Context:
No human probe has travelled much beyond the Solar System, yet we are able to measure distances across billions of light-years.
Cosmic Distance Ladder:
- Distances between the planets and between stars expressed in miles and kilometers.
- Cosmic distances are measured in astronomical units, light years and parsecs.
- Collectively known as the cosmic distance ladder.
Astronomical units (AU):
- 1 AU – distance from the Sun to Earth’s orbit, which is about 93 million miles.
- When measured in astronomical units, the distance from the Sun to Saturn’s orbit, is 9.5 AU.
Light year:
- A light year is the distance a photon of light travels in one year (about 6 trillion miles)
- A light year is how far one can travel in a year if they could travel at the speed of light, which is 186,000 miles per second.
- Nearest star system Alpha Centauri – 4.3 light years away.
- Used to find Sun is about 8 light minutes from Earth.
Parsecs:
- This is the unit used when the number of light years between objects climbs into the high thousands or millions.
- One parsec – 3.26 light years.
- Megaparsecs (1 million parsecs) – scale of distances between the galaxies.
Astronomers talk about distances in terms of how much a galaxy’s light has been shifted toward longer, redder wavelengths by the expansion of the universe, a measure known as redshift.
Source: The Hindu
Previous Year Question
Which one of the following is a reason why astronomical distances are measured in light-years?
[UPSC Civil Services Exam – 2021 Prelims]
(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)
