What’s bigger: LD, AU or Light-Year?

~2 min
What’s bigger: LD, AU or Light-Year?

What does lunar distance mean? What are astronomical units used for? How far is a light-year? In today’s article, we’ll give you brief and simple answers to these questions.

You can also check out our infographic to learn more about the units of distance used in astronomy.

What does lunar distance mean?

In astronomy, lunar distance (LD) is the average distance from the center of the Earth to the center of the Moon, which is 384,400 kilometers (238,855 miles). For reference, this equals about 30 diameters of the Earth.

Why is the average distance used? As the Moon’s orbit is not perfectly circular, our natural satellite is not always the same distance from the Earth. At an average perigee (the closest point to the Earth), the Moon is 363,228 kilometers (225,700 miles) away from our planet, and at an average apogee (the farthest point from the Earth), it’s 405,400 kilometers (251,900 miles) away.

Lunar distance is mainly used to express the distance to near-Earth objects, like asteroids and comets. For example, the asteroid 2001 FO32, which flew by the Earth in March 2021, was five lunar distances away from us.

What is an astronomical unit?

One astronomical unit (AU) is the average distance from the Earth to the Sun, which is about 150 million kilometers (93 million miles). As the Earth’s orbit is elliptical, our planet’s actual distance to the Sun varies by about 3% around the year – that’s why the average distance is used.

The astronomical unit is commonly used to measure distances between objects within the Solar System or around other stars. For example, at their closest to the Earth, Mars and Jupiter are 0.37 AU and 3.9 AU away from it, respectively.

How far is a light-year?

Despite its name, a light-year is a unit of distance, not a unit of time. It is the distance that light travels in a vacuum in one Earth-year, which equals 9.46 trillion kilometers (5.88 trillion miles).

The light-year is used to measure distances to objects outside the Solar System, like other stars and galaxies. For example, the closest star to the Sun, Proxima Centauri, is located 4.24 light-years away from us. The nearest big galaxy to our Milky Way, the Andromeda Galaxy, is about 2.5 million light-years away.

In professional astronomy, instead of the light-year, the unit called parsec (pc) is used more often. One parsec is about 3.26 light-years.

We hope that from now on, you won’t be puzzled by the terms lunar distance, astronomical unit, and light-year while reading astronomical articles. Wishing you clear skies and happy observations!