Alaska Science Forum

This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell, is a science writer at the institute.

In this colorful blur of frenzied fishing trips, sweaty softball games, maniacally maturing vegetables, road-weary relatives, and steadfast sleep deprivation we call summer, it's time to reflect on the season's source---the sun.

At summer solstice, the sun once again bakes Alaskans due to the tilt of the earth's axis that leans us toward the sun. The longest day of the year officially occurred June 21, and varied in length from 24 hours in Barrow to just over 17 hours of possible daylight in Ketchikan.

The word solstice comes from the Latin term "solstitium," which translates into English as "sun standing still." Alaskans upon whom the sun sets this time of year can see the sun standing still for three or four days around solstice, as the sun rises and sets in nearly identical places before continuing its race around the horizon.

Some sunny facts, gleaned from a number of good books:

• The sun is our closest star, at about 93 million miles away from the earth. The next closest star, Alpha Centauri, is more than 250,000 times farther away from us than is the sun.
• The sun is a medium-sized star, measuring about 865,000 miles in diameter; it's about 109 times larger than the earth.
• The sun's energy comes from its highly compressed core, where hydrogen nuclei collide at incredibly high speeds, fusing to form helium nuclei and generating temperatures of 40 million degrees F. This process generates the sun's energy, which gradually works its way to the surface and is radiated off.
• The earth receives only about two billionths of the sun's total energy output, yet that tiny fraction is enough to sustain life.
• Of the sun's energy that reaches the earth without being bounced off by clouds or deflected by other particles in the atmosphere, most is spent evaporating moisture into clouds, and much of the remainder is converted into carbon-based life by plants through photosynthesis.
• The sun is about 5 billion years old, and should continue burning for another 5 billion years.
• Because the earth's path around the sun is an ellipse rather than a circle, the earth is actually closer to the sun in January (at about 92 million miles away) than in July (94 million miles distant).
• Ancient cultures personified the sun with names such as: Ra (Egyptians); Helios (Greeks); Sol (Romans); and Tonatiuh (Aztecs).
• The Romans coined the term "dog days of summer," believing high temperatures from about July 3 to August 11 were the doings of Sirius, the dog star, which is the brightest star in the summer sky (in areas where you can see stars in summer).
• June 21 seems a fitting day for the first day of summer in Alaska because hotter temperatures often come in July and August despite the fact that the amount of solar radiation we receive peaks on the solstice. This heat lag happens because the earth still holds much of the cold of winter, which won't be baked off until the summer wears on.
• If the beginning of summer is defined as the onset of the three hottest months of the year, it begins about May 25 in Arizona, New Mexico and Texas; about May 27 in Nevada, Louisiana, Georgia and Florida; about June 7 in New England, North Dakota, Minnesota, Wisconsin and Michigan; and about June 15 in northern Maine.

To close out this mini-celebration of the sun, a quote from Pliny the Elder, a Roman naturalist who lived from 23-79 AD. Of the sun, Pliny wrote: "He furnishes the world with light and removes darkness; he obscures and he illuminates the rest of the stars; he regulates in accord with nature's precedent the changes of the seasons and the continuous rebirth of the year; he dissipates the gloom of heaven and even calms the storm clouds of the mind of man . . ."