AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Absolute magnitude, M (A-level only): Parsec and light year.
Astronomical distances are so large that the metre is impractical, so two units are defined.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Astronomical distances are so large that the metre is impractical, so two units are defined. A light year is the distance electromagnetic radiation travels through a vacuum in one year; since c = 3.00 × 10⁸ m s⁻¹ and one year ≈ 3.16 × 10⁷ s, 1 ly ≈ 9.46 × 10¹⁵ m. A parsec is defined geometrically: it is the distance at which 1 astronomical unit subtends an angle of 1 arcsecond, so 1 pc = 1 au / tan(1″) ≈ 3.09 × 10¹⁶ m, about 3.26 ly. Astronomers use parallax: a star shifts against the background as Earth orbits, and d = 1/p with d in parsecs when p is in arcseconds. Both units measure distance, not time or brightness, and both must be converted to metres before substitution into formulae such as d = 1/p or flux calculations.
What earns the marks
- Defines the light year as the distance travelled by electromagnetic radiation in a vacuum in one year, and converts using distance = speed × time with c = 3.00 × 10⁸ m s⁻¹.
- Defines the parsec as the distance at which 1 au subtends an angle of 1 arcsecond, linking the unit to the parallax method.
- Uses the small-angle relation d = 1/p with p in arcseconds to give d in parsecs, and converts between parsecs, light years and metres.
- States that 1 pc ≈ 3.26 ly and 1 pc ≈ 3.09 × 10¹⁶ m, using these to compare astronomical distances.
- Distinguishes distance units from time units and from apparent magnitude, avoiding treating a light year as a time.
Common mistakes and how to put them right
Treating the light year as a unit of time.
Put it right by stating it is a distance found from speed × time.
Using the parsec as a time or brightness unit.
Put it right by linking it to the parallax angle of 1 arcsecond.
Substituting a parallax angle in degrees into d = 1/p.
Put it right by converting the angle to arcseconds first.
Exam tips
- Write the defining geometry or the speed–time calculation before substituting numbers, so the examiner sees the reasoning.
- Convert all distances to metres before using them in formulae involving c, G or flux.
- Check whether an angle is given in degrees, arcminutes or arcseconds and convert to arcseconds for d = 1/p.
Try the lesson
The start of the Absolute magnitude, M (A-level only): Parsec and light year. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following is the definition of a light year?
Listen
Astronomical distances are so large that the metre is impractical, so two units are defined. A light year is the distance electromagnetic radiation travels through a vacuum in one year; since c = 3.00 × 10⁸ m s⁻¹ and one year ≈ 3.16 × 10⁷ s, 1 ly ≈ 9.46 × 10¹⁵ m. A parsec is defined geometrically: it is the distance at which 1 astronomical unit subtends an angle of 1 arcsecond, so 1 pc = 1 au / tan(1″) ≈ 3.09 × 10¹⁶ m, about 3.26 ly. Astronomers use parallax: a star shifts against the background as Earth orbits, and d = 1/p with d in parsecs when p is in arcseconds. Both units measure distance, not time or brightness, and both must be converted to metres before substitution into formulae such as d = 1/p or flux calculations. A strong answer earns marks for these points. Defines the light year as the distance travelled by electromagnetic radiation in a vacuum in one year, and converts using distance = speed × time with c = 3.00 × 10⁸ m s⁻¹. Defines the parsec as the distance at which 1 au subtends an angle of 1 arcsecond, linking the unit to the parallax method. Uses the small-angle relation d = 1/p with p in arcseconds to give d in parsecs, and converts between parsecs, light years and metres. States that 1 pc ≈ 3.26 ly and 1 pc ≈ 3.09 × 10¹⁶ m, using these to compare astronomical distances. A common slip: Treating the light year as a unit of time. Instead, stating it is a distance found from speed × time. Exam tip: Write the defining geometry or the speed–time calculation before substituting numbers, so the examiner sees the reasoning.
Key words
Say what each word means before you turn the card.
Light year
Try answering aloud or in your head before turning the card.
Parsec
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Parallax
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d = 1/p
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See every definition
- Light year
- The distance electromagnetic radiation travels through a vacuum in one year, found from speed × time with c = 3.00 × 10⁸ m s⁻¹.
- Parsec
- The distance at which 1 astronomical unit subtends an angle of 1 arcsecond, defined geometrically and linked to the parallax method.
- Parallax
- A star shifts against the background as Earth orbits, and d = 1/p with d in parsecs when p is in arcseconds.
- d = 1/p
- The small-angle relation giving d in parsecs when p is in arcseconds, with the angle converted to arcseconds first.
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