Night shift in the April sky heralds spring's arrival

Of all the signs of the changing seasons, none are as dependable as those in the night sky. The arrivals of various birds and butterflies, the changing peep frog choruses at dusk, the smells of new vegetation - the dates of these depend on flukes of weather (and climate change). But the stars repeat their doings at the same time each year with clockwork certitude. They wheel through their courses with only a very gradual seasonal displacement, which adds up to a shift of just one day per human lifetime.

So when it is early April in New England, you know that Orion will be stalking downward in the southwest as twilight fades, with his three-star belt now horizontal. The Gemini twins stand upright in the west, their heads lit by Castor and Pollux. Leo the Lion walks high across the south after nightfall. Equally high in the north-northeast, the Big Dipper is twisting around to turn upside down.

And the Little Dipper is moving upward underneath, as if to catch the Big Dipper's spilling contents.

A lot of people run into frustration at this point. The Big Dipper is the best-known, easiest-to-spot star pattern in the sky, with the possible exception of Orion. But almost no one can make out its littler counterpart.

One reason is that the Little Dipper would make a lousy dipper. Its bowl is too small, and its long handle is awkwardly bent. But mostly, its stars are too dim. They get washed out of view by light pollution in the skies over the places where most people live. You would need to get well away from Greater Boston to see the whole thing as outlined here.

Two of the Little Dipper's stars are, however, fairly bright exceptions. One is the handle-end: Polaris, the North Star, which everyone ought to know; it is famous for staying fixed in its special spot due north. Find it using the two stars that form the front of the Big Dipper's bowl. These are called the Pointers; they point more or less to Polaris, which is separated from them by about three fist-widths at arm's length.

The lip of the Little Dipper's bowl is Kochab, which matches Polaris in brightness. Can you see its orange tint? Oddly, the lips of both dippers are orange-giant stars whose colors (if you have sharp eyes) differ from the white of the rest. Someone could make up a good constellation-myth story about this, but no one ever has. You could be the first.

When Light Makes Waste

The Big and Little Dippers are part of the constellations Ursa Major and Ursa Minor, the Big and Little Bears. You need to see a lot of other faint stars to piece together the (surprisingly realistic) profile of the Big Bear, and the Little Bear is just a box with a long tail no matter how you look at it. But the darker your sky, the more you see them the way their originators, and perpetuators, viewed them every clear night for most of the history of the human race.

Many people assume that the loss of the stars is an inevitable byproduct of civilization and good outdoor lighting, but it happens because of bad outdoor lighting.

The glow you see in the night sky represents many megawatts of light that is being completely wasted. It comes from all of the poorly designed and improperly aimed outdoor lights for dozens of miles in every direction, fixtures that send some of their light uselessly upward above the horizon, instead of where it will do any good.

Think about it. The burglar you want to deter, the pedestrian you want to avoid hitting, is not flying in the clouds like a bird. He is not tightrope-walking on power lines 40 feet up. He is down on the ground. That is where the light should go. All of it.

That means installing what are called full-cutoff light fixtures. They are designed to direct not just some of their light, but all of it, where it is supposed to go.

No waste means you can get the same illumination on the ground from a lower-wattage bulb. The International Dark-Sky Association (www.darksky.org), founded by astronomers to promote smart lighting design, calculates that the United States wastes well over a billion dollars a year in electric bills illuminating the undersides of clouds, the bellies of airplanes, and outer space.

The wasted money is bad enough. The wasted energy, with the resulting pollution and carbon dioxide emissions, is arguably worse. If saving the stars does not motivate smarter lighting, if even saving on electric bills does not get people's attention, maybe helping to save the Earth will.

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.