Astronomers measuring the scale of the Milky Way estimate that its central disk spans roug… — Armed Services Vocational Aptitude Battery (ASVAB)

Astronomers measuring the scale of the Milky Way estimate that its central disk spans roughly $100{}000$ light-years. What is a light-year?

The total distance that light travels through space in one Earth yearAnswer
B
The amount of time required for sunlight to reach the outer edge of our galaxy
C
The constant speed at which electromagnetic radiation moves through a vacuum
D
The time it takes for Earth to complete one full revolution around the Sun

The total distance that light travels through space in one Earth year

A light-year is defined as the total distance that a beam of light travels through a vacuum in one Julian year, equal to roughly

9.46 \times 10^{12}

kilometers (nearly

6

trillion miles).

Identify the physical quantity measured by a light-year

A light-year is a measure of distance, not time or speed.

Because distances in space are immensely large, astronomers use the distance light travels over time as a practical unit of measurement.

Apply the astronomical definition of the unit

Traveling at roughly

300{}000\text{ km/s}

(

186{}000\text{ mi/s}

), light covers approximately

9.46 \times 10^{12}\text{ km}

(

5.88 \times 10^{12}\text{ miles}

) in one Earth year.

By definition, one light-year equals speed of light multiplied by the duration of one Earth year.

Astronomical units of distance and the definition of a light-year

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