If you visit sites like this one, you can see maps showing which parts of the earth are sunlit and which are in darkness. Normally the maps look something like this:
Notice the wavy band of dawn/dusk snaking its way across the globe. Notice that the poles are completely in night or day.
Today, however, is one of the two days of the year when the picture looks different.
Today is the Equinox, when the tilt of the earth makes no difference to the length of day and night, when the sun sets and rises in the poles.
Notice that the wavy pattern has been replaced by a solid square that moves across the face of the earth.
Over the next few days, this solid square will bleed upwards to cover the north pole, bathing the Arctic in darkness.