There is no "now" in the night sky

Above: An illustration of the time lag in viewing an object, in this case, the Moon, from Earth, caused by the finite speed of light and the great distance between the two bodies. (James O'Donoghue, CC BY 3.0, via Wikimedia Commons)

One of the more fascinating things about visual astronomy is that we are literally time travelling when observing. Although the speed of light is fast—nothing can go faster than the speed of light in a vacuum—there is still a long time lag because of the tremendous distances light travels across the universe. 

On Earth, everything seems to happen instantaneously. I can talk to someone in Tokyo about 6,800 miles away and there is only an imperceptible lag to our conversation because the electromagnetic waves of our transmission are going through a conductor that won't let them travel as fast as light in a vacuum, maybe 50 to 90% of it. But it seems instantaneous or close to it because that's still really fast in our limited experience. So we think of everything on Earth as happening "now," and for all practical purposes there is a "now."

Above: One man's visualization of electromagnetic waves, slightly more poetic than current theory. (Attributed to Vittorio Matteo Corcos, 1859-1933, Public domain, via Wikimedia Commons)

Earth is tiny. The world of our immediate experience is tiny and our lifespan is an incredibly small fraction of the age of the universe. (To visualize the events on Earth, just one third the age of the universe, on a relative scale of one year, see this fascinating article in The Conversation.) To us there is a "now." But look out into space, into the night sky, and "now" becomes a very parochial term. Light travels at about 186,000 miles per second in a vacuum. Our view of the Moon is about 1.2 seconds old, viewed from Earth. That's almost "now." Light from the Sun, 93 million miles away, is about 8.2 minutes old when I observe it in my telescope with a solar filter. That's not "now," but fairly close.

The Einstein Cross is the image of a quasar 8

billion light years away broken up into four images

by a foreground galaxy 400 million light years away

through a process known as gravitational lensing.

(NASA Hubble, CC BY 2.0, via Wikimedia Commons)

Just within our solar system, we see or hear things as they were minutes or even hours ago. Currently any radio signal from the spacecraft on Mars is about 6 minutes old, as is my view of the Red Planet in my telescope. At opposition, our view of Jupiter is about 32 minutes old.  At its farthest, our view of Jupiter is about 54 minutes old. 

Because everything is moving, the age of our view changes. Neptune is so much farther out that we see it as it was pretty much four hours ago, give or take 8.2 minutes depending on which side of the Sun we are from it. It doesn't even make a complete orbit, 165 years, in a person's lifetime. Signals from Voyager 1, out in interstellar space now, beyond the influence of the Sun, take around a day to arrive here in Earth. Our view of M31, the Andromeda Galaxy, is about 2.5 million years old. Our view of M104, the Sombrero Galaxy, is around 30 million years old. If I can find Quasar 3C 273 in my telescope, I view it as it was 2.4 billion years ago.

But it's a hodgepodge. Let's look at the belt of Orion, the stars Zeta (Alnitak), Epsilon (Alnilam), and Delta (Mintaka). They are all about the same brightness and they're in a roughly straight line. Simple, huh? But no, depending on the source, the middle star, Alnilam, could be nearly three times farther than the other two. (Left: Orion by Tsuruta Yosuke, CC BY 2.0, via Flickr)

When we look up at the sky or in our telescopes, we are looking at a nearly endless number of "nows." Stars at different distances, clusters, nebulae, galaxies...viewed all at the same time, our "now," but all as they appeared at greatly varying times in the past. How confusing, yet how exhilarating! We're looking at a very jumbled canvas on which history is painted at wildly varying intervals. Fascinating!

This all brings up an interesting thought. If electromagnetic signals and light take so long to get to us, does anything really ever cease to exist? Does history every completely vanish? We can see history, and in fact can "now" see way back billions of years to just a couple hundred million years after the Big Bang with instruments like the James Webb Telescope.

Turn it around. Do we ourselves ever totally disappear from the universe? Let's face it, we're probably not as advanced as we could be. Hopefully we've got a long way to go before we realize our full potential in observing and understanding our universe, but there's no guarantee of that. However, if someone well beyond our level of development on a planet orbiting another star had an instrument powerful enough to detect us, we might well have been dead for thousands of years before they do. 

(Cheesy alien image created using Microsoft Copilot. 2025)

We could all be, in a sense, immortal, at least the record of our existence might well be. Yet it all depends on some sentient form, creature, or machine being sophisticated enough to detect the signs and signals of our existence. The pursuit of scientific knowledge and its application to cosmology, throughout the universe, may be our true Fountain of Youth.

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.