Equator twilight
Q: Why is twilight shorter at the equator than at the
poles?
[© 2001 Dennis Kortyka, used with permission] The night sky.
A: The sun rises and sets, of course, because Earth rotates. To we folk, riding Earth, though, the Sun and stars appear to actually move. (Figure 1). Each of the circular streaks in the long-exposure photo is the path of a star during the hour or so it took to take the picture. The stars appear to rotate about the North Star, Polaris, (the center dot).
Polaris never rises or sets and neither do the nearby stars. The ones farther away from Polaris, however, rise up from the horizon and set below it as their circular path dips below the horizon. So does the Sun, just another star.
Polaris sits directly above the North Pole on a line that is an extension of Earth's axis of rotation. So, if you stand on the North Pole and look straight up, you see Polaris shining overhead and all the stars spinning around Polaris. As Earth spins, it carries you in a complete circle. Polaris, overhead, stays motionless because it's on the axis of rotation. If you fix your gaze on any other star, it makes a circle.
At the North Pole on the Summer Solstice, the Sun never sets.
Let's imagine we're standing on the North Pole at the Summer Solstice (the longest day of the year). Figure 2. The Sun (yellow dot) never sets. It's like one of those stars that make a complete circle about Polaris. Now let's stroll down to the Arctic Circle a few days after the Summer Solstice. The Sun sets so briefly that our day is barely interrupted. But for a short time, we can't see the Sun and the Sun glides almost tangentially to the horizon as it disappears and reappears, like a star trail that grazes the horizon.
That's the crux of the matter: the angle the Sun makes with the horizon as it drops below the horizon. An oblique angle allows the sun to stay close to the horizon for a longer period of time so its gloaming colors linger.
Let's amble on south, checking the sunset now and then.
Figure 3. The farther south we go
toward the equator, the
lower Polaris appears in the sky and the longer the
nightthat is, the longer the Sun stays below the horizon
and the steeper the angle it makes with the horizon as it
sets and, consequently, the shorter the twilight. The Sun
sets faster when it drops at a steeper angle because its path
is shorter.
Further south, the Sun sets obliquely.
Finally, we arrive at the Equator. Figure 4. Here Polaris is
on the horizon and the Sun descends 
vertically at any date.
That's why the twilight is always shorter at the equator
than anyplace else. The Sun drops below the horizon quicker.
At the equator, the Sun sets vertically.
(Answered by April Holladay, science journalist, April 4, 2001)
Further Surfing:
How round is squished Earth?, WonderQuest
