Lightyear vs. AU: The Guide on Units of Distances in Astronomy

Have you ever asked what a light year is? Contrary to what the name and popular culture seem to imply, it’s a measurement of distance, usually between stars, and not of time. The same goes for a parsec. Specialized astronomical measurements eliminate some of the confusion between the Imperial and metric systems and allow us to indicate both the distances to deep-space objects as well as their relation to one another in the sky.

When we talk about the AU between planets, the light-years between stars, or the kiloparsecs between galaxies, we are venturing into the realm of physical distances. These measurements provide a framework—a cosmic ruler, if you will—that helps us chart the vast territories of the universe. They give substance to our understanding of the cosmos, anchoring celestial bodies in a fabric of space that, while expanding, can be quantified and mapped.

Distances between celestial objects

In the domain of science and, by extension, astronomy, the metric system (officially the International System of Units, or SI) serves as the primary foundation for measurements. Adopted for its consistency and ease of use, the metric system utilizes base units and prefixes to scale measurements up or down.

In everyday life, we are familiar with meters and kilometers, but the vastness of the universe demands larger units of distances for astronomy, which are fairly unfamiliar. Some astronomers use exponential digits to represent vast distances, but it’s far more convenient to simply use the more standardized large units of measurement.

You might not know it, but in addition to the common milli- and kilo- prefixes, the International System of Units (SI) employs a large series of prefixes to indicate multiples and submultiples of base units of distances. These prefixes create a standardized system that allows for ease of understanding and conversion across different scales. Here’s a brief rundown:

  • Pico- (p): Represents one-trillionth (10^-12) of the base unit.
  • Nano- (n): Signifies one-billionth (10^-9) of the base unit. Nanotechnology operates on this incredibly tiny scale.
  • Micro- (μ): Denotes one-millionth of the base unit (10^-6). A common example might be the diameter of bacteria, which can be a few micrometers.
  • Milli- (m): Indicates one-thousandth of the base unit (10^-3). So, 1 milliliter (ml) is 0.001 liters.
  • Deci- (d): Indicates one-tenth of the base unit. For instance, 1 deciliter (dl) is 0.1 liters. 
  • Deca- (da): Represents ten times the base unit. So, 1 decameter (dam) is 10 meters. It’s less commonly used compared to other SI prefixes but is still part of the system.
  • Kilo- (k): Represents a factor of 1,000 (10^3). For example, 1 kilometer (km) is 1,000 meters.
  • Mega- (M): Denotes a million-fold increase or 1,000,000 (10^6). For instance, 1 megabyte (MB) equates to 1,000,000 bytes.
  • Giga- (G): Represents a billion times the base unit or 1,000,000,000 (10^9). A familiar example might be a 1-gigahertz (GHz) processor in a computer, which means it can execute a billion cycles per second.
  • Tera- (T): Denotes a trillion times the base unit, or 1,000,000,000,000 (10^12).
  • Peta- (P): Denotes a quadrillion times the base unit, or 10^15.

In astronomy, both ends of this scale are used with both SI units and to indicate multiples of astronomical units of both angle and physical distance, such as milliarcseconds or gigameters, respectively.

Units Mostly Used For Interplanetary Distances

The distances to nearby planets, asteroids, and comets are often gauged using radar ranging with very large ground-based antennas such as the Very Large Array. By sending a radar signal to a target (like Venus) and measuring the time it takes for the signal to bounce back, astronomers can directly determine the distance to that object.

Note: We’ve a separate article on how distances are measured in space, be it between planets or galaxies.

First up, we encounter the megameter (Mm), representing one million meters (1 x 10^6 meters). While not as frequently cited as kilometers, the Earth’s circumference, for instance, is about 40 Mm. It’s a useful metric for conceptualizing some large-scale terrestrial and atmospheric phenomena, both on Earth and other planets.

The gigameter (Gm) takes us to the next level, representing one billion meters (1 x 10^9 meters). As an illustrative example, consider that the distance from the Earth to the Moon varies from about 363 to 405 Gm, depending on their relative positions in their orbits. When we speak of distances within our Solar System, especially those between planets and minor bodies, gigameters start to become a relevant scale.

The Astronomical Unit (AU) serves as a benchmark for distances within our Solar system and is likewise used when we define the orbits of exoplanets or multiple-star systems. Defined as the average distance between the Earth and the Sun, it’s approximately 93 million miles, or 149.6 gigameters. For example, Jupiter is roughly 5.2 AU from the Sun.

Pushing beyond, we have the seldom-used terameter (Tm), equivalent to one trillion meters (1 x 10^12 meters). Though not frequently invoked, it can be used in lieu of the more popular astronomical unit and is equivalent to about 66.8 AU. For instance, the outer edge of the Kuiper Belt—a region beyond Neptune filled with small icy bodies and dwarf planets like Pluto—extends up to about 2 Tm from the Sun. If you play spaceflight simulators like Kerbal Space Program, these games typically don’t use AU or light-years on account of the computing power required to constantly update metric-to-AU conversions, so you might see terameters or even petameters used instead.  

Common Interstellar Distances Units

Even with the terameter’s expansive reach, it remains inadequate for encompassing the larger scales found in interstellar space. 

What is a light-year?

The light-year (ly) is another unit of astronomical distance, denoting how far light travels in the vacuum of space over a year – equivalent to 9.461 petameters, 9461 terameters, 63241 AU, or about 5.88 trillion miles! It’s a handy unit for interstellar distances. The nearest star, Proxima Centauri, is about 4.24 light-years away from us. Sometimes, AU is used in lieu of light-years when referring to relatively close interstellar distances; for instance, the orbits of distant icy comets or wide double stars may be expressed in thousands of AU instead of light-years for better precision.

What is a parsec?

A parsec (pc) is, contrary to the implications of the line uttered by Han Solo in Star Wars, not a unit of time but instead a unit of measurement. The name is derived from parallax and second, relating to the method by which it’s determined.

One parsec is approximately 3.26 light-years or about 3.086 x 10^13 kilometers. It’s defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which is a useful reference point when making parallax measurements of distant stars. Parsecs are often used when describing distances to nearby stars or galaxies, but since it lacks a close physical benchmark, it’s often not as easy to conceptualize. The nearest brown dwarf system to us, Luhman 16, is almost exactly two parsecs away.

Units Commonly Used For Intergalactic Distances

The kiloparsec (kpc) and megaparsec (Mpc) scale these measurements up for discussions involving larger cosmic structures. A kiloparsec is 1,000 parsecs, and a megaparsec is a million parsecs. For instance, the Andromeda Galaxy, our Milky Way’s neighboring spiral galaxy, is about 0.78 megaparsecs or 2.537 million light-years away.

Even larger units such as the gigaparsec (Gpc, 1 billion parsec/3.26 billion ly) are occasionally used for measuring the distance for very far-away galaxies and other objects in the wider universe, but really, these units aren’t necessary over just using megaparsecs.

Redshift isn’t a unit of distance per se, but it’s closely related to understanding cosmic distances, especially for very distant galaxies. As the universe expands, light from remote galaxies gets stretched, or “redshifted.” A galaxy’s redshift can indicate its distance and the rate of the universe’s expansion. The more significant the redshift, the farther away the galaxy is and the faster it’s moving away from us. Redshift is today used as the main form of distance measurement for galaxies when Cepheids and supernovae are not available.

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME Magazine, National Geographic, Sky & Telescope, La Vanguardia, and The Guardian. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars.

Leave a Comment