What causes perpetual daylight and darkness

We often hear about those exciting places where the sun never sets or where the sun never rises, for months at a time. For example, Norway advertises itself to tourists as the land of mid night Sun where you can see day light for continuous 24 hours. (They of course have a period where the Sun never rises but they don't advertise that!). Then there are places not so extreme but with bizarre day:night length ratios, say 22 hours day and 2 hours night. 

For us normal folks who live a normal world, day and night are almost the same length or off by at most three to four hours. This post takes a stab at explaining why Norway has mid night Sun while the Unites States doesn't. 

Two things to learn before we get into why some places have day light for 24 hours - 

Earth's Axial Tilt - Courtesy: WikiPedia
1. First is to realize that the Earth is tilted with respect to its orbital plane. In other words, if one draws a line that passes through the North and South poles of our planet (the axis of rotation) it cuts the orbital plane at an angle of 23.439281°. The axial tile diagram should illustrate this. (click to enlarge)

One can see how the rotational axis is at an angle to the ecliptic plane (ecliptic plane or orbital plane can be thought of as the imaginary plane that passes through the Sun's equator on which all planets orbit)

2. Another thing to note is that Earth revolves and rotates around the Sun at the same time. Revolution is Earth moving around the Sun (which takes an year) and rotation is Earth spinning on its axis (which takes a day)

With this knowledge about the revolution and rotation at a titled axis, it is easy to see why some places have days and nights for months at a time.

Earth's orbit around the Sun and the bright/dark sides as the planet revolves
Image not to scale. Note the red, blue and green dots
Courtesy: NDAA
The above diagram illustrates clearly what is happening. Lets just talk about the North pole (red dot) for demonstration purposes. As you can see, in December, the Earth is tilted in such a way that even after one complete rotation North pole will remain dark as it is tilted away from the Sun. However after six months in June when Earth revolves and reaches the other side of the Sun, North pole is tilted towards the Sun and thus always receives sunlight.
(This of course doesn't mean North pole has 6 months of daylight and 6 months of darkness. You can imagine other phases as Earth revolves where North pole sees only twilight or has regular day/night phases. In fact North pole remains completely dark only for about 11 weeks)

Poles are an extreme case. Places that are closer to the poles (blue dot) like Alaska, Norway etc. have similar day/night conditions but to a lesser extent while countries closer to the equator (green dot) like India or Brazil and situated such that they enter the bright and dark sides on every rotation.

Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.

1 comment:

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