To the east, Pegasus gallops into view

Every season, every month, even every week of the year has its own particular arrangement of stars and constellations in the evening sky. Each of these shifting arrangements is as sure a sign of the calendar as autumn leaves, winter snow, or baking summer heat. More so, in fact - because the stars are immune to the vagaries of weather and changing climate.

Fall brings a starry landmark that's famous ahead of all others for its season. Although fall doesn't start until Sept. 22 this year, summer is over in practice for everyone with a schedule tied to Labor Day. So it's appropriate that, if you step out and look east after dark this week, you'll find the Great Square of Pegasus, celestial backdrop to chill winds and falling leaves, already looming up into view above your neighborhood houses and trees.

Look for the Great Square due east a little less than halfway from horizontal to straight up. It's made of four modestly bright stars forming a pretty good square balancing on one corner. It's rather big; your fist held out at arm's length fits inside it.

The Great Square is supposedly the body of Pegasus, the flying horse of Greek myth. If you have a moderately dark sky, more stars will be visible, and along with the Square they can be made into a stick-figure profile of a horse's front half, as shown here. The horse is currently upside down, with its head and nose to the right and its prancing forefeet above. Turn the diagram upside down to see Pegasus better.

This stick figure is probably how the ancients actually regarded Pegasus. For instance the name of the cartoon horse's nose star, Enif, comes from the Arabic for "nose." And ancient sky depictions showed only the front half of Pegasus as it leaps out of a cloud.

In the coming weeks and months, Pegasus will rise higher in the evening sky, finally crossing overhead and descending down to the west in winter. Next September Pegasus will be right back there in the east again, and every September for the rest of your life, and the lives of your children's children's children. Who knows what the September climate will be like then - but beyond Earth, many things are very slow to change.

Not-so-eternal stars
The horse-shaped pattern of Pegasus hasn't changed for tens or hundreds of thousands of years (long before horses were domesticated) because its stars are so far away. The Great Square's stars are, counting clockwise from the top, 200, 140, 300, and 97 light-years from Earth. Each is drifting through space in its own random direction at a speed of several miles per second. But at those distances, it takes many millennnia for stars to appear to move appreciably on our sky. So the constellations have remained essentially the same throughout human history.

However, their places in Earth's cycle of seasons are not quite so constant. If, 70 years from now, your children's children's children make careful measurements, they'll find that Pegasus and all other constellations have shifted a day forward in the timetable of the seasons. This effect is due not to the stars themselves but to a slow, slight change in the direction that Earth's axis points.

This change, called the "precession of the equinoxes," is caused by the gravity of the moon and sun pulling with a twisting force on the slight bulge around Earth's equator. The only change is in Earth's orientation with respect to the rest of the universe; it doesn't cause the North Pole to move with respect to the ground.

And in the really long run?

In a million years, most of the stars that we know will have drifted this way and that into the far distance out of sight, while new ones will be passing by to take their place. The sky will be filled with entirely new starry arrangements, which our far descendants (or perhaps our successors from another species) can make into new constellations of their own.

In Earth's 4.6-billion-year history, a complete constellation turnover has happened several thousand times. Not even the stars are really forever.

Whole-sky maps
Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/howto/basics/3308331.html.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.