How far is it? How astronomical distances are described

Light-year, megaparsec, astronomical unit...what does all that mean, and is there any way we can really understand how far away astronomical objects are?

Distances in the universe range from the Moon, averaging about 239,000 miles from Earth, to the farthest galaxies billions of light-years away. A light-year is almost 5.9 trillion miles. Already you can see the problem. Even 239,000 miles, about 30 Earth diameters, is difficult to comprehend, but for even a close galaxy like M31, the Andromeda Galaxy, the distance in miles becomes such a ridiculously large number as to be meaningless.

Above: The Earth and Moon photographed by the Nozomi spacecraft, launched in 1998 but failed to achieve Mars orbit. (NASA/NSSDC-KSC, Public domain, via Wikimedia Commons)

Units of measurement

A light-year is the distance light travels in a vacuum in the course of one Earth year, traveling at a speed of about 186,000 miles per second. This is a somewhat arbitrary and Earth-centric unit, being partially based on our little planet's movement around its star. Still, for most of us, this is about the best we can do to comprehend what is essentially incomprehensible.

Professional astronomers use the astronomical unit (AU) to describe closer distances in space, such as those within the solar system or around other stars. An astronomical unit, another Earth-centric measurement, is the mean Earth-Sun distance, or just under 93 million miles. (Diagram: nagualdesign, CC BY-SA 4.0, via Wikimedia Commons)

They use the parsec to measure larger cosmic distances. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A megaparsec (mpc) is a million parsecs. Easy to visualize, huh?

While none of these can really give us a true sense of the vast distances in the universe, I think most amateur astronomers and regular folks are better off using light-years, primarily because it introduces the element of time into the equation, which makes it relatable both spatially and temporally. But you'll find various measurements used in the literature. For example, Sky Safari Pro, my preferred star charting app, shows the distances to galaxies in megaparsecs (Mpc) and megalight-years (Mly). Mega=million.

My recommendation? Stick to light-years and just use the measurements to compare distances or the visual time delay between objects. For example, the Pleiades star cluster is listed in Sky Safari at 430 light-years and the Hyades cluster at 147 light-years, or almost three times closer to Earth. Aldebaran, which appears to be part of the Hyades but is actually a foreground star, is listed at 66.6 light-years, more than two times closer than the Hyades.

Above: The bright foreground star Aldebaran in the lower left, superimposed over the more distant Hyades cluster, with the hot blue stars of the Pleiades (upper right) three times farther away than the Hyades. (Jiří Bubeníček, CC BY-SA 4.0, via Wikimedia Commons) See also Taurus in 3D.

Arriving at an actual number

Once you have some idea of the measurement units used, then you have to try to understand how astronomers come up with the numbers for each object. This can vary considerably, and is why you often see quite different distance estimates from different sources. 

Let's take the galaxy M81 in Ursa Major as an example. Sky Safari lists its distance as 12 Mly. Wikipedia says 11.8 Mly, citing several technical studies, and the NASA/IPAC Extragalactic Database (NED) says 11.98. As you get farther away, the numbers tend to diverge even more. So who is right?

Left: Galaxy M81. (KeithSteffens, CC BY-SA 4.0, via Wikimedia Commons)

It depends on how it was measured. There's a lot of information out there, much of which is outdated, and that includes a lot of the information in apps like Sky Safari. Refinements of distances are continually being made, as scientists conduct research and obtain new data, as well as reinterpret older data. 

NED lists some of the methods used to determine distances to galaxies, which include the redshift—the amount light from the galaxy is shifted into the red part of the spectrum because of the expansion of the universe, 10 primary non-redshift methods, and 26 mostly lesser known and highly specialized methods. For M81, the database includes 67 measured redshifts and 99 distances measured by non-redshift methods. No wonder we can't agree! 

I won't go into all the different methods for measuring cosmic distances, but some of the most common include parallax, the shifting of a relatively close object's position relative to the background at different points in the Earth's orbit; luminosity of objects considered "standard candles," such as stars known as Cepheid variables, X-ray bursts from neutron stars, Type Ia supernovae; and calculations such as the Tully-Fisher relation, a correlation between the luminosity and rotational velocity of spiral galaxies. 

Left: Henrietta Swan Leavitt, who discovered the relationship between the period and luminosity of Cepheid variables and first used that to determine the distance to galaxies. (William Henry credited as photographer in the Woman Citizen issue where this photo appeared, Public domain, via Wikimedia Commons)

The lesson here is that science is a dynamic process and our knowledge and understanding of even apparently simple things, like distance, changes depending on how we observe it and what we use to measure it. So we take the commonly accepted number and run with it. For now.

Practical application

Long ago I gave up trying to conceptualize cosmic distances. Instead, I look at them relative to each other and try to get a sense of perspective that way. 

For example, when viewing Saturn and its largest and brightest moon, Titan, in the telescope, I consider that Saturn orbits the Sun about 9.6 times farther out than the Earth, and as I write this, sunlight reflecting off the top of its outer layer of gas takes about 77 minutes to reach our eyes. Titan averages about 746,000 miles from Saturn in its orbit, so that gives me a relative sense of the distance I am looking at between Saturn and Titan in the telescope when it is at its furthest from Saturn in my line of sight (greatest elongation). (Image: Saturn with Titan to the upper right; Kevin M. Gill, CC BY 2.0, via Wikimedia Commons)

You can also look at distance as a function of time, which is why I like to use light-years as the measuring unit. The farther away an object is, the farther in the past you are looking at it, and the number is the same. Right now, if it were clear, I would see the Moon as it was about 1.2 seconds ago (1.2 light seconds away), the Sun 8.2 minutes ago, Jupiter about 35 minutes ago, the Pleiades about 430 years ago, and galaxy M81 about 12 million years ago. Visual time travel.

Above: Supernova in M101. Seen by us in 2023. Actually happened about 21 million years ago. (Kheider, CC BY-SA 4.0, via Wikimedia Commons)

My point is that you don't have to do more than look at a couple of numbers and do simple arithmetic to understand the distance and time relationships between astronomical objects and appreciate what you are seeing when you observe.