HOW COME? The tropical sun's fast disappearing act

Why does the sun seem to set faster in the tropics? asks Elizabeth Bailey, of Garden City.

Night falling with an abrupt crash? You must be near the equator. While sunsets in most parts of the United States seem to be followed by a pleasantly long twilight, the sun's glow slowly fading in the sky, twilight near the equator seems strangely brief. In 1878, British naturalist Alfred Russel Wallace wrote: "Travellers usually exaggerate the shortness of the tropical twilight, it being sometimes said that if we turn a page of the book we are reading when the sun disappears, by the time we turn over the next page it will be too dark to see to read."

Wallace notes that twilights at the equator can last about two-thirds as long as those in temperate latitudes. So a 30- minute twilight in New York might mean a 21-minute period of semidarkness in the tropics. So why does it seem like night follows right on the heels of the setting sun? And why are twilights briefer near the equator anyway?

Although we think of twilight as the time just after the sun sets in the evening, it's twilight time just before the sun rises in the morning, too. During twilight, although the sun is below the horizon, the sky is still aglow with light, gradually dimming (after sunset) or intensifying (before sunrise). Twilight is in-between time, the peaceful "blue hour" that marks the transition from day to night, night to day.

Twilight's glow comes courtesy of the upper atmosphere. Light rays from below the horizon are bent (refracted) as they pass from empty space into the gas molecules of the atmosphere. So sunlight still lights up the sky for a time after sunset and before sunrise. Since the light is also scattered every which way by the air's gas molecules, part of it reaches the ground. So it's the still-illuminated sky that creates the twilight landscape.

How long twilight lasts depends on the time of year and your location. In the continental United States, "civil" twilight lasts 30 to 40 minutes. But at the equator the show is over in as little as 20 minutes. Why the difference? What counts most is the angle the setting (or rising) sun makes with the horizon. From the vantage point of the northern United States, the angle is oblique. So the sun remains closer to the horizon, even after it's set.

But near the equator, the sun sets at a steep angle, perpendicular to the horizon. This makes it appears as if the sun has dropped like a stone. And after the sun sets on its straight path, light disappears more quickly.

In addition, sunlight seems brighter at the equator because the sun's more overhead path during the day means its light streams through thinner layers of the atmosphere. Combine a fast disappearing act with the contrast between the day's dazzling brightness and the encroaching gloom, and the impression is that darkness really falls. Likewise, equatorial birds have barely begun chirping before the sun has popped up like morning toast.