Let's look at a simple, yet important regulation that is often forgotten or misunderstood.
Regulation
§ 91.209 Aircraft lights.
No person may:
(a) During the period from sunset to sunrise (or, in Alaska, during the period a prominent unlighted object cannot be seen from a distance of 3 statute miles or the sun is more than 6 degrees below the horizon)—
(1) Operate an aircraft unless it has lighted position lights.
Interpretation
Position lights must be illuminated between sunset and sunrise (except in Alaska). Let's define position lights. Position lights are often referred to by pilots and aircraft manufacturers as navigation lights or "nav" lights. They are the red (left wingtip) and green (right wingtip) lights with white aft sections and sometimes an all white light on the aft empennage.
Explanation
The purpose of position lights is to make aircraft visible to other pilots at night or in reduced visibility and to assist pilots in determining the orientation of nearby traffic. For instance, if we see three dim white lights, that indicates the traffic is most likely moving directly away from us (if they're bright white lights, the opposite might be true because we might be seeing landing lights). If we see a single green light, we're looking only at the aircraft's right wingtip, which means the aircraft is crossing from left to right. Most importantly, if we see a green light on our left and a red light on our right, it means we're converging with the traffic head-on.
It's obvious why it's a requirement to have position lights turned on when light conditions are dim or dark. We use position lights not only so we can see other traffic (anti-collision lights and beacons help with this, too), but more importantly so that we can tell which direction the other guy is traveling. During daylight this is easy to do because we're able to see the other guy's wings, nose, and tail. At night the lights have to provide this information.
Some pilots are confused about what the law states regarding when position lights must be illuminated. As the regulation simply states, the lights must be on from sunset to sunrise. Don't confuse this with the FAA's definition of night which is most often used in consideration of one's night currency for passenger carrying (one hour after sunset and one hour before sunrise). Position lights are required to be illuminated for an additional two hours -- one additional hour either side of the definition used for night currency. That means if your airplane isn't equipped with position lights or its position lights are inoperative, you must land before sunset or delay an early morning takeoff until after sunrise.
And one final note. The military occasionally conducts special "lights out" night operations in Military Operations Areas (MOA). That means there are high performance fighter jets flying around with no external lights on, so they're basically impossible to see. That's one more reason to avoid active MOAs when flying VFR at night or at a bare minimum be in contact with air traffic control receiving VFR Flight Following.
