# Did the night sky ever change in recorded history?

I wonder whether there has ever been a major change of the firmament in recorded history, like changes in the positions of stars, changes in constellations, or stars disappearing after going supernova. There've been visible supernovae but had the star in question been visible to the naked eye before the supernova? If so, how did it change what constellation?

• Wikipedia has a page on History of Supernova Observations which is almost certainly relevant to you. – StephenG Oct 25 '20 at 13:54
• @StephenG There's no mention of stars being visible to the naked eye before the nova from which I conclude the answer to my questions is 'no', right? Even what stop-firing-the-good-guys is describing actually isn't a change to the night sky when watched from the same angle relative to the firmament. – John Oct 25 '20 at 14:04
• I glanced at a NASA website on this nebula indicates the original star was a white dwarf. It was unlikely to be visible to the naked eye itself, but they believe it had a companion star which possibly was. – StephenG Oct 25 '20 at 14:13
• The nearest historical supernova was SN 1054. With a probable 10-solar mass progenitor and a distance of 2 kpc, it might have been visible as an unremarkable 4th or 5th magnitude star. The Chinese astronomical records mention the SN as a "guest star" (i.e., not a previously known star), so it's very unlikely anyone identified it with a pre-existing star. – Peter Erwin Oct 25 '20 at 15:06
• There have been occasional suggestions that the Pleiades -- with six visible stars but a frequent tendency to be associated with seven mythological figures -- might have originally had seven visible stars, with one having (somehow) faded or disappeared. This is very unlikely, for a number of reasons. (Most likely, some cultures saw an analogy between the Pleiades and the Big Dipper; since the latter has seven main stars, some people though the Pleiades ought to have seven, too; sometimes the missing seventh star is said to be Alcor, next to Mizar in the Big Dipper.) – Peter Erwin Oct 25 '20 at 15:26

1. Do the stars change positions in the sky ... such that the layout of say... major stars in constellations appear to move over time?

2. Are there events that cause sudden changes (changes you might notice overnight ... or within a few weeks or months)?

3. How long does it take to notice a change?

Precession

The Earth's axis mostly stays oriented in the same direction as we orbit the Sun. This is why we can get away with saying that Polaris is the "North Star" ... because it appears to be almost directly above Earth's North Pole regardless of what time of the day or night ... or what day of the year you choose to observe (it is roughly within 2/3rds of a degree from the pole).

But it turns out the Earth's axis does "wobble" like a spinning top. This wobble is very slow and takes thousands of years to complete a cycle (nearly 26,000 years). At the end of this century (roughly the year 2100) Polaris will be a little closer to the pole ... only 1/2° away from the true pole (today it is about 2/3rds° away from the pole). That is as close as it will get ... and then our pole's precession cycle will start taking it farther away from the pole.

This "precession" shift causes all stars in the sky to shift their position very slightly from year to year and it's mostly not enough to notice in a single human lifetime ... but it is easily noticed when measured across thousands of years.

There are other more subtle cycles, but Precession is noticeable across large amounts of time (thousands of years). The position of the Sun in the sky at the equinoxes changes every year and this causes the Sun to appear to be moving across the zodiac over thousands of years.

The sky has a coordinate system used to catalog the locations of stars and other objects. That coordinate system uses a "declination" to measure where a star is located relative to Earth's North/South ... it works much in the same way that latitude measures geographic locations on Earth. There is also a "right ascension" value that works a bit like longitude on Earth. Except Earth is spinning (and longitudes remain fixed to the planet) ... so we needed a way to mark a fixed position in space to have a sort of celestial longitude. That measurement is called Right Ascension and it needs a "0" position. So the zero position is established by the location of the Sun in the sky at the Vernal Equinox (the start of Northern Hemisphere Spring). It even got a name... the "First point of Aries" ... named because, when it was established a few thousands years ago, it was in Aries. Today ... it's migrated all the way over to Pisces (and is getting closer to the boundary of Aquarius).

These are changes caused by Precession and noticeable ... but mostly only when measuring over hundreds of years or thousands of years. It is actually noticeable just from year to year ... but only with precise measurements.

Precision causes the entire sky to shift in the same direction (and by a very subtle amount). This means that the locations of the constellations will shift ... but it wont explain changes to their shapes. For that, we have to look at other factors. In reality it isn't the "sky" that is shifting ... but rather the orientation of Earth's axis that changed.

Proper Motion

Stars are moving ... and they don't all move in exactly the same direction. But the stars are so distant that the movement is mostly only noticeable with precise instruments. Regardless, they do move.

Here for example, is the Big Dipper asterism, but I've turned on Proper Motion vectors ... the vectors (shown in a light brown color) indicates the direction of that star's motion (relative to us) and longer vectors indicate great proper motion.

Notice that many of the stars of the Big Dipper are all moving in the same general direction at about the same speed ... including Alcor/Mizar, Alioth, Megrez, Phecda, and Merak. In this diagram, those stars are all moving up and to the left (relative to this frame).

But more importantly... notice that Alkaid and Dubhe are going in roughly the opposite direction. These stars are moving down and to the right. This means that over thousands of years, this asterism will become distorted and will eventually not resemble the "Dipper" shape anymore.

Incidentally, the stars in the dipper that are going in the same direction are all part of an Open Cluster called the Ursa Major Moving Group. It isn't as obvious that it's an open cluster because it is relatively close (not all stars in Ursa Major are members of the cluster.)

Stars that physically closer to our Sun will may appear to have much faster proper motion. It isn't so much that the stars are necessarily physically moving faster ... but since they are close, the changes are easier to spot.

Here, for example, is the proper motion of Rigel Kentaurus (aka Alpha Centauri).

In this frame (current time) the star is in the upper-left section of this chart. It is in Centaurus (hence its name) but near Circinus. But notice it has a VERY long proper motion vector which is headed toward the right side of this chart in the direction of Hydra.

Its proper motion vector is much longer than the other stars ... but Rigel Kentaurus is just a little over 4 light years away (one of our closest neighbors) -- so its relative motion is more noticeable.

While Rigel Kentaurus makes the top-20 list for stars with the highest proper motion, it's not the winner ... that spot goes to Barnard's Star (in Ophiucus). But Barnard's Star isn't very bright... at magnitude 9.5 it isn't noticeable to ordinary human vision and requires a telescope to see it. But it is moving just a little under 3x faster than Alpha Centauri A & B.

If you're interested, here's a top-20 list: https://www.cosmos.esa.int/web/hipparcos/high-proper-motion

Transient Events

Certainly events such as bright comets would change the sky -- but those aren't stars. Super-Novae events, on the other hand, are stars ... and these are easily noticeable if they happen in our home galaxy. They don't happen very often. The estimate is that, on average, they happen in a galaxy our size about once every hundred years. But's not like a clock where they happen with periodic regularity ... huge amounts of time may pass without any of them ... and then several could occur timed much closer together.

Since Super-Novae events can be quite bright, they can even be visible in the daytime sky. They don't last particularly long ... they quickly brighten up ... then begin to dim over weeks and ... after a few months they may no longer be noticeable (but they will leave behind a Super Nova Remnant -- deep sky nebulae such as the Crab Nebula and Veil Nebula are examples of Super Nova Remnants.)

• But the question was about "major changes" in "recorded history" (so, in the last 5,000 years or so); none of the visible stars' proper motions are large enough to be noticeable to non-telescopic observers over that short a period of time. – Peter Erwin Oct 25 '20 at 22:46
• While your post doesn't answer my question, I accepted it because it's very good and informative. My question is answered anyway: as you also mention, in the years of recorded history, the Crab Nebula appeared which wasn't there before the supernova on the firmament. – John Oct 26 '20 at 6:38
• What's the scale of the proper motion vectors in the charts? how much time do the arrows represent? For example, at what time will Rigil Kentaurus reach Hydra (assuming of course that the motion vector remains constant)? Is this more in the range or thousands of millions of years? – Stephan Matthiesen Jun 15 at 23:34
• Rigil Kentaurus has a proper motion of about 4 arcsec per year, so if I calculate this right it will need roughly 1000 years for one degree. That's about a fifth of the distance between Alpha and Beta Centauri, so Centaurus visually will change somewhat but it would be hard to notice, I guess. – Stephan Matthiesen Jun 15 at 23:57

I don't see a mention yet of variable stars. There are stars whose brightness, as observed from earth, changes to an extent noticeable to naked-eye observers. The first two that we know historical people noticed provide examples of the two main types.

Algol, in the constellation Perseus, is an eclipsing binary where a dim companion periodically blocks some of the light from the brighter star. Its brightness (as observed from earth) varies from magnitude 2.1 to 3.4 over a time scale of days. The light output of the group of stars does not change, but the amount of light that reaches us changes enough to be visually detectable. See the Wikipedia article https://en.wikipedia.org/wiki/Algol
Note, that this same effect is used to hunt for exoplanets - as the planet periodically blocks some of the light from the start; but to detect planets, this requires exquisitely sensitive instruments.

Mira, in the constellation Cygnus, is a variable star whose brightness ranges from about 3.5 or 4 to 14 - that is easily visible to needing a very good telescope to detect it - over a period of months. The actual light output by the star varies over time. See the wikipedia entry at https://en.wikipedia.org/wiki/Mira Note that this star's output actually pulsates as it varies in brightness.

To quote the first little bit of a long Wikipedia entry on variable stars:

    An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.[2][3][4]

Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught. In this way, the discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries.

The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669; John Goodricke gave the correct explanation of its variability in 1784. Chi Cygni was identified in 1686 by G. Kirch, then R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known. John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850 the number of known variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography.


So, you can see that besides planets, comets, and novae, variable starts have been observed for hundreds of years, maybe thousands.

The axial precession precession of the Earth has a relatively short cycle of about 25,000 years, which made Beta and Gamma Ursae Minoris the "twin pole stars" to ancient Egyptian and Chinese astronomers.

If a star bright enough to be notable to ancient astronomers had gone supernova, the brightness of the explosion would have been much much larger than what we have on record.

• Do we have any records of sailors using the twin stars for navigation in that period? – John Oct 25 '20 at 11:25