Question: My cousin moved to Alaska last year, and he tells me he's now seeing the Midnight Sun. I have no idea what that is, and didn't want to sound dumb, so I just said "That's cool." But what's a Midnight Sun? — TW, Vancouver, British Columbia
Answer: Don't feel dumb about that. Most people on this planet have never seen the Midnight Sun. And if it gets covered in school at all, maybe in a geography class, there's never much explanation. Usually they just say something about "the tilt of the Earth" and how the Sun stays up for 24 hours.
The best way to understand what's happening is to watch an animation of the phenomenon. What you see above was created with a nifty open source app called Celestia. You can visit any planet in the Solar System, script motions, and record videos of what you choreograph.
I chose the date June 21, which is Summer Solstice in the northern hemisphere. This is the date with the greatest amount of daylight and shortest amount of night. It's also when the Midnight Sun happens — if you're above the Arctic Circle (latitude 66°33′46.3″). You didn't say what city in Alaska your cousin moved to, but there's several above that latitude. Fort Yukon is right on the Arctic Circle. Check out this map.
Watch my animation and you'll see the Earth rotate twice (the speed-up factor is 7680X). As you can tell from the terminator (day/night line) the Sun is off to the right. Note how the northern hemisphere is tilted toward the Sun on the Summer Solstice. Note also how there's a region north of the Arctic Circle that stays in daylight continuously and never enters into night. This where the Midnight Sun is happening. And this is what it looks like.
So it's not about the Sun itself looking different — it's about what the Sun does during a 24 hour period. That photo I linked you to is a 5-hour time-lapse centered around midnight. You can see the Sun dips low in the sky, but never drops below the horizon. The Sun is "up" at midnight, hence the term Midnight Sun.
On the Winter Solstice (Dec 21) the northern hemisphere will be tilted away from the Sun, and the opposite effect (I like to call it the Noon Darkness) will happen — 24 hours with no Sun. You can see that happening south of the Antarctic Circle in this animation. Not much in the way of cities down there, but the staff at research stations can enjoy the show.
Both the Midnight Sun and the Noon Darkness last for a number of days that depends on your latitude. At the north city limits of Fort Yukon, you get one day each year to observe these phenomena. Farther north, like in Barrow, Alaska, the Midnight Sun lasts from May 15 to July 28, and the Noon Darkness from November 18 to January 16.
You might wonder why Earth's tilt changes at all. That's a topic for another post, but here's the short answer: The tilt of Earth's rotation axis is actually fixed in space (like a spinning gyroscope) at an angle of 23.5° to the plane of its orbit. So over the course of one orbit, the tilt with respect to the Sun will vary between ±23.5°. You can watch a nice simulation here.
Next Week in Sky Lights ⇒ The Geometry of Angular Size