Correct Answer - Option 3 : Distance; Time
Length is a measure of the size or distance between two points.
- The SI unit of length is the meter (m).
- A light-year is a unit of distance.
- A light-year is the distance a beam of light travels in a single Earth year, or 6 trillion miles (9.7 trillion kilometres).
- One light-year is the distance travelled by light in a vacuum in one year.
- A light-year is a convenient unit of measurement of large distances.
- Generally, a light-year is used to represent astronomical distances.
- 1 light year = 9.46 × 1015 m.
Thus, the light-year is a measure of distance, not time.
Parsec (parallactic second):
- It is the largest practical unit of distance used in astronomy.
- It is defined as the distance at which an arc of the length of 1 astronomical unit subtends an angle of 1 second of arc.
- 1 parsec = 3.08 × 1016 m
- It is defined as the mean distance of the earth from the sun.
- It is used in astronomy to measure the distance of planets.
- 1 astronomical unit = 1 AU = 1.496 × 1011 m