I know that north of the artic circle that days and nights last six months each. My question is how far south of the artic circle do you have to be before the days return to “normal” lenghth. Is it a sudden change or is it gradual?
Gradual. But it’s incorrect to say that north of the Arctic Circle days and nights last six months; that is true of the North Pole only. If you are exactly on the Circle, you’ll only have one day without a night (the summer solstice), and one night without a day (the winter solstice) per year, and as you progress gradually toward the Pole, the day (sunrise to sunrise) equals the year (trip around the sun).
It’s gradual, of course. Take a look at this map.
The shape of the day/night curve changes with the season. Right now, it’s still rather steep but it oscillates between that and more of a sine-like shape. It’s at one extreme on the soltices, and the other at the equinoxes.
OK, then the next question is how much does the lenghth of the day change by latitude?
Sorry, I mean longitude.
No, actually you mean lattitude. The length of day doesn’t change by your longitude.
Figure it this way.
At the Arctic circle ( 66.67 degrees) you have 1 day of 24 hour sun/dark.
At the North Pole (90 degrees) you have 180 days of 24 hour sun/dark.
180 days/23.33 (degrees between the pole and the circle) = 7.71 days per degree of lattitude north.
OK, I grew up in Fairbanks, Alaska, about 100 miles south of the Arctic Circle. In mid-summer the sun sets, but the sky never gets dark, we just have an hour or two of dusk/dawn and then the sun rises again.
We always have a few hours of light even in mid-winter. The sky might start to get light at 9:00, rise at 11:00, set at 1:00, and the sky would be dark by 3:00.
The thing to remember is that the sun isn’t going straight down towards the horizon, but moving “sideways”. So, you have very very long sunrises, sunsets, dusks and dawns. At least at the solstices, at the equinoxes you get 12 hour days and 12 hour nights like everywhere else. You get a gradual increase or decrease of the amount of daylight every day, about 7 minutes. In fact, in the winter the radio stations announce daylight hours and how much they’ve changed along with the weather. “The sun rises at 8:53 and sets at 3:22 today, a gain of 6 minutes and 12 seconds over yesterday…”
This phenomenon is easy to understand if you get a globe and a bright light. See how the globe is tilted? If you point the north pole towards the light, see how everything above the arctic circle is always lit? And conversely, everything below the antarctic circle is always dark. That’s northern hemisphere summer, southern hemisphere winter. Turn the axis the other way and you get the reverse.
It is a surprise for American friends who visit England in winter to find out how short our days are. Our climate is quite similar to Washington State but we are much further north. Hours of daylight are down to about six and a half a day- sunrise as late as 8.30am and sunset as early as 4pm. However we do have full sunlight at 4.30am in the height of summer and light into the evening that allows you to see well until 10.30pm- eighteen hours of useful daylight.
The other side of this is that the days don’t vary by more than about 10 minutes in length if you live on the Equator. Once in a rural Congo (ex-Zaire) village, we’d been discussing satellites, snow and Americans walking on the moon with the people who lived there. They’d seen Equatorial satellites pass overhead for years, especially after sunset, and had a Swahili term “nyota wa kutembea” or “stars that walk” for them.
On the other hand, when I tried to describe that at my home the summer days lasted until 10 p.m., the response was, “Oh, now monsieur, you’re trying to make fun of us. Who would believe that days get longer or shorter?”