Jean Ward, age12, of Duluth, Minnesota, for her question:
Why does the sun rise and set at different times?
Tomorrow the sun will rise a little earlier and set a little later than it did today. This will happen every day until June 22. Then every sunrise until December 21 will be a bit later and every sunset a bit earlier. This is because the earth's axis is not perpendicular to its orbit.
Our planet is a dizzy dancer, spinning around its axis as it races around its orbit. The sun is always shining on half the globe and each side keeps spinning from daylight to darkness. The axis is a line through the globe from pole to pole. You would expect the dividing line between night and day to pass straight through the poles. If it did, all the days and nights would be equal. Sunrise would be at the same time every day and so would the sunset. But throughout the world, except at the equator, the days and nights are equal only twice each year.
This is because the earth '.s axis is tipped. The dizzy dancer leans a little as it spins on its way around the sun. The axis is tipped 23 1/2 degrees from the perpendicular. This 23 1/2 degree angle does not change. But the orbiting earth makes the axis point in different directions in relation to the sun. In June, the North Pole bows toward the sun and more of the Northern Hemisphere is on the sunny side of the globe. Our summery days are longer than the nights. The South Pole is pointing away from the sun and more of the Southern Hemisphere is on the shady side of the globe. Their winter nights are longer than the days.
The earth races around its orbit at about 66,600 miles an hour. The pointed axis is shifted a little with each day's journey. For six months, the days grow longer and for six months they grow shorter. We get our earliest sunrise and our latest sunset on June 22, our longest day of the year. Then the North Pole begins to move back to the shady side of the globe. For six months, our days grow shorter. Each sunrise comes a bit later and each sunset a bit sooner. Meantime, the South Pole begins its bow to the sun and south of the equator, the days grow longer. On September 23, the line dividing the sunny and shady halves of the globe runs through the poles of the axis and all over the world, days and nights are equal.
On December 21, the North Pole starts back to the sunny side. Our short days grow longer, each sunrise is earlier and each sunset later. Meantime, below the equator, the long days grow gradually shorter until on March 21, the days and nights are equal again from pole to pole.
At the equator, the days and nights are always equal or almost equal. The farther we go from the equator, the more they vary through the year. In June, the entire Arctic Circle is on the sunny side of the globe and through the summer months no part of it spins onto the shady side. Through the long Arctic summer, the sun never sets. In December, the Arctic Circle is all on the shady side and through the cold, dark winter months, the sun never rises.