# Measurement Units in Space

## Distances

Distances in space are so large that the measurements we use on Earth aren't often practical. We can use kilometres or miles to measure distances within our Solar System, but the rest of the Universe requires a step up.

Note: Metric is the official measurement system of astronomy. The base unit of distance is the metre. In academic or engineering situations, metres are often shown using scientific notation, e.g. 2.5 × 107 m, but you don't need to worry about that if you don't want to. For everyday use, kilometres (km) are often used for distances up to the size of the Solar System, but astronomers tend to prefer to use astronomical units, light-years and parsecs.

### Astronomical Unit (au or AU)

One AU is the average distance from the Sun to the Earth, approximately 150 million km. For example, Saturn orbits the Sun at a distance of 9 au, or nine times as far away from the Sun as Earth.

### Light-year (ly)

The most common measurement unit for astronomical distances is the light-year. The name often confuses people—remember that a light-year is a measurement of length/distance, not time. It means the distance that a photon of light can travel through the vacuum of space in one Earth year. That's roughly 9.5 trillion km (9,460,000,000,000 km or 9.46 × 1015 m or 5.9 trillion miles).

We can also use other units based on the travel time of light, for example:

• Light-seconds = The distance a photon travels in one second.
• Light-minutes = The distance a photon travels in one minute.
• Light-days = The distance a photon travels in one Earth day.

### Parsec

The parsec is an unusual unit. It's based on trigonometry and seems like a strange way to measure distance. It's also only about 3.3 times as long as the light-year, so it's used for the same kind of measurements, which almost makes it redundant. However there is a good reason for astronomers to like the parsec, as it makes certain calculations based on telescope observations easier. We don't need to concern ourselves with that at this stage though. All you need to know is that one parsec is roughly equal to 31 trillion km (19 trillion miles), or about 210,000 astronomical units, or 3.3 light-years.

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