THE DEEP BLUE sky of a frigid late afternoon in winter sets an astronomer's pulse to running -- or so it always has mine. Night comes early. The arctic air shows no sign of haze or humidity, promising the darkest skies of the year. Studding the icy blackness will be such bright riches as Orion, Canis Major, Gemini, Auriga, Perseus, and Cassiopeia.

And yet I hear amateurs say their scopes are "in storage;" that this is the season one reads about astronomy rather than practices it. Do these people shiver too hard to keep a steady eye? Do they think Orion can be viewed only through the pain of frozen fingers and toes? In fact you can enjoy winter nights comfortably for hours on end if you dress properly and heed a few cold-weather tips that everyone should know.

Clothing

The first principle of cold-weather dressing is to trap layers of warm air near your body. Studies by the U.S. Army have found that "dead air space," air held in place by tiny fibers, is the only effective body insulator. It doesn't really matter what the fibers are, whether thrift-shop cotton, finest goose down, or exotic synthetics -- only how many inches you put on.

Of course some insulators are lighter than others, per inch of dead air space provided, and have other desirable properties. Vigorous hikers and skiers need light, flexible materials that wick perspiration away from the skin so it can evaporate without leaving a clammy, cold feeling. Special winter outfits are designed for these needs. Skywatching, on the other hand, is hardly athletic. So you can do fine by piling on layers of ordinary clothes that are already around the house.

What matters is how you wear them. Many thin layers are often better than a single thick one. Remember, you want to trap air. The outermost layer should be windproof to keep cold air out. It should also have elastics or ties to close off the waist, sleeves, and the face of a parka hood.

The second principle is to cover your whole body evenly. Three sweaters and a down parka won't keep you warm if there's nothing on your legs but blue jeans. Long underwear and an extra pair of pants -- perhaps heavy wool hunter's pants or insulated snow overalls -- are just as important as a coat. Pajama pants make good "long underwear." Two pairs of pajama pants are better. Your neck and head are major areas of heat loss, so a thick, warm hat and scarf or a thick parka hood are essential.

Where different items of clothing meet at ankles and wrists, prevent bare spots by interleaving the layers. Pull your inner socks up over the legs of long underwear, your pants down over the socks, and your outer socks up over the pants. Whenever it's mildly chilly it helps just to tuck your pant cuffs into the tops of your socks to keep cold air from blowing up your legs.

The third principle is to protect your extremities. Fingers, ears, toes, and nose freeze first. Good footgear is crucial. Your boots should be heavily insulated, but since you won't be scrambling up rockslides they needn't be rugged. Many observers swear by the large, puffy snow boots ("Moon boots") used by snowmobilers. Much warmth is lost from the feet to the ground by conduction through the soles of ordinary boots, so an insulated bottom liner or insole will help. Boots should be large enough to allow you to wear heavy wool socks over your regular socks without any feeling of tightness. Circulation to hands and feet must be kept completely free; anything that feels tight will soon feel frozen.

Protecting fingers is a problem because you have to manipulate eyepieces, charts, pencils (pens freeze up), and so on. One strategy is to wear thin skier's gloves inside loose, more heavily insulated mittens. The mittens can come off briefly as needed. My little finger is always the first to turn painful unless I keep it in the same finger of a glove as my ring finger; then it's no problem. Better alternatives are shooter's mittens with flap-covered slots that allow you to stick your fingers out. You might make cuts in the fingertips of an old pair of gloves.

A ski mask with holes for your eyes and mouth protects the face, if you don't mind looking like a terrorist. Don't use the kind with no mouth hole; your humid breath will come out the eyeholes and fog the eyepiece.

Since you'll be standing still, dress for 20° to 30° Fahrenheit colder than the actual temperature. Studies for Canada's National Research Council indicate that this is the clothing-requirement difference between walking briskly (what most people normally do outdoors in winter) and standing still for long periods.

Eat, Drink, and Act Merry

You can prolong your time in the cold by eating a good meal beforehand and by nibbling carbohydrates, which raise blood sugar and provide heat energy. Hard candy is convenient, but too many sweets can cause a sudden jump in blood sugar followed by an equally abrupt crash. A sandwich gives a steadier lift.

A thermos of hot coffee may feel comforting, but caffeine restricts circulation in the extremities. So does tobacco. A thermos of hot cider or other sweet drink will be better for you. Avoid alcohol; it not only reduces night vision but makes you lose heat by dilating capillaries in the skin.

Once any part of you gets cold, warming it is very hard without an external heat source. So as soon as something begins to feel chilled, run in place for a while or do some jumping jacks. You produce several times more heat during mild exercise than at rest, and good circulation will carry this heat all the way to your toes and fingertips.

Elderly and very thin people have lower metabolism (production of body heat) and are especially vulnerable to cold. Women produce less heat on average than men. People with good muscle tone generate more, even at rest. Vigorous exercise raises anyone's metabolism for up to six hours afterward, so late afternoon or early evening would be a good time for a workout. Beware of exhaustion, however, which leaves you prone to rapid chilling.

A little-known cause of chills, headaches, and ill feeling in winter is dehydration. You lose a lot of water breathing dry winter air, while cold depresses the thirst mechanism so you don't drink enough. When the body runs low on water it conserves fluid by reducing circulation to the extremities, which means your hands and feet freeze quicker. Guzzle water before going out under the stars.

Safety

Cold can kill. If you observe from a remote, lonely site in winter, think through the entire chain of events that will happen if your car won't start. Have you told someone where to come looking for you if you don't show up by breakfast? How will you keep warm until then?

Car batteries lose power in the cold. Even if you normally run your equipment off the car battery and have enough juice left to start the engine, don't assume you can do this in unusually low temperatures. If in doubt, run the engine for five or 10 minutes per hour while observing. This is a good reason to power your equipment from a separate 12-volt battery -- which in an emergency might recharge the car battery enough to get you started.

Any car that is driven in cold rural areas should carry certain emergency items under the seat: extra old sweaters or blankets, an extra hat or two, hard candy, matches, and candles. A candle in a car will provide much warmth if you huddle over it, but open the window a crack. If the engine will start but the car won't move, check the gas gauge. Conserve gasoline by running the engine and heater only 10 minutes or so out of each half hour or hour. If the area is snowy, check that the tailpipe is clear so exhaust won't be trapped under the car. Police report that exhaust poisoning is a major cause of death for motorists whose cars get stranded in deep snow.

No matter how thirsty you get, never eat snow when in danger from cold. Snow requires so much body heat to melt that rapid hypothermia can result. Instead, melt it in a hubcap over your candle. Times like these make a cellular phone or mobile ham radio look mighty good.

Another piece of advice from cold-weather state police: don't leave your car. It provides by far the driest, most windproof, most comfortable shelter you could possibly devise in the wilds. It's also highly visible. Poor judgment leading to gross stupidity is a classic effect of oncoming hypothermia. When things look bad it would be very easy to walk a half mile down the road from your car at 3 a.m. in hopes of finding the house that you think maybe you passed, become disoriented, and forget to turn around. If you stay in your car, sooner or later -- whether in hours or days -- someone will always come.

Such emergencies, of course, are unlikely. With a little planning and common sense, winter nights under the stars will be as pleasant as any -- as well as darker and more exciting.

Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.