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A few things ya gotta know: part of learning astronomy is learning the language.

Like anything else, astronomy has its own jargon. Newcomers quickly run into terms like "arcsecond," "4th magnitude," and "right ascension." But they're easy to learn. Here's a quick rundown of some important astronomy terms you need to know.

Sky Measures

People often have trouble describing distances on the sky. You might get into a conversation that sounds like this:

"Do you see those two stars? The ones that look about eight inches apart?"

"Yeah, but they look more like 10 feet apart to me."

The problem here is that distances on the sky can't be expressed in linear measures like feet or inches. The way to do it is by angular measure.

Astronomers might say the two stars are 10 degrees (10[degrees]) apart. That means if lines were drawn from your eye to each star, the two lines would form a 10[degrees] angle at your eye.

Hold your fist at arm's length and sight past it with one eye. Your fist from side to side covers about 10[degrees] of sky. A fingertip at arm's length covers about 1[degrees]. The Sun and Moon are each 12[degrees] wide. The Big Dipper is 25[degrees] long. From the horizon to the point overhead (the zenith) is 90[degrees].

There are finer divisions of angular measure. A degree is made up of 60 arcminutes (60'), and each arcminute is made up of 60 arcseconds (60").

The brightest planets usually appear just a few dozen arcseconds across as seen from Earth. A 5-inch telescope can resolve details 1 arcsecond (1") across. This is the width of a penny seen 4 kilometers (2 1/2 miles) away.

Sky Coordinates

Seen from Earth, the night sky looks like a huge dome with stars stuck on its inside surface. If the Earth beneath us vanished, we'd see stars all around us--and we'd have the breathtaking sensation of hanging at the center of an immense, star-speckled sphere.

Astronomers designate the positions of stars by where they appear on this celestial sphere.

Picture the Earth hanging at the center of the celestial sphere. Imagine the Earth's latitude and longitude lines expanding outward and printing themselves on the celestial sphere's inside. The lines now provide a coordinate grid on the sky that tells the position of any star, just as latitude and longitude tell the position of any point on Earth. In the sky, "latitude" is called declination and "longitude" is called right ascension.

Declination is expressed in degrees, arcminutes, and arcseconds north (+) or south (-) of the celestial equator.

Right ascension is expressed not in degrees but in hours (h), minutes (m), and seconds (s) of time, from 0 to 24 hours. Astronomers set up this arrangement long ago because the Earth completes one turn in about 24 hours, so the celestial sphere appears to take about 24 hours to complete one turn around Earth.


The brightness of a star (or anything else in the sky) is called its magnitude. You'll meet this term often.

The magnitude system began about 2,100 years ago when the Greek astronomer Hipparchus divided stars into brightness classes. He called the brightest ones "of the 1st magnitude," simply meaning "the biggest." Those a little fainter he called "2nd magnitude," and so on down to the faintest ones he could see: "6th magnitude."

With the invention of the telescope, observers could spot even fainter stars. Thus 7th, 8th, and 9th magnitudes were added. Today good binoculars will show stars as faint as 9th magnitude, and an amateur's 6-inch telescope will go to 12th or 13th.

It turned out that some of Hipparchus's "1st-magnitude" stars are a lot brighter than others. To accommodate them, the scale now extends into negative numbers. Vega is zero (0) magnitude, and Sirius, the brightest star in the sky, is magnitude -1.4. Venus is even brighter, usually magnitude -4. The full Moon shines at magnitude -13 and the Sun at -27.


The Earth orbits (circles around) the Sun once a year at a distance from the Sun averaging 150 million kilometers, or 93 million miles. This distance is called one astronomical unit (a.u.). This is the basic yardstick of the solar system. Jupiter, for instance, is 5 a.u. from the Sun. Pluto averages 40 a.u. from the Sun.

The distance that light travels in a year--9.5 trillion kilometers, or 6 trillion miles, or 63,000 a.u.--is called a light-year. Note that the light-year is a measure of distance, not time ... just like kilometers or miles. The nearest star, Alpha Centauri, is 4 light-years away. Most of the brightest stars in the sky lie a few dozen to a few hundred light-years away.

Professional astronomers often use another unit for big distances: the parsec. One parsec equals 3.26 light-years. A kiloparsec is 1,000 parsecs, and a megaparsec is a million parsecs.

Our Place in Space

To understand astronomy, you need a framework to know where everything fits. Here's a quick rundown.

The planets of our solar system are actually very small compared to the huge distances between them. In a scale model, if the Sun were a beach ball 31 inches wide, the Earth would be a pea 93 yards away from it, and Venus, the planet that comes closest to Earth, would be another pea no closer to us than 25 yards. Pluto would be a sesame seed 2.1 miles away.

The nearest star (Alpha Centauri) would be another beach ball 15,000 miles away. Clearly, most of space is very empty indeed.

Our solar system, all the stars we see, and about 200 billion other stars make up our Milky Way galaxy. The galaxy is a great pancake-shaped swarm of stars about 100,000 light-years wide and a few thousand thick.

The next-nearest large galaxy is the Great Andromeda Galaxy, 2.5 million light-years away. Others are scattered in groups and galaxy clusters through space as far as we can see--and no doubt much farther beyond.

Star Basics

Stars are the most basic constituents of the universe. Even the nearest stars look like mere pinpoints, but they come in an incredible range of sizes, brightnesses, and temperatures. The Sun, middling size and yellow-white-hot, is a more or less average main-sequence star. Stars spend most of their lives burning quietly on the "main sequence" of stellar evolution. Those in this stage of life range from dim red dwarfs (actually pale orange) tens of thousands of times dimmer than the Sun to brilliant, blue-white-hot orbs nearly a million times brighter that the Sun. All stars are classified by their spectral type, which depends mostly on a star's temperature but also on its size, age, and other characteristics.

As a star ages, it eventually swells up and becomes a red giant or supergiant (again, more orange than red), like Betelgeuse or Antares. A very massive star ends its life by exploding as a supernova; a less-massive star such as the Sun will end up as a white dwarf.

Telescope Basics

And what about the tools to see these wonderful sights? The most important thing about a telescope is its aperture, which is the diameter of its large, main objective lens or mirror. The bigger the aperture, the fainter the telescope can see, and the sharper the images it can provide (assuming its optics are good).

The focal length of the objective is, more or less, the distance from the objective to the image that it forms. The focal ratio or f/number (such as f/4 or f/8) is simply the focal length divided by the aperture.

The eyepiece is the little magnifying-lens assembly you look at the image with. You can change a telescope's power (magnification) by switching eyepieces. An eyepiece has its own focal length. To find the telescope's power with any given eyepiece, divide the objective's focal length by the eyepiece's focal length.

For instance, a 4-inch f/10 telescope has a focal length of 40 inches. Used with an eyepiece of 1-inch (25-millimeter) focal length, it will magnify 40x.

Now, was that so hard?

A Sky & Telescope senior editor, Alan MacRobert took forever to learn stuff like this when he was a kid.
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Title Annotation:observing
Author:MacRobert, Alan M.
Date:Jan 1, 2004
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