Distances in Space

The idea

Space is mostly space. Ordinary units fail immediately, so astronomers use units scaled to what they are measuring.

Unit	Length	Used for
Astronomical unit (AU)	$1.50\times 10^8\mathrm{km}$	Within the solar system
Light-year (ly)	$9.46\times 10^{12}\mathrm{km}$	Between stars
Parsec (pc)	$3.26\mathrm{ly}$	Professional astronomy

A light-year is a distance, not a time

$$d=vt=(3.00\times 10^{5}km/s)(3.15\times 10^7\mathrm{s})=9.46\times 10^{12}\mathrm{km}$$

Scale it down to something you can walk

Shrink the Sun to the size of a basketball (24 cm):

Object	Scaled distance
Earth	26 m away, and 2 mm across
Jupiter	134 m away
Neptune	775 m away
Proxima Centauri	7 000 km away — Toronto to Lisbon

This is the fact to take away

On a scale where the entire solar system fits on a school field, the nearest other star is on another continent. Every science-fiction voyage quietly ignores this.

Looking out is looking back

Light takes time. The Andromeda galaxy is 2.5 million light-years away, so you see it as it was 2.5 million years ago. Telescopes are time machines pointed the only direction they can go.

Curriculum connection

E2.5

quantify distances in the solar system and the universe by applying an understanding of relative distances and sizes and using appropriate units of measure

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E2.4

describe major components of the solar system and the universe and compare their characteristics

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