Astronomy For Dummies. Maran Stephen P.

, or a complete circle around the sky. A minute of RA, called a minute of time, is a measure of angle on the sky that makes up

of an hour of RA. So you take

, or

. A second of RA, or a second of time, is 60 times smaller than a minute of time.

      ● Dec is measured in degrees, like the degrees in a circle, and in minutes and seconds of arc. A whole degree is about twice the apparent or angular size of the full Moon. Each degree is divided into 60 minutes of arc. The Sun and the full Moon are both about 32 minutes of arc (32’) wide, as seen on the sky, although, in reality, the Sun is much larger than the Moon. Each minute of arc is divided into 60 seconds of arc (60”). When you look through a backyard telescope at high magnification, turbulence in the air blurs the image of a star. Under good conditions (low turbulence), the image should measure about 1” or 2” across. That’s 1 or 2 arc seconds, not 1 or 2 inches.

A few simple rules may help you remember how RA and Dec work and how to read a star map (see Figure 1-3):

      ❯❯ The North Celestial Pole (NCP) is the place to which the axis of Earth points in the north direction. If you stand at the geographic North Pole, the NCP is right overhead. (If you stand there, say “Hi” to Santa for me, but beware: You may be on thin ice because there’s no land at the geographic North Pole.)

      ❯❯ The South Celestial Pole (SCP) is the place to which the axis of Earth points in the south direction. If you stand at the geographic South Pole, the SCP is right overhead. I hope you dressed warmly: You’re in Antarctica!

      ❯❯ The imaginary lines of equal RA run through the NCP and SCP as semicircles centered on the center of Earth. They may be imaginary, but they appear marked on most sky maps to help people find the stars at particular RAs.

      ❯❯ The imaginary lines of equal Dec, like the line in the sky that marks Dec of 30° North, pass overhead at the corresponding geographic latitudes. So if you stand in New York City, latitude 41° North, the point overhead is always at Dec 41° North, although its RA changes constantly as Earth turns. These imaginary lines appear on star maps, too, as declination circles.

      © John Wiley & Sons, Inc.

       FIGURE 1-3: Decoding the celestial sphere to find directions in space.

      Suppose you want to find the NCP as visible from your backyard. Face due north and look at an altitude of x degrees, where x is your geographic latitude. I’m assuming that you live in North America, Europe, or somewhere in the Northern Hemisphere. If you live in the Southern Hemisphere, you can’t see the NCP. You can, however, look for the SCP. Look for the spot due south whose altitude in the sky, measured in degrees above the horizon, is equal to your geographic latitude.

      In almost every astronomy book, the symbol ″ means seconds of arc, not inches. But at every university, a student in Astronomy 101 writes on a lab report, “The image of the star was about 1 inch in diameter.” Understanding beats memorizing every day, but not everyone understands.

      Here’s the good news: If you just want to spot the constellations and the planets, you don’t have to know how to use RA and Dec. Just consult a star map drawn for the current week or month (you can find these on the website of Sky & Telescope or one of the other magazines that I mention in Chapter 2, in the magazines themselves, or using a desktop planetarium program for your home computer or a planetarium app for your smartphone or tablet; I recommend programs, websites, and apps in Chapter 2 as well). But if you want to understand how star catalogs and maps work and how to zero in on faint galaxies with your telescope, understanding the system helps.

      And if you purchase one of those snazzy and surprisingly affordable telescopes with computer control (see Chapter 3), you can punch in the RA and Dec of a recently discovered comet, and the scope points right at it. (A little table called an ephemeris comes with every announcement of a new comet. It gives the predicted RA and Dec of the comet on successive nights as it sweeps across the sky.)

      Gravity: A Force to Be Reckoned With

      Ever since the work of Sir Isaac Newton, the English scientist (1642–1727), everything in astronomy has revolved around gravity. Newton explained gravity as a force between any two objects. The force depends on mass and separation. The more massive the object, the more powerful its pull. The greater the distance, the weaker the gravitational attraction. Newton sure was a smart cookie!

      Albert Einstein developed an improved theory of gravity, which passes experimental tests that Newton’s theory flunks. Newton’s theory was good enough for commonly experienced gravity, like the force that made the apple fall on his head (if it really hit him). But in other respects, Newton’s theory was hit or miss. Einstein’s theory is better because it predicts everything that Newton got right, but also predicts effects that happen close to massive objects, where gravity is very strong. Einstein didn’t think of gravity as a force; he considered it the bending of space and time by the very presence of a massive object, such as a star. I get all bent out of shape just thinking about it.

      Newton’s concept of gravity explains the following:

      ❯❯ Why the Moon orbits Earth, why Earth orbits the Sun, why the Sun orbits the center of the Milky Way, and why many other objects orbit one object or another out there in space

      ❯❯ Why a star or a planet is round

      ❯❯ Why gas and dust in space may clump together to form new stars

      Einstein’s theory of gravity, called the General Theory of Relativity, explains everything that Newton’s theory does plus the following:

      ❯❯ Why stars visible near the Sun during a total eclipse seem slightly out of position

      ❯❯ Why black holes exist

      ❯❯ Why gravitational lensing is found when we observe deep space

      ❯❯ Why Earth drags warped space and time around with it as it turns, an effect that scientists have verified with the help of satellites orbiting Earth

      ❯❯ How a collision of two black holes produces gravitational waves that shake things up even billions of light-years away

      You find out about black holes in Chapters 11 and 13, and you can read up on gravitational lensing in Chapters 11, 14, and 15 without mastering the General Theory of Relativity.

      You’ll get smarter if you read every chapter in this book, but your friends won’t call you Einstein unless you let your hair grow, parade around in a messy old sweater, and stick out your tongue when they take your picture.

      Space: A Commotion of Motion

      Everything in space is moving and turning. Objects can’t sit still. Thanks to gravity, other celestial bodies are always pulling on a star, planet, galaxy, or spacecraft. Some of us are self-centered, but the universe has no center.

      For example, Earth

      ❯❯ Turns on its axis – what astronomers call rotating – and takes one day to turn all the way around.

      ❯❯ Orbits around the Sun – what astronomers call revolving — with one complete orbit taking one year.

      ❯❯ Travels with the Sun