The circle of illumination intersects the North Pole on the spring and autumnal equinoxes, March 20 or 21 and September 22 or 23, when the Earth's North and South Poles are not tilted toward or away from the sun, but are both at 90 degrees to the sun's rays. The sun is directly above the equator, and every location on Earth has 12 hours of daylight.
March 21 and September 22 are dates of equinox. On this date, not only do the seasons change, but at the poles, the sun either rises or sets for the year, depending on the pole. As well, on these dates, the polar circles -- about 66.5 degrees N and S -- experience the single 24-hour period with no sunset or no sunrise, again depending on the polar region. Between the circles and the poles, the period of annual no sunrise/ no sunset varies from the single 23-hour period at the circles to six months at the poles.
The circle of illumination passes through both the north and south poles only twice each year, on the spring and autumn equinox. The spring equinox occurs around March 20 and the autumn equinox occurs around September 22.
A diameter is a segment that passes through the center of a circle and has both endpoints on the circle.
This is the diameter of the circle.
That's a diameter of the circle.
The Arctic Circle passes through both Norway and Sweden but it does not pass through Denmark.
The Diameter
This is a line segment called a diameter of the circle.
This is referred to as a chord. If the chord passes through the center of the circle, it represents the diameteror width of the circle.
Both Canada and the US (through Alaska).
That's a 'chord' of a circle. If the chord happens to pass through the center of the circle, then it's called a 'diameter', and no chord of the same circle can be longer.