Cosmic Distances

No human probe has travelled much beyond the Solar System, yet we are able to measure distances across billions of light-years.

The space beyond Earth is so incredibly vast that units of measure, which are convenient in everyday lives, can become gigantic.
Distances between the planets, and especially between the stars, become so big when expressed in miles and kilometers.
Hence, for cosmic distances other types of units such as astronomical units, light years and parsecs are used.

All these methods are collectively known as the cosmic distance ladder.

Astronomical units (AU)

They are a useful unit of measure within our solar system.

One AU is the 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.

Therefore, astronomical units are a great way to compress truly astronomical numbers to a more manageable size.

Light year

A light year is the distance a photon of light travels in one year, which is 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.

The nearest star system to ours is the triple star system of Alpha Centauri, at about 4.3 light years away.
Light years also provide some helpful perspective on solar system distances: the Sun is about eight 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 is 3.26 light years.

Astronomers use megaparsecs, a megaparsec is 1 million parsecs, for intergalactic distances, or the scale of distances between the galaxies.
When distances between galaxies become so epic that even megaparsecs get unwieldy, redshift is used to measure the distance.

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.

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