OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Cosmology: (ly) and parsec
The parsec (pc) is a cosmological distance unit defined using parallax: one parsec is the distance at which one astronomical unit subtends an angle of one arcsecond.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 17
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The parsec (pc) is a cosmological distance unit defined using parallax: one parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. It equals about 3.09 × 10¹⁶ m, or about 3.26 light-years. The light-year (ly) is the distance light travels in a vacuum in one year, about 9.46 × 10¹⁵ m. To convert, multiply parsecs by 3.26 to get light-years, or multiply light-years by 0.307 to get parsecs. For example, a star at 10 pc is about 32.6 ly away, or about 3.09 × 10¹⁷ m. Parsecs are preferred for stellar distances because they connect directly to parallax angle measurements.
What earns the marks
- One parsec (pc) is the distance at which one astronomical unit subtends an angle of one arcsecond.
- One parsec equals approximately 3.09 × 10¹⁶ m, or about 3.26 light-years.
- One light-year (ly) is the distance light travels in a vacuum in one year, approximately 9.46 × 10¹⁵ m.
- Convert parsecs to light-years by multiplying by 3.26, and light-years to parsecs by multiplying by about 0.307.
- The parsec is defined through parallax geometry, linking distance to the measured parallax angle.
Common mistakes and how to put them right
Treating the parsec as a unit of time or as a parallax angle.
Put it right: The parsec is a distance derived from the parallax angle of one arcsecond.
Using 1 pc = 1 ly.
Put it right: 1 pc ≈ 3.26 ly, so the two units differ by a factor of about 3.26.
Confusing arcseconds with degrees when applying the parallax definition.
Put it right: One arcsecond is 1/3600 of a degree.
Forgetting to convert between parsecs and light-years before comparing distances.
Put it right: Express both distances in the same unit first.
Exam tips
- Learn the approximate conversions 1 pc ≈ 3.26 ly and 1 pc ≈ 3.09 × 10¹⁶ m for quick estimates.
- When a question gives a parallax angle in arcseconds, remember d (in pc) = 1 / p (in arcseconds) for small angles.
- State which distance unit you are using in the final answer to avoid ambiguity.
Try the lesson
The start of the Cosmology: (ly) and parsec lesson, made from checked teaching notes for this specification point.
Listen
The parsec (pc) is a cosmological distance unit defined using parallax: one parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. It equals about 3.09 × 10¹⁶ m, or about 3.26 light-years. The light-year (ly) is the distance light travels in a vacuum in one year, about 9.46 × 10¹⁵ m. To convert, multiply parsecs by 3.26 to get light-years, or multiply light-years by 0.307 to get parsecs. For example, a star at 10 pc is about 32.6 ly away, or about 3.09 × 10¹⁷ m. Parsecs are preferred for stellar distances because they connect directly to parallax angle measurements. A strong answer earns marks for these points. One parsec (pc) is the distance at which one astronomical unit subtends an angle of one arcsecond. One parsec equals approximately 3.09 × 10¹⁶ m, or about 3.26 light-years. One light-year (ly) is the distance light travels in a vacuum in one year, approximately 9.46 × 10¹⁵ m. Convert parsecs to light-years by multiplying by 3.26, and light-years to parsecs by multiplying by about 0.307. A common slip: Treating the parsec as a unit of time or as a parallax angle. Instead, the parsec is a distance derived from the parallax angle of one arcsecond. Exam tip: Learn the approximate conversions 1 pc ≈ 3.26 ly and 1 pc ≈ 3.09 × 10¹⁶ m for quick estimates.
Key words
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Parallax and the parsec
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Light-year as a distance unit
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Interstellar distance scales
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Unit conversion between parsecs, light-years and metres
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See every definition
- Parallax and the parsec
- The parsec is a cosmological distance unit defined using parallax: the distance at which one astronomical unit subtends an angle of one arcsecond.
- Light-year as a distance unit
- The distance light travels in a vacuum in one year, about 9.46 × 10¹⁵ m.
- Interstellar distance scales
- Both units are used for interstellar distances, with parsecs common in professional astronomy and light-years common in popular science.
- Unit conversion between parsecs, light-years and metres
- Multiply parsecs by 3.26 to get light-years, or multiply light-years by 0.307 to get parsecs; one parsec equals about 3.09 × 10¹⁶ m.
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