What does that Mean?
By Alan MacRobert
Originally featured in SkyWatch 2001, the ultimate guide for beginning astronomers
Part of learning astronomy is learning the language.
Astronomy, like any other endeavor, has its own jargon. Newcomers can get thrown by such arcane-sounding terms as "arcsecond," "4th magnitude,"and "right ascension." But they're not as tough as they sound. Here's a quick rundown of the most important astronomy terms for you to know.
Beginners often have trouble describing distances in the sky. You may 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 six feet apart. . . ."
The problem here is that distances on the sky can't be expressed in linear units like feet or inches. The way to do it is by angular measure.
Astronomers might describe the two stars as 10 degrees apart. That means if lines were drawn from your eye to each star, the two lines would form an angle of 10 degrees at your eye.
Hold your fist at arm's length and sight past it with one eye. Your fist covers about 10 degrees of sky from side to side. A fingertip held at arm's length covers about 1 degrees. The Sun and Moon are each 1/2 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, and each arcminute is divided into 60 arcseconds.
If two objects lie a quarter degree apart, astronomers might note that as 15 arcminutes, abbreviated 15'. The brightest planets usually appear just a few dozen arcseconds across as seen from Earth. A 5" telescope can resolve details 1 arcsecond (1") across. This is the width of a penny seen at a distance of 2.5 miles.
From the Earth's surface, the night sky looks like a huge dome with stars stuck on its inside surface. If the Earth were swept out from under us, 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 lie on this celestial sphere.
Picture the Earth hanging at the center of it. Imagine the Earth's lines of latitude and longitude expanding outward and printing themselves on the inside of the celestial sphere. They 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.
There's a slight complication. These coordinates change slightly over the years, due to a slow shift of the Earth's orientation in space known as precession. When right ascension and declination are given in books and atlases, you might see them accompanied with a year date such as 1950 or 2000. This is the year for which the position is strictly correct.
The brightness of a star (or anything else in the sky) is called its magnitude. You'll encounter 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 "1st magnitude." Those a little fainter he termed "2nd magnitude," and so on down to the faintest ones he could see, "6th magnitude."
With the invention of the telescope in the 17th century, observers could see even fainter stars. Thus 7th, 8th, and 9th magnitudes were added. Today binoculars will show stars as faint as 9th magnitude, and an amateur's 6" telescope will go to about 13th. The largest and most sensitive telescopes used by professional astronomers can reach to about 29th magnitude.
It turns out that some of Hipparchus's "1st-magnitude" stars are 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 is -13, and the Sun shines at magnitude -27.
The Earth orbits (circles around) the Sun once a year at a distance averaging 93 million miles. That distance is called one astronomical unit (a.u.).
The distance light travels in a year, 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.
Most of the brightest stars in the sky lie a few dozen to a couple thousand light-years away. The nearest large galaxy beyond our own Milky Way is 2.5 million light-years distant.