What's the other plane doing?

Position lights

At night, an airplane in flight shows position lights (14 CFR 91.209). A steady red light is on the left wingtip, a steady green light is on the right wingtip, and a white light is on the tail. The red and green lights shine forward and out to their own side. The white light shines backward. Most airplanes also carry an anti-collision light, a flashing red beacon or white strobes, visible from every direction.

The steady lights tell you which way the other airplane is pointed. The flashing light only tells you there's an airplane out there.

What you can see here

The green light means you are looking at the airplane's right side. The flashing red beacon sits on the fuselage, and here it appears just to the right of the green wingtip light. The only way the fuselage shows up to the right of the right wingtip is when the nose is pointed toward you. The red light on the far wingtip is there too, but it's lost in the glare of the landing light.

No relative motion

The light holds one spot on the windscreen and slowly gets larger. The AIM says any aircraft that appears to have no relative motion and stays in one scan quadrant is likely to be on a collision course, and if a target shows no lateral or vertical motion but increases in size, take evasive action (AIM 8-1-8, Collision Course Targets). The controller's call adds one more reason to act: altitude unknown means ATC can't tell you whether that airplane is at your altitude.

The other options

• Crossing right to left would show you the left wingtip, a steady red light, not the green.
• Overtaking would put you behind the other airplane. You'd see the white tail light and no green.
• Crossing left to right would show the green, but the beacon wouldn't appear to the right of the green light, and a crossing airplane drifts across the windscreen.