Nobody marks the arrival of summer the scientific way any more.

There are truer barometers: the opening of outdoor swimming pools, the stifling sweet scent of the blossoms on Russian olive trees and the abundance of bare legs - shapely, gangly and pudgy, all winter-white and summer-shy.Summer's fanfare gets here long before she does. The bill for the first month of air conditioning has already arrived. Nordstrom's much-hailed fall sale is only a month away. Most sunbathers have achieved their tans and now worry only about maintaining them.

But if you are one of the few entranced by the earth's relationship with the sun, you will know that summer arrives at 9:57 p.m. tonight. At exactly that moment, the sun's rays are straining as far north as they can get. Tonight, while most of us are shepherding children to bed, turning off sprinklers and turning on the news, the earth will tilt just a bit further south and the sun will be positioned directly over the Tropic of Cancer - ushering in summer.

Tomorrow is the first official day of summer. Despite her anti-climatic, if not downright late, arrival, tomorrow brings a gift no other day in the year offers: that splendid arch of sunlight that stretches from the earliest dawn to the latest sunset in the year.

June 21 is the longest day in the year - but not by much.

The sun will set tonight at 9:02 p.m. as it has for four days, said William J. Alder, meteorologist in charge of the National Weather Service's Salt Lake Office. But tomorrow, it sets at 9:03 p.m., giving Utah one extra minute of sunlight.

The longest day of the year will bring the state 907 minutes - or 15 hours and 7 minutes - of daylight, he said.

By June 22, the days will already have started to shorten. Between June 22 and 25, the sun will again set at 9:02 p.m. After that, the sunsets will get earlier and earlier.