Spacewatch 101: Tips, terms and basic skywatching information

A pair of binoculars is all you need to explore the night sky, and even the old naked eye can perform pretty well. Some basic information to help you begin:

What's visible | Degrees | Stellar Movement | Viewing Tips

Apparent Brightness

The brightness of stars and other celestial objects is measured on a scale of apparent brightness. Smaller numbers are brighter (negative numbers are the brightest). The scale assumes dark skies.

What's Visible Here are the planets and some of their moons, all visible with a good telescope, some visible with the naked eye. To find out which ones are favorably positioned for viewing tonight, see our Sky Calendar. Visible with naked eye Binoculars needed Telescope needed Large telescope needed (8" or bigger aperture) *Difficult to spot because of closeness to Mars.		OBJECT	BRIGHTNESS
		Mercury	-1.9
		Venus	-4.4
		Mars	-2.0
		---Phobos*	+11.3
		---Deimos*	+12.4
		Jupiter	-2.7
		---Io	+5.0
		---Europa	+5.3
		---Ganymede	+4.6
		---Callisto	+5.6
		Saturn	+0.7
		---Rhea	+9.7
		---Tethys	+10.2
		---Titan	+8.3
		---Iapetus	+10.2
		---Dione	+10.4
		---Enceladus	+11.7
		---Mimas	+12.9
		Uranus	+5.5
		Neptune	+7.8
		---Triton	+13.5
		Pluto	+13.6

Degrees

Astronomers use degrees to measure the apparent size of objects in the sky and the distances between them.

An easy way to estimate the size of a single degree is to begin by visualizing the full Moon. It's half a degree across, so two full Moons edge-to-edge equal a degree. The tip of your pinkie finger may just be about the right size when viewed along your outstretched arm.

To measure greater spans in the sky use your outstretched fist. From thumb to little finger, the average fist covers about 10 degrees.

Sense of scale, scope and movement

If you have dark skies, you've probably seen stars that are thousands of light-years away. Others are less than 10 light-years away. A light-year is the distance light travels in one year, moving at 186,000 miles per second.

Our Milky Way Galaxy is more than 100,000 light-years across. It contains billions of stars, but the most adept skywatchers can see no more than 6,000 of them without a telescope.

Skywatching can teach you about celestial mechanics. By watching the movement of the planets and stars between, say, dinner and bedtime, you can observ how the Earth spins on its axis.

The Moon, planets, and stars traverse the night sky (and the Sun the day sky) from east to west because the Earth rotates from west to east.

By noting the changing positions of stars and planets over the course of two or more nights, you can witness how the Earth's yearly motion around the Sun alters the positions of objects in the night sky. Stars near the sky's equator rise (and set) four minutes earlier each night, explains John Mosley of the Griffith Observatory near Los Angeles.

There's some nifty magic to the math here: Stars rise (and set) two hours earlier each month, and 24 hours earlier each year -- when the Earth has completed one orbit around the Sun.

The four-minute rule works for the planets Jupiter and Saturn, and often for Mars, Mosley says. Mercury and Venus orbit the Sun relatively quickly, though, and their paths are more complicated. Stars near the sky's north pole remain above the horizon throughout the year, and the North Star remains in the same position -- above the North Pole of the Earth.

Viewing Tips

The best viewing is always under the darkest skies. But even in a city, with a decent amateur telescope, you can see Saturn's rings, find Jupiter's moons, or explore the craters of our own Moon.

Whether you view the night sky with your naked eye, binoculars, or a telescope, here are a few tips to enhance the experience:

No matter how much light surrounds you, more is always worse. Turn off any lights you can. Also, allow 15 minutes or more for your eyes to adapt to the darkness.

One thing you can't control is the atmosphere. Even a cloud-free night isn't always great for viewing. When you look directly overhead, you're peering through about 7 miles of the troposphere, the densest portion of Earth's atmosphere; the light from an object nearer the horizon filters through dozens of miles of the troposphere. The result can be blurry stars.

Turbulent air, something you can't see or predict, can have an effect, too. It makes objects appear to jump around or twinkle. The bottom line: Telescopes will typically yield better results when a target is higher in the sky and when the atmosphere is calm.

Only experience will tell you which nights are best for viewing.

-- Robert Roy Britt, Senior Science Writer