Longest day of the year: Summer solstice arrives today

Early this morning the sun passed over the Tropic of Cancer, the northern limit of its yearlong path. The result is the summer solstice, the longest day of the year for anyone living in the northern hemisphere.

The solstice occurs because of the Earth’s tilted axis – a feature that can been seen in most classroom globes, which hold their model planets at an angle of 23.4 degrees. As a result of the tilt, the northern hemisphere of the world leans toward the sun through half of its yearlong elliptical orbit around the sun, or the summer, and away from it during the other half, the winter.

Here in Spokane, the solstice means a day of around 16 hours of sunlight. For our Alaskan neighbors to the north, who lie closer to the sun because of the tilt, the sun may dip below the horizon for a few hours, no more.

Alternately, the winter solstice – which falls this year on Dec. 21 – will see a day of around 8 hours of daylight.

Interestingly, the solstice has been growing steadily longer over the estimated 4.5 billion years since the planet first formed. The rotation of the Earth itself is slowing due to a number of countervailing forces, the drag and friction of the Earth’s great bodies of water most prevalent among them. At its formation, one day on earth lasted only about six hours.

Don’t expect to see a noticeable change in solstice length this year, however – the slowing occurs at a pace of only about 3 milliseconds a century.