By the astronomy standard, summer begins at 11:24 p.m. next Tuesday, when the Earth's orbit is positioned so that the north polar end of its axis leans at the full 23 ½-degree angle toward the sun, giving all parts of Earth north of the Arctic Circle constant daylight.

The year's long­est span of daylight is called the summer solstice. We in the Twin Cities area have 15 hours, 36 minutes of daylight. Winona, to the southeast, will have 15 hours, 30 minutes of daylight. Hallock, in the extreme northwest, will have 16 hours, 15 minutes. The Twin Cities area's 15 hours, 36 minutes of daylight will start Saturday and continue until June 24. We lose one minute June 25. From our summer solstice until December's winter solstice, the nights will slowly lengthen and daylight hours diminish.

Even though the sun is highest in the sky on the June solstice — 68 ½ degrees above the horizon at noon in the Twin Cities — it will take about five more weeks until we get our warmest days. July 26 is statistically our warmest day of the year. This is because of the lag time it takes to heat the Earth's surface to the maximum, for it is radiation given off by the surface, rather than the sunlight itself, that heats the atmosphere.

Yes, from the astronomical point of view, the first day of summer here in the Northern Hemisphere is June 20 or 21, yet meteorologists and climatologists continue to point to June 1 as the critical date in Minnesota.

This earlier date is in better accord with the normal course of plant development locally.

Jim Gilbert is the author of five books on nature in Minnesota. He has taught and worked as a naturalist for 50 years.