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How big is the Solar System, and what units do we use to measure cosmic distances?

The scale of the Solar System and the astronomical distance units: the astronomical unit (AU), the light year (l.y.) and the parsec (pc).

A focused answer to Edexcel GCSE Astronomy statements 7.5 and 7.6, covering the scale of the Solar System and how to use the astronomical unit (1 AU = 1.5 x 10^8 km), the light year and the parsec to express astronomical distances, with conversions between them.

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  1. What this dot point is asking
  2. The scale of the Solar System
  3. The astronomical unit
  4. The light year and the parsec
  5. How Edexcel examines this
  6. Try this

What this dot point is asking

Edexcel statements 7.5 and 7.6 want you to use information about the scale of the Solar System, and to use the astronomical unit (1 AU=1.5×108 km1\,\text{AU} = 1.5 \times 10^8\,\text{km}), the light year (l.y.) and the parsec (pc) to express and convert astronomical distances.

The scale of the Solar System

Grasping the scale means recognising two jumps: from the Earth to the outer Solar System (tens of AU) and from the Solar System to the nearest stars (light years). This is why two different families of units are needed: AU for inside the Solar System, light years and parsecs for between the stars. The data sheet table of planetary distances (in AU) supports scale questions.

The astronomical unit

Using kilometres for Solar System distances would mean carrying around figures like 1.5×1081.5 \times 10^8 everywhere; the AU replaces that with simply "1" for the Earth. Historically the AU was pinned down using transits of Venus (Topic 11). It is the bridge unit: 11 light year is about 63000 AU63000\,\text{AU}, so the AU is far too small for interstellar distances.

The light year and the parsec

The light year is intuitive (a distance defined by light's travel time), and it also tells you how far back in time you are looking: light from a star 1010 light years away left it 1010 years ago. The parsec comes from parallax (Topic 13) and is the astronomer's preferred unit; the key conversion is 1 pc=3.261\,\text{pc} = 3.26 light years. You should be able to move between AU, light years, parsecs and kilometres, keeping numbers in standard form.

How Edexcel examines this

This is naked-eye Paper 1 content (and reused in Paper 2) with steady calculation marks. Expect conversions between AU, light years, parsecs and kilometres using the data-sheet values (1 AU=1.5×108 km1\,\text{AU} = 1.5 \times 10^8\,\text{km}, 1 l.y.=9.5×1012 km1\,\text{l.y.} = 9.5 \times 10^{12}\,\text{km}, 1 pc=3.1×1013 km=3.26 l.y.1\,\text{pc} = 3.1 \times 10^{13}\,\text{km} = 3.26\,\text{l.y.}), nearly always in standard form. A common explanation question asks why different units are used: large units keep huge numbers manageable, the AU for Solar System scales and the light year or parsec for interstellar distances. Scale questions use the planetary distances in AU from the data-sheet table. Synoptic links run forward to parallax and the parsec (Topic 13) and to comets in the Kuiper Belt and Oort Cloud (Topic 11). The biggest errors are treating a light year as a time and dropping standard form, so define the light year as a distance and keep the powers of ten tidy.

Try this

Q1. State what one astronomical unit (AU) is, and its value in kilometres. [1 mark]

  • Cue. The mean Earth-Sun distance, 1.5×108 km1.5 \times 10^8\,\text{km}.

Q2. State how many light years there are in one parsec. [1 mark]

  • Cue. 3.263.26 light years.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 1AS0 20224 marksA star is 4.0 parsecs from the Earth. Using 1 pc = 3.26 light years and 1 light year = 9.5 x 10^12 km, calculate the distance to the star in light years and in kilometres.
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First convert parsecs to light years: 4.0×3.26=13.04≈13 light years4.0 \times 3.26 = 13.04 \approx 13\,\text{light years} (2 marks). Then convert light years to kilometres: 13.04×9.5×1012=1.24×1014 km13.04 \times 9.5 \times 10^{12} = 1.24 \times 10^{14}\,\text{km} (accept about 1.2×1014 km1.2 \times 10^{14}\,\text{km}) (2 marks). Markers reward the parsec-to-light-year conversion giving about 13 light years and the light-year-to-kilometre conversion giving about 1.2×1014 km1.2 \times 10^{14}\,\text{km}, with correct use of standard form. Working directly from 1 pc=3.1×1013 km1\,\text{pc} = 3.1 \times 10^{13}\,\text{km} to get 4.0×3.1×1013=1.24×1014 km4.0 \times 3.1 \times 10^{13} = 1.24 \times 10^{14}\,\text{km} is also full marks.

Edexcel 1AS0 20213 marksExplain why astronomers use the astronomical unit (AU) for distances within the Solar System but the light year or parsec for distances to other stars.
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Distances in space are so large that kilometres become unwieldy, so astronomers use larger units to keep the numbers manageable (1 mark). The astronomical unit (the mean Earth to Sun distance, 1.5×108 km1.5 \times 10^8\,\text{km}) is a convenient size for distances within the Solar System, where separations are a few AU (1 mark). The light year (9.5×1012 km9.5 \times 10^{12}\,\text{km}) and parsec (3.1×1013 km3.1 \times 10^{13}\,\text{km}) are far larger and suit the huge distances to other stars, which are many light years away, so even the nearest star is over 4 light years off (1 mark). Markers reward using larger units to manage huge numbers, the AU for Solar System scales, and the much larger light year and parsec for interstellar distances.

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