A funny thing happens to the sky each year, starting now in late August. Although few seem aware of it, the heavens suddenly freeze. The pantheon of constellations becomes a slow-motion drama. To grasp what's going on, you need a little background...

Earth's clocks do not ticktock in rhythm with the stars. There's a daily four-minute discrepancy between them. In other words, stars set four minutes earlier each night. This adds up to a half hour per week, or two hours every month. So a constellation seen halfway up the western sky at 8 p.m. sets two hours earlier a month later, pushing it into the day sky and rendering it invisible. It's fallen into the glare of dusk's twilight, gone for the season.

Everything rises two hours earlier per month too. Orion, now standing in the east just before dawn, will be up at 2 a.m. in late September, midnight in October, 9 p.m. in November (the end of Daylight Savings jumps the sky an extra hour), 7 p.m. in November, and as soon as night falls in December. That's why constellations are deemed winter, spring, summer or fall groupings, depending on the time of year in which they appear at a convenient viewing hour, or for that matter can even be seen at all!

Starting in late August and continuing through the fall, an extra effect is at play: The time of sunset rapidly retreats. Each evening, darkness falls at an earlier time, and this partially offsets the earlier times that the stars rise and set.

What are we getting at? Merely this: The combination of the stars setting earlier and, in the fall, darkness occurring earlier, makes constellations now appear to freeze in nearly the same position each evening at dusk. The summer and fall constellations never seem to go away. The Summer Triangle and its brightest star, Vega, stand high overhead at nightfall, just as they did a month ago. The "W" of Cassiopeia floats in the northeast when darkness falls, and it'll still be there after Halloween.

The wonderful patterns of Scorpius and the Teapot of Sagittarius linger in the south at nightfall. The glorious Milky Way keeps splitting the sky in half from north to south. And the characters of Greek mythology - which dominate only at this time of year - are like the relatives who came for dinner. Hopefully you're fond of them, because they're not going away.