This is absolutely the last weekend of summer even though signs of autumn are everywhere.

Astronomical summer switches over to astronomical autumn at 10:21 a.m. Thursday, the moment of autumnal equinox. At that time the sun’s daily arc across the sky from east to west slips below the celestial equator, a projection in the sky of the Earth’s terrestrial equator.

That basically means that the sun’s daily path in the sky will stay completely in the southern half of the sky, and will continue to progress lower and lower until the first day of winter in December.

As that happens the east-to-west arc also will become shorter and we’ll lose even more daylight hours.

That suits me just fine, as the night for stargazing becomes longer and the stars are out earlier in the evening.

You’ve no doubt heard that the first day of autumn also means that days and nights are equal in length. That’s why it’s called the equinox.

Actually, that’s not exactly true. Here in Butler on Thursday sunrise is at 7:08 a.m. and sunset is at 7:16 p.m. We still have more than eight more minutes of daylight than nighttime.

The Earth’s atmosphere causes this to happen. The sunlight along the horizon has to plow through a lot thicker layer of air than it does higher in the sky. The thicker atmosphere near the horizon bends the sun’s rays so much that the sun may appear above the horizon when it’s actually below it. This is called astronomical refraction. Days and night actually become equal on Sept. 25.

A great way to kick off the autumn stargazing season is to make a date to get out in the dark skies of the countryside, if you’re not already living out there.

I would hold off this stargazing adventure until later this week or next weekend because for the next several nights we still have a waning full harvest moon whitewashing the early evening skies.

I guarantee that this will be a treat that you’ll remember for a long time, no matter if you’re by yourself or with family or friends. Bring the blankets, binoculars, star charts, snacks and beverages, and be prepared to sleep in the next morning. Even better, turn this into an overnight campout.

It will be a great show. The clear autumn skies are more transparent because there’s much less humidity in the air.

When you settle under the autumn evening heavens, make an attempt to estimate how many stars you can see with your naked eye.

Traditional astronomy textbooks say you can see about 3,000 stars with the naked eye, but I’m sure there’s a lot more than that.

Don’t even try to count or you’ll fall asleep and the show will be over. If the skies are dark enough, you can’t help but notice the bright band of milky light that bisects the sky from roughly north to south like a cosmic artist’s stroke.

You may have been taught at an early age that the band is the Milky Way galaxy.

While that’s certainly true, what you have to realize is that every single star you see anytime, including the sun, is a member of our home galaxy, the Milky Way.

The stars that aren’t in the band are relatively close by. The stars in the band are much more distant and make up the plane of our home galaxy, the thickest part of the Milky Way.

There are so many stars in the band that are so far away that all we can see is a collective ghostly glow.

From all that’s been learned over the last 100 years, astronomers have concluded that the Milky Way galaxy is made up of about 400 billion stars that form a nearly circular disk that’s broken up into spiral arms.

There’s a huge hump of stars in the center. The Milky Way is a little more than 100,000 light-years in diameter and 10,000 light-years thick, with one light-year equal to about 6 trillion miles. Our sun is about 30,000 light-years from the center of the galaxy in one of the arms.

When you see that milky band of light across the sky you are looking edgewise into our galaxy.

All of the stars we see obediently orbit around the center of the Milky Way. Our sun takes more than 200 million years to make one circuit.

In case you’re wondering, the center of the Milky Way is in the direction of the constellation Sagittarius the Archer in the low southwest sky.

Sagittarius’s nickname is “the Teapot,” because that’s what it really looks like. The Milky Way band is not really all that much brighter around the teapot, because there is a lot of dark interstellar gas and dust that blocks the “hump” at our galaxy’s center. It’s been said that if we could see the Milky Way’s central region unobstructed, that area of the sky would be much brighter than a full moon.

Lie back on the ground or in a reclining lawn chair and roll your eyes from one end of the Milky Way band to the other.

Just a decent pair of binoculars will greatly enhance the celestial treasures you come across like bright patches, dark rifts, and star clusters. You might even see a few human-made satellites rolling across the heavens.

While you’re enjoying our own galaxy keep in mind that our Milky Way is only one of millions of other galaxies out there. Astronomers have spotted billions and billions of galaxies more than 13 billion light-years away.

It’s a big sky out there for you to enjoy. Don’t miss it!

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/Paul and is author of the book, “Stars; a Month by Month Tour of the Constellations” published by Adventure Publications available at bookstores at http://www.adventurepublications.net