Astronomy Unit 2 Practice Test

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Anastronomical unit is the distance between Earth and the sun and measures distances on the scale of star systems. A parsec is a unit used to measure vast distances in interstellar space, such as distances between stars and galaxies, and is partially defined using an AU. One parsec is about 19 trillion miles (31 trillion km). It takes light 8.3 minutes to travel between Earth and the sun but 3.26 years to travel one parsec.

The AU remains the baseline for any trigonometric parallax measurements, so nearly all distances measured in the Milky Way (MW), such as from the ESA [European Space Agency] Gaia mission, are calibrated to the AU. Outside the MW, distances based on standard candles like Cepheids are also calibrated by parallax so also depend on the AU.

Because Earth's orbit around the sun is elliptical (oval-shaped), it isn't always the same distance from the sun. An astronomical unit represents a practical average, rather than a precise measurement, of our distance from the sun.

Another unit is the parsec, which is equal to about 3.26 light-years, according to NASA. The parsec is a more technical measurement that is derived from an astronomical unit and is used mainly by scientists.

Before 2012, the definition of an astronomical unit was not defined as a constant and depended on several factors. The IAU, the international group that defines astronomical constants, decided to make the measurement simpler in August of that year.

Why the change? One reason was that the previous method of calculating an AU depended on knowing the mass of the sun, but that measurement is always changing as the sun converts its mass into energy, Nature reported. Another is related to Einstein's theory of general relativity, which posits that space-time is relative to the observer's location. The current definition addresses this problem by basing the distance on the speed of light in a vacuum, which always remains constant.

The solar system extends for thousands of astronomical units away from the sun. Mercury, the closest planet to the sun, gets as near as 29 million miles (47 million km) in its elliptical orbit, while some objects in the Oort cloud, the solar system's icy shell, are thought to lie as far as 100,000 AU to 200,000 AU from the sun.

The first known person to measure the distance to the sun was Greek astronomer Aristarchus of Samos, who lived from about 310 B.C. to 230 B.C. He used the phases of the moon to measure the sizes and distances of the sun and moon.

He postulated that when the half moon appears in Earth's sky, the center of our planet and the center of the moon create a line in space that forms a 90-degree angle with another line that could be drawn through space from the moon's center all the way to the sun's center. Using trigonometry, Aristarchus determined the hypotenuse of a triangle based on those two imaginary lines. The value of the hypotenuse provided the distance between the sun and Earth.

Although his measurement was imprecise, Aristarchus provided a simple understanding of the sizes and distances of the three bodies, which led him to conclude that Earth goes around the sun about 1,700 years before Nicolaus Copernicus proposed his heliocentric model of the solar system.

In 1653, astronomer Christiaan Huygens calculated the distance from Earth to the sun. Much like Aristarchus, he used the phases of Venus to find the angles in a Venus-Earth-sun triangle. His more precise measurements for what exactly constitutes an AU were possible thanks to the existence of the telescope.

Guessing (correctly, by chance) the size of Venus, Huygens was able to determine the distance from Venus to Earth. Knowing that distance, plus the angles made by the triangle, he measured the distance from Earth to the sun. However, because Huygens' method was partly guesswork and not completely scientifically grounded, he usually doesn't get the credit.

In 1672, Giovanni Cassini used a method involving parallax, or angular difference, to find the distance to Mars and, at the same time, figured out the distance to the sun. He sent a colleague, Jean Richer, to Cayenne, French Guiana (located just northwest of the modern-day Guiana Space Center, near Kourou) while he stayed in Paris. At the same time, they both took measurements of the position of Mars relative to background stars, and triangulated those measurements with the known distance between Paris and French Guiana. Once they had the distance to Mars, they could also calculate the distance from Earth to the sun. Because his methods were more scientific, Cassini usually gets the credit.

"Expressing distances in the astronomical unit allowed astronomers to overcome the difficulty of measuring distances in some physical unit," Nicole Capitaine, an astronomer at the Paris Observatory, told Space.com in 2012. "Such a practice was useful for many years, because astronomers were not able to make distance measurements in the solar system as precisely as they could measure angles."

Mercury, the planet closest to the sun, gets as close as 29 million miles (47 million km) in its elliptical orbit, while objects in the Oort Cloud, the solar system's icy shell, are thought to lie as far as 9.3 trillion miles (15 trillion km).

The distance to the nearest star, Proxima Centauri, is about 268,770 AU, according to NASA. However, to measure longer distances, astronomers use light-years, or the distance that light travels in a single Earth-year, which is equal to 63,239 AU. So Proxima Centauri is about 4.25 light-years away.

Adam Riess is an astrophysicist who studies physical cosmology, measuring the universe using distance indicators such as supernovas (exploding stars) and Cepheids (pulsating stars). He also studies the expansion of the universe and was co-awarded the Nobel Prize in physics in 2011 for his role in discovering that the expansion rate of the universe is accelerating. "}), " -0-10/js/person.js"); } else console.error('%c FTE ','background: #9306F9; color: #ffffff','no lazy slice hydration function available'); Adam RiessProfessor of physics and astronomy at Johns Hopkins University, distinguished astronomer at the Space Telescope Science Institute, 2011 Nobel laureate in physicsAdam Riess is an astrophysicist who studies physical cosmology, measuring the universe using distance indicators such as supernovas (exploding stars) and Cepheids (pulsating stars). He also studies the expansion of the universe and was co-awarded the Nobel Prize in physics in 2011 for his role in discovering that the expansion rate of the universe is accelerating.

Watch a video explaining Aristarchus' approach to calculating the distance from Earth to the sun. NASA's sun fact sheet provides basic statistics about our star and its solar system exploration page offers details about solar science and missions studying the sun.

Rebecca Sohn is a freelance science writer. She writes about a variety of science, health and environmental topics, and is particularly interested in how science impacts people's lives. She has been an intern at CalMatters and STAT, as well as a science fellow at Mashable. Rebecca, a native of the Boston area, studied English literature and minored in music at Skidmore College in Upstate New York and later studied science journalism at New York University. "}), " -0-10/js/authorBio.js"); } else console.error('%c FTE ','background: #9306F9; color: #ffffff','no lazy slice hydration function available'); Rebecca SohnSocial Links NavigationContributing WriterRebecca Sohn is a freelance science writer. She writes about a variety of science, health and environmental topics, and is particularly interested in how science impacts people's lives. She has been an intern at CalMatters and STAT, as well as a science fellow at Mashable. Rebecca, a native of the Boston area, studied English literature and minored in music at Skidmore College in Upstate New York and later studied science journalism at New York University.

Before you take the Armed Services Vocational Aptitude Battery (ASVAB), study this selection of sample test questions. Each question corresponds to one of the topics you can expect to find on a real ASVAB test.

General science tests the ability to answer questions on a variety of science topics drawn from courses taught in most high schools. The life-science items cover botany, zoology, anatomy and physiology, and ecology. The earth and space science items are based on astronomy, geology, meteorology and oceanography. The physical science items measure force and motion mechanics, energy, fluids, atomic structure and chemistry.

Arithmetic reasoning tests the ability to solve basic arithmetic problems encountered in everyday life. One-step and multistep word problems require addition, subtraction, multiplication, division and choosing the correct order of operations when more than one step is necessary. The items include operations with whole numbers, operations with rational numbers, ratio and proportion, interest, percentage and measurement. Arithmetic reasoning is one factor that helps characterize mathematics comprehension, and it also assesses logical thinking.

Paragraph comprehension tests the ability to obtain information from written material. Students read different types of passages of varying lengths and respond to questions based on information presented in each passage. Concepts include identifying stated and reworded facts, determining a sequence of events, drawing conclusions, identifying main ideas, determining the author's purpose and tone, and identifying style and technique.

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