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.