Question

Which of the following has its unit as Parallactic second?
${\text{A}}{\text{.}}$ Time
${\text{B}}{\text{.}}$ Speed
${\text{C}}{\text{.}}$ Distance
${\text{D}}{\text{.}}$ None of these

Answer 1

- Hint- Here, we will proceed by defining the scientific term astronomy. Then, we will be discussing the measurement of astronomical distances and the units in which these are measured. We will also be defining one parallactic second.

Complete step-by-step solution -

Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation).
Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, originally for calendrical and astrological purposes and later for navigational uses and scientific interest. The catalogue of objects now studied is much broader and includes, in order of increasing distance, the solar system, the stars that make up the Milky Way Galaxy, and other, more distant galaxies.
A central undertaking in astronomy is the determination of distances. Without a knowledge of astronomical distances, the size of an observed object in space would remain nothing more than an angular diameter and the brightness of a star could not be converted into its true radiated power, or luminosity. Astronomical distance measurement began with a knowledge of Earth’s diameter, which provided a base for triangulation. Within the inner solar system, some distances can now be better determined through the timing of radar reflections or, in the case of the Moon, through laser ranging. For the outer planets, triangulation is still used. Beyond the solar system, distances to the closest stars are determined through triangulation, in which the diameter of Earth’s orbit serves as the baseline and shifts in stellar parallax are the measured quantities. Stellar distances are commonly expressed by astronomers in parallactic seconds.
1 parallactic second = 3.086$ \times {10^{16}}$ m = 3.26 light-years = 1.92$ \times {10^{13}}$ miles.
Therefore, parallactic second (parsec) is the unit of distance.
Hence, option C is correct.

Note- The distance at which a star would make parallax of one second of arc or the distance at which an arc equal to the length of one astronomical unit makes an angle of 1' (i.e. one second) is called parallactic second. The physical meaning of parallactic second is the distance at which the line drawn through Earth's centre and the Sun will intersect subtending an angle of 1’ (i.e., one second) with each other.