One parsec in astronomical units (A.U.) is about
- (a)2 × 10³ A.U.
- (b)2 × 10⁴ A.U.
- (c)2 × 10⁵ A.U.
- (d)2 × 10⁶ A.U.
Correct — C, about 2 × 10⁵ A.U. A parsec is the distance at which one astronomical unit (the Earth–Sun distance) subtends an angle of one arcsecond. Working that out gives 1 parsec ≈ 206,265 A.U., which rounds to about 2 × 10⁵ astronomical units.
- (a)2 × 10³ A.U. — This is a hundred times too small — 2,000 A.U. is far short of the roughly 206,000 A.U. in a parsec.
- (b)2 × 10⁴ A.U. — 20,000 A.U. is about ten times too small; a parsec is of the order of 200,000 A.U., not 20,000.
- (d)2 × 10⁶ A.U. — 2,000,000 A.U. is about ten times too large — that overshoots the roughly 206,000 A.U. that make up one parsec.
Astronomers use several distance units. The astronomical unit (A.U.) is the mean Earth–Sun distance, about 1.5 × 10⁸ km. A parsec is defined by parallax: it is the distance from which one A.U. would appear to span one arcsecond of angle. That geometry fixes it at roughly 206,265 A.U.
The question is really about orders of magnitude. Because a parsec comes from a very small angle (one arcsecond, i.e. 1/3600 of a degree), the distance must be very large in A.U. — of the order of 10⁵. Only option (c) is in that range; the others are off by factors of ten to a thousand.
- One parsec is about 206,265 astronomical units, i.e. roughly 2 × 10⁵ A.U.
- A parsec is the distance at which one A.U. subtends an angle of one arcsecond (parallax definition).
- One parsec is also about 3.26 light-years, or about 3.09 × 10¹³ km.
- One astronomical unit is the mean Earth–Sun distance, about 1.496 × 10⁸ km.
A parsec comes from a one-arcsecond parallax angle, making it about 2 × 10⁵ astronomical units — hence option (c).
- Judge by order of magnitude — a parsec is about 10⁵ A.U., so options off by ten or a thousand are wrong.
- Do not confuse the parsec (about 3.26 light-years) with the light-year itself.
Distance units are asked as 'convert or rank A.U., light-year and parsec', usually rewarding a sense of their orders of magnitude.
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
Same family of concepts — the astronomical unit, which is the very yardstick a parsec is built from. UPSC pinned the A.U. to the Earth–Sun distance; the NDA question then asks how many such A.U. make one parsec.
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.
A companion distance-unit concept — the light-year. UPSC tested why light-years are used (the constant speed of light); a parsec is about 3.26 light-years, so the two units go hand in hand.
- practice — not a real PYQ
One parsec is approximately equal to
- (a)1 light-year
- (b)3.26 light-years
- (c)0.5 light-year
- (d)10 light-years
Answer(b) 3.26 light-years — a parsec is about 3.26 times a light-year.
- practice — not a real PYQ
The astronomical unit (A.U.) is defined as the average distance between
- (a)the Earth and the Moon
- (b)the Sun and Jupiter
- (c)the Earth and the Sun
- (d)the Sun and the nearest star
Answer(c) the Earth and the Sun — about 1.5 × 10⁸ km.