Light year is a unit for measurement of
- (a)age of universe
- (b)very small time intervals
- (c)very high temperature
- (d)very large distance
Correct — D, very large distance. A light year is the distance light travels in vacuum in one year — about 9·46 million million kilometres, or roughly 9·46 × 10¹² km. The word 'year' in the name is doing the damage: it names the time over which the travelling is counted, not the quantity being measured. Multiply a speed by a time and you get a distance, which is all that is happening here. Astronomers use the unit because the speed of light in vacuum is a fixed constant everywhere in the universe, so a light year is the same length whoever measures it and wherever they are — UPSC has set a prelims item on exactly that reasoning.
- (a)age of universe — The age of the universe is a time, quoted in years — currently estimated at about 13·8 billion years. It is easy to confuse with distance because the two are linked in cosmology, since light from an object 13 billion light years away left it 13 billion years ago, but the light year itself is a length.
- (b)very small time intervals — Small intervals are measured in milliseconds, microseconds, nanoseconds and so on, and in the laboratory in femtoseconds. Nothing about the light year is small, and nothing about it is a time.
- (c)very high temperature — Temperature is measured in kelvin, and stellar interiors run into millions of kelvin. No unit built from a speed and a time can measure temperature.
Astronomy needs several length units because the scales involved differ so enormously. The astronomical unit, about 149·6 million kilometres, is the mean Earth-Sun distance and is used inside the Solar System. The light year, about 9·46 × 10¹² kilometres, is used for stars and galaxies. The parsec, about 3·26 light years, is the professional astronomer's unit and is defined through stellar parallax. All three measure length; none measures time.
This is the classic trick question of units, and it works because of a linguistic accident rather than any real difficulty. Two related traps sit close by. First, the light year is not a unit of speed either — the speed is already fixed, and it is the year that is being multiplied in. Second, the astronomical unit is often misremembered as an Earth-Moon distance; it is Earth to Sun. A useful ladder to fix in memory: light takes about 1·3 seconds to reach us from the Moon, about 8 minutes from the Sun, and about 4·2 years from Proxima Centauri, the nearest star beyond the Sun. That last figure is why interstellar distances need a unit of this size at all — expressing them in kilometres would mean writing thirteen digits every time.
- One light year is about 9·46 × 10¹² kilometres — the distance light travels in vacuum in a year.
- One astronomical unit is about 149·6 million kilometres, the mean Earth-Sun distance.
- One parsec is about 3·26 light years and is defined through stellar parallax.
- Light takes about 8 minutes to reach the Earth from the Sun and about 4·2 years from the nearest star beyond it.
Every unit in the top three rows is a length, however time-like its name sounds.
- Reading 'year' in the name and answering with a time.
- Confusing the astronomical unit with the Earth-Moon distance.
- Treating a light year as a unit of speed.
This is one of NDA's most repeated single-line items, appearing in 2017-II, 2018-I and 2021-I within five years, sometimes as a statement code.
Which one of the following is a reason why astronomical distances are measured in light-years?
- (a) Distances among stellar bodies do not change.
- (b) Gravity of stellar bodies does not change.
- (c) Light always travels in straight line.
- (d) Speed of light is always same.
Answer(d) Speed of light is always same.
UPSC assumes the light year is a distance and asks the next question — why that unit is chosen — which is the constancy of the speed of light.
One Astronomical Unit is the average distance between
- (a) Earth and the Sun
- (b) Earth and the Moon
- (c) Jupiter and the Sun
- (d) Pluto and the Sun
Answer(a) Earth and the Sun
The other astronomical length unit and its own standing trap, useful to learn alongside the light year so the two are never swapped.
Light year is a measure of
- (a) time
- (b) distance
- (c) total amount of light falling on the Earth in a year
- (d) average intensity of light falling on the Earth in a year
Answer(b) distance
The same question in the previous session, with 'time' as the leading distractor — the identical trap this 2018 item rebuilds.
Consider the following statements about Light year: 1. Light year is a unit for measurement of very large distances. 2. Light year is a unit for measurement of very large time intervals. 3. Light year is a unit for measurement of intensity of light. Which of the statements given above is/are correct?
- (a) 1, 2 and 3
- (b) 2 and 3 only
- (c) 1 and 2 only
- (d) 1 only
Answer(d) 1 only
The statement-code version three years later, which shows NDA returns to this unit again and again and always with time and intensity as the wrong options.
- practice — not a real PYQ
One astronomical unit is the average distance between
- (a)the Earth and the Moon
- (b)the Earth and the Sun
- (c)the Sun and the nearest star
- (d)two neighbouring galaxies
Answer(b) the Earth and the Sun — about 149·6 million kilometres.
- practice — not a real PYQ
One parsec is approximately equal to
- (a)3·26 light years
- (b)1 light year
- (c)9·46 light years
- (d)365 light years
Answer(a) 3·26 light years — the parsec is the larger of the two units.