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 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 the year 964. But it may have been commonly known to Persian astronomers at Isfahan as far back as 905, 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.
The light from that "little cloud" is actually the total accumulation of light from more than 400 billion stars. It is listed as Messier 31 (or M31) in Charles Messier's famous catalog — a list of hazy objects that resembled 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.
Bigger than the Milky WayM31 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) that are much fainter and placed much farther away, close to the border of nearby Cassiopeia.
As you look at the Andromeda Galaxy 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 good reason why this patch of light appears so faint to the naked eye. When you see it, consider that this light has been traveling 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-mile journey earthward (that's a 15, followed by 18 zeros!), 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.
Nebula or galaxy?For a long time, M31 was popularly referred to as the Andromeda "Nebula." Although big reflecting telescopes such as Lord Rosse's 72-inch at Birr Castle in Ireland were in operation during the mid-19th century, its true nature was not confirmed until astronomer Edwin P. Hubble finally resolved M31 into individual stars with the 100-inch telescope at Mount Wilson Observatory in 1923.
Nevertheless, 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."
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.