At star parties we’re often asked: “How powerful is your telescope?” It’s a good question, but one we have to answer with a brief explanation about telescopes. And we often start by talking about binoculars, which many people have and already something know about.

Unlike telescopes, binoculars usually have information about power printed on them, expressed as two numbers, like 6x30 or 7x40.

The first number, which gives the instrument’s power (also called magnification), tells how much larger and nearer an object will seem to appear. With 7x40s, for example, 7x means seven-power, thus these binoculars make things look seven times larger and nearer than when viewed with the naked eye.

For most binoculars, the power is fixed and cannot be changed. So, the question about power can be easily answered for binoculars. (Zoom binoculars are an exception.)

The second number gives the aperture, which tells the size (in millimeters) of the lens at the big end. So the aperture of 7x40 binoculars is 40 mm.

The big lens, however, doesn’t do the magnifying. It gathers light from the object being viewed and focuses an image of the object on the lens nearest your eye. That lens, called the eyepiece, does the magnifying.

Now let’s look at telescopes. They, too, have a lens (or mirror) to gather and focus light. (Refracting scopes have a lens at the front end while reflecting scopes have a mirror at the back end — both serving the same purpose.) The diameter of the lens or mirror is the telescope’s aperture. So my 8-inch reflecting telescope has an 8-inch mirror to gather and focus light.

And as in binoculars, the light is focused on the eyepiece, which magnifies the image. But here’s where telescopes have an advantage over binoculars: While binocular eyepieces are fixed, and thus so is their power, telescopes are made so eyepieces, and therefore power, can be easily and quickly changed.

For example, I have several eyepieces, which give me a choice of powers from 77x to nearly 300x.

So the next time you’re at a star party looking through a telescope, impress your friends by asking the star party host: What power are we seeing with this eyepiece?  The host will be impressed, too.

* Next two weeks — Average sunrise: 7:11 a.m.; average sunset: 7:37 p.m. Tonight, a half-hour after sunset, Mercury is below fainter Mars, with brilliant Venus to their right, all near the western horizon; the moon is two moonwidths below Scorpius’ brightest star, Antares.

Sunday the moon is at first quarter. Tuesday evening, the moon is below Jupiter in the south. Early Thursday evening, Venus is less than a moonwidth above much fainter Mars, with Mercury to their lower left low in the west.

The Sept. 15 full moon is this year’s Harvest Moon, being the full moon nearest the fall equinox. The evening of Sept. 19, Venus, Mars, Mercury and the star Spica are grouped near the western horizon a half-hour after sunset; later that evening, the moon is three moonwidths below the Pleiades star cluster when they rise around 10:30 p.m.

* Naked-eye planets — (The sun, moon and planets rise in the east and set in the west due to Earth’s west-to-east rotation.) Evening: Venus, Mercury and Mars are low in the west, with Jupiter dominating the south.

Stargazer appears every other Saturday in the Brazos Living section. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at (254) 753-6920 or at paulderrickwaco@aol.com. See the Stargazer Web site at stargazerpaul.com.