7 June 2013
The sun has been hidden by fog for a few days, but we can still tell it’s no longer setting over Tara’s deck. Rather, it slowly descends towards the horizon, then immediately rises again in the sky. This permanent daylight, called polar day or midnight sun, is due to the complex motions of the Earth around the sun.
To understand the phenomenon, imagine a light bulb attached to the ground representing the sun. Now take a spinning-top transpierced by a metal rod from top (north pole) to bottom (south pole). This represents the Earth, revolving around the light bulb in an almost perfect circle. It will take 365 days to go around the sun, and at the same time turns on itself every 24 hours. At every moment, only half of the top receives light, while the other half remains in darkness.
The length of day can be explained by another analogy using the top. The metal rod, which corresponds to the Earth’s axis of rotation, is not perfectly perpendicular to the ground. In other words, the top is slightly tilted – at an angle of about twenty degrees.
At a certain moment during the rotation around the bulb (at the summer solstice), the upper part of the top is pointed towards the light: it’s summer for the northern hemisphere and the days lengthen. Six months later (during the winter solstice) the lower part of the top points toward the sun: winter days get shorter in the northern hemisphere, but south of the equator it’s summertime.
Finally, during the summer solstice, when the northern hemisphere is pointing toward the light, let’s observe the area around the metal rod emerging from the top (the North Pole). Because its rotation axis is slightly tilted, we see that this area is constantly in the light, even when the top turns on itself: this is what we call “polar day”, when the North Pole never gets dark. At the same moment, the area around the metal rod at the bottom of the spinning-top (the South Pole) is constantly plunged in darkness – “polar night”.
At the two poles, the polar day lasts six months, while the night extends the other six months of the year. The farther we move away from these extreme latitudes, the less time the phenomenon lasts. The Arctic Circle is defined as the lowest latitude where the sun doesn’t set for at least 24 hours (the day of summer solstice), and doesn’t rise on the day of winter solstice. In the other extreme case – at the equator – each day has the same length throughout the year. During most of this expedition, Tara will be navigating inside the Arctic Circle, and the polar day will be chasing the night for many weeks.