Which side of the plane should you sit on to avoid the sun?

You'll often hear a simple rule for this: sit on the left flying east, and on the right flying west. It makes it sound as if every route has one shady side, but it doesn't. Which side gets less sun depends on the direction you fly and the time you take off. Across the world's busiest routes, with departures sampled from 6 am to 9 pm on one day in every month, about two in three switch sides between a morning and an evening flight. Once you picture where the sun is in the sky, you can see why the simple rule only works some of the time.

Left and right here mean what they mean in your seat, facing the front of the plane. The shady side is the one that gets less direct sun over the whole flight, not one that never sees it.

Flying east or west: the hemisphere decides

North of the tropics, which covers most of North America, Europe and Asia, the sun is to the south of you around the middle of the day. Heading east, south is on your right, so that's where the midday sun comes in and the left side gets the shade. Heading west, it's the other way around.

That's where the simple rule comes from, and on a late-morning flight it works: Denver to Chicago at 11 am in March keeps the shade on the left the whole way, and Chicago to Denver at the same time keeps it on the right.

South of the tropics, the midday sun is to the north instead, so the same direction gives you the other side. Buenos Aires to Santiago at 11 am in March heads west, just like Chicago to Denver, at the same hour and in the same month. Its shade is on the left.

Flying north or south: the clock decides

On a north-south route, the time of day matters more than the hemisphere. The sun rises in the east and sets in the west wherever you are, so the clock decides which side of the plane it ends up on.

Take Atlanta to Miami, a short flight that heads south. On an 8 am departure, the sun is still low in the east, and when you're facing south, east is on your left. The sun comes in through the left windows, and the right side stays in the shade. Take the same flight at 4 pm and the sun has moved across to the west, which is on your right. Now the shade is on the left. The route didn't change at all, only the time on your boarding pass. Flying north, it's the other way round.

Most routes don't run neatly east-west or north-south but somewhere in between, so they get a bit of both. That's why, on so many of them, the shady side on a morning flight isn't the shady side on an evening one.

Around midday, minutes matter

Around the middle of the day, the sun swings from the eastern half of the sky to the western half. On a north-south flight it sits almost straight ahead or behind you, and only drifts a little to one side, so a short change in departure time can tip it from one side to the other.

Atlanta to Miami at noon on June 15 has its shade on the right. Leave half an hour later and the shade is on the left. If your flight leaves around midday, a short delay can change which side you want.

When the date matters too

On some flights even the time of day isn't enough, because the sun's path across the sky shifts with the seasons.

In the tropics, the midday sun passes almost straight overhead, and for part of the year it passes a little to the north. Bangkok to Hong Kong heads northeast, and at 11 am on December 15 the shade is on the left, just as the rule says. Take the same flight at 11 am on June 15 and the sun is north of the plane, so the shade moves to the right.

Far to the north, the summer sun also sets somewhere unexpected: in June it goes down in the northwest, well away from due west. Helsinki to Stockholm heads west, and on a December afternoon the shade is on the right, as expected. At 8:30 pm on June 15, the low evening sun is on the right for almost the whole flight, so the shade is on the left.

Long flights add one more twist, because the sun can set and rise again while the route curves underneath you. Hong Kong to San Francisco is an eastbound flight, so you'd expect the shade on the left. Leave at 2 pm on June 15, though, and most of the sun that reaches a window comes in on the left: behind you and to the left through the afternoon as you head northeast, then, after the night, ahead on the left as the route bends down toward California. So the shade is on the right.

Your flight, worked out

So on almost every busy route, either side can turn out to be the shady one, and it all comes down to when you fly. The quickest way to know for yours is to check your flight on FlightSide with your route, date and departure time.