In our day-to-day life, we usually measure distance in meters or kilometers or in miles. And it is easily possible with our regular scales or measuring tape. But if we talk about space and the universe, there we cannot use our regular distance measuring units (m, km, etc.) because space is very vast even we don’t know the endpoint of our universe so right now we can say that our universe is infinite. In this infinite universe, we cannot use meters and kilometers for distance measurement between two celestial points, at this scale m & km become very small units. That’s why we have to introduce new distance measurement units, especially for astronomical measurements.

**ASTRONOMICAL UNIT (AU)**

- Astronomical Unit (AU) is known as the distance between the Earth and Sun.
- 1 AU = 93,000,000 miles or 150,000,000 km
- In the reference of distance between Earth and Sun which is 1 AU, we can calculate distance of other planets from the sun. As depicted below,

Planets | Distance in AU |
---|---|

Mercury | 0.39 |

Venus | 0.72 |

Earth | 1.00 |

Mars | 1.52 |

Jupiter | 5.20 |

Saturn | 9.53 |

Uranus | 19.18 |

Neptune | 30.06 |

- But as technology increased and due to that we can able to explore more outside our solar system. So now AU also become small unit for distance measurement in space and new unit comes into the list known as “Light Year”.

2. **Light Year (ly)**

- One light-year is known as the distance travel by the Light in a period of 1 Year.
- To calculate that we need to know the speed of light, which is 186,000 miles/sec OR 300,000 km/sec. It means,
- 1 ly = 9.46 x 10^12 km
- Time is taken by the light to travel from the sun to the nearest celestial body is shown in the below table,

Planets | Time taken by light to travel from Sun |
---|---|

Mercury | 3.2 minutes |

Venus | 6.0 minutes |

Earth | 8.3 minutes |

Mars | 12.6 minutes |

Jupiter | 43.2 minutes |

Saturn | 79.3 minutes |

Uranus | 2.66 hours |

Neptune | 4.10 hours |

- And if we talk about the nearest star to our solar system is Proxima Centauri, which is 4.3 light-years away from us. It means if we start to travel at the speed of light in the year 2021, then it takes around 4.3 years (2025) to reach that star.

In conclusion, These are the two basic units which we use for distance measurement in the field of astronomy. which ease our calculations then meters and kilometers.

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