nucleus is the hazy patch that is visible to the unaided eye. Like our own galaxy, M31 has several attendant satellite galaxies. Two of these: M32 and M110 can be picked out with low magnification in a small-to-medium sized telescope, in the same field of view as M31. There are yet two other smaller companions (NGC 147 and 185) which are much fainter and placed much farther away, close to the border of nearby Cassiopeia.

As you look at the Andromeda Galaxy tonight you'll be doing something that no one else in the world except a stargazer can do; you will actually be looking back into the distant past.

There is a very good reason that this patch of light appears so very faint to the naked eye. When you see it tonight, consider that this light has been traveling some 2.5 million years to reach you, traveling all that time at the tremendous velocity of 671 million mph.

The light you are seeing is around 25,000 centuries old and began its journey around the time of the dawn of human consciousness. The light you are now getting is at least 480 times older than the Pyramids; the distance it has traveled is so inconceivable that even to write the number of miles is all but meaningless.

When it began its nearly 15-quintillion (15, followed by eighteen zeros!)-mile journey earthward, mastodons and saber-toothed tigers roamed over much of pre-ice-age North America and prehistoric man was struggling for existence in what is now the Olduvai Gorge of East Africa.

For a very long time, M31 was popularly referred to as the Andromeda "Nebula." But although big reflecting telescopes such as Lord Rosse's 72-inch at Birr Castle in Ireland were in operation during the mid-19th century, it was not until astronomer Edwin P. Hubble finally resolved M31 into individual stars with the 100-inch telescope at Mount Wilson Observatory in 1923.

Yet there were those who many decades earlier suspected that M31 was much more than just a luminous cloud. Read this prophetic comment out of W.H. Smyth's A Cycle of Celestial Objects written back in 1844:

"Sir John Herschel . . . concludes that it is a flat ring, of enormous dimensions, seen very obliquely. It consists probably, of myriads of solar systems at a most astounding distance from ours, and affords a distinct lesson that we must not limit the bounds of the universe by the limits of 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 other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.