Caoimhín's Content

We have used Stellarium to time travel in other videos already. In fact, almost every video looks a little bit into the future, so that we are prepared to take a look at whatever is coming up. In previous videos we travelled years into the future, to watch how the planets will move and to see future eclipses. A few years is nothing compared to how far Stellarium lets you time travel. We’re going to take advantage of that to travel even further back.

The constellations that we see today are often named in Latin, the language of the Roman Empire, even though a lot of them were named already in Ancient Greek by then. We’re going to skip over most recent history and jump back 2023 years to year 0. Year zero in astronomy is equal to 1 CE or 1 AD. In this video I use the term “before the common era” or BCE, which means the same thing as BC, with CE for “the common era” being equal to AD. At this time, the Roman Empire was already an Empire, under Augustus, having already been a Republic for hundreds of years before.

Jumping back a couple of thousand years produces an immediate and noticeable change, light pollution! There wasn’t much of a problem with light pollution before modern cities started to light themselves up, first with gas or oil and eventually electricity. In ancient times, the night sky was pristinely dark even in large settlements, unless they were burning down. This is one of the major factors in viewing the constellations as ancient people did, they didn’t have light pollution to obscure their view.

The Romans of course only renamed the constellations and the planets, we owe most of them to the Greeks. There is a good chance that a lot of the constellations originated with the Babylonians, but their records are a little sparser. To get an idea of how the Greeks saw the sky, we travel back to the year -320, or 321 BCE.

This little jump of only a few hundred years doesn’t make too much of a difference. Light pollution can’t get any lower, and the stars certainly haven’t moved much from a human perspective. We can see how the Ancient Greeks and later Romans were seeing the same patterns of stars that we see today. Of course, some things have changed, the planets have changed positions, but that will happen on comparatively shorter timescales as well. The motion of the planets against the apparently motionless background of the stars is a big part of how they were recognized as different to the stars before telescopes.

The next jump is much much bigger, a little less than three thousand years. In Stellarium, this is as easy as adding a “0”, and it brings us back to the year 3000 BCE, 3000 years before year zero, already a staggering 5000+ years before the modern day. Now we get to see some big differences, but the biggest difference still isn’t the positions of the stars relative to each other. The big change here is the Earths axial tilt. Over the course of thousands of years, where exactly the Earths north pole points changes. The axial tilt of the Earth can vary up to almost 4 degrees over the course of a cycle thousands of years long. We’re only seeing a little of this change in tilt here, but it’s enough to bring the constellation of Scorpius above the horizon for those of us here in Ireland. It’s comparable to the change in view between modern day Cork City and the view from the Mediterranean. However, our viewpoint hasn’t changed, all that’s changed is the tilt of the Earth.

Some stars have changed position a little, we can see that Sirius, the brightest star in the sky and in the constellation Canis Major, has drifted a little. Some stars do drift even faster, for example Barnard’s Star appreciably moves in the space of just a few years, but a telescope is necessary, seeing the same kind of proper motion with the naked eye is a lot harder for us here on Earth. Other planets, in other parts of the Milky Way might be able to see stars “zipping” across their sky in just a few decades. For us, even thousands of years produces only slight changes in the positions of visible stars in the sky.

Different stars do drift at different speeds, and this will be more obvious as they begin to drift further from what we are used to today. Looking into the North, it appears that the North Star and the Plough have drifted a huge amount, but relative to each other they are in pretty much the same places, what has changed is where the North Pole is pointing. Changes in our axial tilt changes what star would actually be useful for finding North, and south for that matter. At the time when the Upper and Lower Egyptian empires were unified, about 3000 BCE, our North Pole is pointing practically into Draco, the constellation next to Ursa Minor, where our modern day North Star lies.

The Egyptian empire began a little under 3000 years before the Greek Empire, which then was then superseded by the Roman empire a bit more than 2000 years before today. We can keep going back by thousands, through the 6th century BCE, through the 7th and into the 9th, 9000 years before year zero, over 11,000 years before today. Even that far back, we have records of settlements such as Jericho in the Middle East. This is believed to be the time when farming and long term settlements began. All the constellations that we would be familiar with today remain recognizably the same. Some stars have moved a little, and our North Star would have been closer to the constellation of Hercules, but things haven’t changed too much. We’re looking at a sky that pre-dates written history or even the concept of writing, we are truly into prehistory now. In fact, history, written history at least, is considered to have begun with the Egyptians around 3200 BCE, still centuries in the future for anyone looking at this sky. In fact, for the early settlers of Jericho, the Egyptian empire was further in the future than we are from those same Egyptians.

Things haven’t changed enough so the last jump is another big one, we’re adding another “0”, bringing us all the way back to 90,000 BCE, over 92,000 years before the modern day. Now things have really changed! Parts of the sky that still look a little familiar, like the front of Scorpius, are now far out numbered by stars that have moved drastically. Almost no part of the sky is recognizable. This is almost as far back as Stellarium can take us. We could push further, back to 99,999 BCE, but the differences would be comparable to the difference between year 0 and 9000 BCE, which as we saw was rather slight barring the changes in tilt. We are already looking back to a time potentially before modern humans first left Africa.

This should give you an idea of the kind of time depth necessary to see changes in the positions of stars thanks to proper motion. Even though we can see some changes “only” around 10,000 years ago, we don’t see drastic changes until closer to 100,000, or 10 kya for kilo-years-ago. As such, we’ll be waiting a comparatively long time into the future before we see similar changes. Although this video is much shorter than the timescales we’ve observed, it’s still a bit long to try and squeeze in a look at the future, so for that I hope you’ll join me here again in the much nearer future to have a look.