Astronomy For Dummies. Maran Stephen P.
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Название: Astronomy For Dummies

Автор: Maran Stephen P.

Издательство: Автор

Жанр: Зарубежная образовательная литература

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isbn: 9781119374381

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СКАЧАТЬ them from the wandering planets. But in fact, stars are in constant motion as well, both real and apparent. The whole sky rotates overhead because Earth is turning. The stars rise and set, like the Sun and the Moon, but they stay in formation. The stars that make up the Great Bear don’t swing over to the Little Dog or Aquarius, the Water Bearer. Different constellations rise at different times and on different dates, as seen from different places around the globe.

      Actually, the stars in Ursa Major (and every other constellation) do move with respect to one another – and at breathtaking speeds, measured in hundreds of miles per second. But those stars are so far away that scientists need precise measurements over considerable intervals of time to detect their motions across the sky. So 20,000 years from now, the stars in Ursa Major will form a different pattern in the sky. (Maybe they will even look like a Great Bear.)

      In the meantime, astronomers have measured the positions of millions of stars, and many of them are tabulated in catalogs and marked on star maps. The positions are listed in a system called right ascension and declination – known to all astronomers, amateur and pro, as RA and Dec:

      ❯❯ The RA is the position of a star measured in the east–west direction on the sky (like longitude, the position of a place on Earth measured east or west of the prime meridian at Greenwich, England).

      ❯❯ The Dec is the position of the star measured in the north–south direction, like the latitude of a city, which is measured north or south of the equator.

      Astronomers usually list RA in units of hours, minutes, and seconds, like time. We list Dec in degrees, minutes, and seconds of arc. Ninety degrees make up a right angle, 60 minutes of arc make up a degree, and 60 seconds of arc equal a minute of arc. A minute or second of arc is also often called an “arc minute” or an “arc second,” respectively.

      DIGGING DEEPER INTO RA AND DEC

      A star at RA 2h00m00s is 2 hours east of a star at RA 0h00m00s, regardless of their declinations. RA increases from west to east, starting from RA 0h00m00s, which corresponds to a line in the sky (actually half a circle, centered on the center of Earth) from the North Celestial Pole to the South Celestial Pole. The first star may be at Dec 30° North, and the second star may be at Dec 15° 25’12” South, but they’re still 2 hours apart in the east–west direction (and 45° 25’12” apart in the north–south direction). The North and South Celestial Poles are the points in the sky – due north and due south – around which the whole sky seems to turn, with the stars all rising and setting.

      Note the following details about the units of RA and Dec:

      ● An hour of RA equals an arc of 15 degrees on the equator in the sky. Twenty-four hours of RA span the sky, and

, 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!

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