In aviation, lights are used not only to illuminate, but signal as well. For example, ATC Towers use a multi-color “Light Gun” when radios aren’t working. On aircraft, lights are used in specific ways, signaling intentions or actions. This is a 737 perspective, but lights are similar for other fleets. They’re controlled by the pilots & not usually automatic; maybe half the switches are installed on the Captain’s Overhead Panel, w/ the other half on the FO’s. With the exception of the “Logo” light, they’re used the same way day & night. Let’s pretend we’re pushing back from the gate to fly: The “Logo” light is always on at night on the ground & below 18k’, even at the gate. It’s often mounted in the horizontal stabilizers & angled at the vertical stabilizer, illuminating the airline’s logo. It gives you a slightly better view of the tail when doing an exterior inspection & makes you easier to see overall; it’s the only light we don’t use during the day. The rotating, red “Anti Collision” lights (sometimes called beacons) are installed on the top AND bottom of the aircraft. When on, they signal the airplane is in motion or engines are running; they’re on as we leave the gate & only come off when parked w/ engines stopped. The steady red & green, wingtip mounted “Position” lights are used to help an observer orient the airplane in space. The red light is on the left wing & green on the right – like maritime rules. In addition, a steady white light is either mounted on the tail or facing backward on the wings & is only visible from the aft. Taken together, these lights can tell you if the jet is coming or going & what side of it you’re on. They come on as we leave the gate & only come off as we park. Next is the “Taxi” light; it’s mounted on the nose wheel strut & turns w/ it, lighting the direction you’re steering. It’s angled slightly down. When ground crew disconnects the tug & indicates they’re clear of the aircraft, a quick flash of the taxi light acknowledges. The taxi light is turned on when we intend to or are taxiing, showing the plane is in motion. When stopped, we turn it off, signaling we’re at least temporarily parked. We also turn it off when turning toward the gate for parking, to avoid blinding the Marshalers. Once we’re cleared onto a runway, white, wingtip mounted Anti Collision lights (sometimes called the “Strobes”) come on. These flash at a specified rate & can be seen quite a way. We also turn on the “Runway Turn Off” lights at this point. They’re white lights mounted on both sides of the fuselage & angled left/right; they illuminate taxiways as we slow down on landing & look for an exit. The runway turn off lights are sometimes flashed in flight when ATC is calling out traffic; as a technique, if you know it’s you ATC is advising others of, you light one on the side the traffic is approaching to help them see you. The “Wing Inspection” lights also come on when taking a runway. These fuselage mounted lights are pointed down & out along the wing & help us see if the wing has iced up or taken damage, but also increase visibility. Together, the runway turn off, strobes & wing lights indicate we’re on a runway, but NOT taking off, making us visible to anyone trying to land behind us. Even if just crossing a runway, the lights come on & are turned off on the other side, or when exiting the runway after landing. We’re trying not to ruin other’s night vision! When cleared for takeoff, “Landing” lights come on. They’re like headlights, angled ahead, mounted in the wing root. They illuminate the runway, but also increase visibility. All lights are on anytime we’re below 18,000’, because we’re mixed in w/ aircraft not controlled by ATC. Above 18,000’, all aircraft have to be controlled by ATC, but the beacon, strobes & position lights stay on. A technique is to not turn on the taxi light on descent until cleared to land. If you can’t remember if you got clearance or not, you’ll be able to tell.