Everyone knows that the stars we see at night are very far away. Trying to express their distances in miles involves using values in the trillions, and except for the national debt, not a lot of us use trillions to describe anything.

So, it's easier to express stellar distance in light-years,

the distance a beam of light travels in one year's time. The actual speed is just over 186,000 miles per second.

If you do the calculations, that works out to about 6 trillion miles for just one light-year.

Most stars you see in the Western Pennsylvania sky are about 100 light-years - about 600 trillion miles - away.

A good example of a star that's 101 light-years away is Alkaid, the star at the end of the handle of the Big Dipper. Since light travels at a set speed, astronomers can calculate when the light you are seeing actually left that star. If you gaze at Alkaid tonight, the light you see left that star in 1904. At that time, less than 15 percent of American homes had bathtubs!

Many other stars in the sky are a lot farther away than Alkaid. Some stars are thousands of light-years away.

One such star is Deneb, the brightest star in the Cygnus the Swan constellation, high in the southwestern sky. Deneb is about 3,200 light-years away, so we're seeing Deneb as it was around 1800 B.C.

Another heavenly illusion comes into play, this time of year. The autumnal equinox, otherwise known as the first day of fall, is this Thursday.We've all been taught that on that day we have equal amounts of sunlight and darkness. However, if you look at the newspaper on Thursday, you'll see that we actually have 12 hours and 6 minutes of daylight. Where did that extra 6 minutes come from? Well, it's all due to the illusion of astronomical refraction, caused by the Earth's atmosphere.The sun's light is actually bent by our atmosphere, making it appear higher in the sky than it actually is. When we see the sun near the horizon, we have to look through a lot more of our Earth's atmospheric shell. That atmosphere bends the sun's light so much so that the sun appears to be above the horizon when it's actually below, giving us the extra few minutes of daylight.

The harvest moon will be climbing into the eastern horizon this weekend and lasting through the early part of next week. It wipes out serious stargazing with all the light, but it is a joy to see. But why is the moon so huge when it rises?The truth is that it's not really any bigger. It's just an optical illusion that we see whenever the moon or sun is close to the horizon because we're comparing it with objects we see on the landscape.The moon is no smaller when it's high in the sky. You can prove this by using a paper clip. When the moon is first rising, hold out a paper clip at arm's length and bend it apart so that the moon's disk fits exactly between the two ends of the clip. A couple of hours later, when the moon seems smaller, hold out that same bent paper clip at arm's length and you'll see that the moon is exactly the same size.Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the new book, "Pennsylvania Starwatch," available at bookstores and at his Web site www.lynchandthestars.com