The universe keeps turning out to be bigger than we thought.

What this statement means, not even the astronomers know, when you come right down to it. For millennia, it was a pretty well-established fact that the whole universe consisted of five planets, the moon, the sun, and the stars, all of which (most learned people thought) revolved around the Earth. Few people thought of them as solid, physical objects.

When Galileo in the early 1600s aimed one of the first telescopes at Jupiter, he was stunned to track points of light orbiting around it — they turned out to be moons. In the 1600s and 1700s more and more natural philosophers got better and better telescopes and spotted more moons, and more people started thinking the planets were solid, like Earth. The universe got bigger.

There were already larger ideas. In the late 1500s, the rabble-rousing philosopher-theologian Giordano Bruno wrote a tract called “On the Infinite Universe and Worlds” in which he argued that the universe is infinite, that it contains an infinite number of worlds, and that the worlds are inhabited by intelligent beings. Maybe these ideas contributed to his being burned at the stake as a heretic, but no one knows for sure. Later in the 1600s, Henry More, a Cambridge University professor, in 1646 published “An Essay upon the Infinity of Worlds” (short title), one among a number of such writings on the topic in Europe. In the 1700s, Immanuel Kant described in his rigorous logic how the nebulae in the night sky might well be “island universes,” an early speculative phrase for what we call galaxies.

Around the same time, in 1781, William Herschel expanded what was visibly verifiable when he spotted another planet — eventually named Uranus. In 1801 a tiny body was seen between Mars and Jupiter which turned out to be Ceres, the largest of what we now call the asteroids. In the 1840s, several astronomers were convinced there must be yet another planet beyond Uranus and tracked it down — Neptune. In 1930, Pluto was detected on photographic plates. Clearly, the solar system was larger than anyone had thought. But did we really live in just one of many island universes?

The American astronomer Edwin Hubble verified in 1924 that a type of star called a Cepheid variable — whose unusual characteristics are so stable it makes a reliable marker of stellar distances — was present in one of the nebulae, indicating the nebula was far beyond our own galaxy. There are, it turned out, “island universes.” A lot of them.

Better and better telescopes and photography in the 20th century revealed more and more galaxies, more and more distant. They vary in size from a few million to hundreds of billions of stars, and more. Our Milky Way Galaxy, where we live, probably has something like 100 billion stars. Some astronomers think more, some a little less. The galaxy 2.5 million light-years away known as M31, where Hubble spotted the extragalactic Cepheid variable, may have up to 1 trillion stars.

How many galaxies are there? The answer is that no one has ever known for sure. By the year 2000, it was estimated that there are somewhere around 125 billion galaxies. About 10 years later, studies of the Hubble Space Telescope’s deep field views raised this estimate to around 225 billion galaxies.

Before I tell you the punch line of all this, let’s stop and get straight about what these numbers mean. Here’s a pragmatic way of thinking about it: Let’s say you were alive on the day Christ was born (plus you were immortal). On that day, someone gave you $1 million. They told you to spend $1,000 a day. How long would the money last? In less than three years, your million would be gone.

Now let’s say in that same year, zero A.D., you were given $1 billion and started spending $1,000 a day. How long would your billion last? The answer is: Today, in 2016, you would still have more than a quarter of a million dollars left.

A billion is a thousand million. And a trillion, just for clarity’s sake, is a thousand billion.

In the past 25 years, astronomers have more or less agreed that most stars have at least one planet revolving around them. So if there are 225 billion galaxies with, for the sake of an average, about 10 billion stars each, and each star has one planet, that’s enough stars and planets, really, to call it even and say Bruno and More were for all practical purposes right. Our universe contains what’s tantamount to an infinity of worlds.

Punch line: Some astronomers in Great Britain recently spent a couple of years analyzing Hubble Space Telescope and other data from studies of galaxies, and last month reported that most likely, there are not 225 billion galaxies — there are upwards of 2 trillion galaxies in the universe.

And they still haven’t been able to infer with certainty that that’s the best ball-park total, partly because “the distribution of galaxies can vary significantly as structure is highly clustered, and thus some regions are more over dense than others,” and partly because they accounted only for galaxies with a redshift up to 8. Redshift is a measure of how fast an object like a galaxy is moving away from us because the universe is expanding. The most distant galaxies (the word “distant” here means really nothing like the spatial distance you’re trying to picture) are moving away from us the fastest, like dots on an inflating balloon. The highest confirmed spectroscopic redshift of a galaxy is z = 11.1, in astronomical notation. So they’re out there, but after about redshift z = 8, they become too fast/distant to locate reliably.

The universe literally keeps getting bigger. And bigger.

Dana Wilde lives in Troy. His book on the planets, stars and galaxies “Nebulae: A Backyard Cosmography,” <http://booklocker.com/books/6276.html> is available in paperback and e-book editions. You can contact him at [email protected] Backyard Naturalist appears the second and fourth Thursdays each month.