Dealing with precession
We note some of the difficulties in writing historical fiction (among other things).
We’ve recently been rereading Shakespeare’s Julius Caesar, one of our favorite works by that author. In Act III, scene 1, Caesar makes the claim, “I am as constant as the Northern Star,” comparing his own steadfastness to Polaris; all other stars describe circles through the sky while Polaris stays fixed. It is an effective speech and does much to establish his character. Unfortunately, in Caesar’s lifetime there was no northern star. Polaris was then more than ten degrees from the pole, and quite obviously moved in its own circle.
The reason is the precession of the equinoxes. Earth wobbles a bit as it spins. The place in the sky where its axis points, about which all the stars seem to circle daily, shifts with time. It describes its own circle with a radius of about 23 degrees, taking something like 26,000 years to do so. That’s a long time, but an astronomer wishing to be precise about a star’s position (which is tied to the direction of Earth’s axis) must make a correction of something like 50 seconds of arc a year. To give some idea how big this is, it’s larger than any of the planets appear to be as seen from Earth (except Venus sometimes), though too small to be detected with the unaided eye.
Precession was known even in Caesar’s time, as astronomers noticed that star positions had shifted from those recorded centuries before. It can be predicted for long periods in the future and calculated far back into the past. In fact our astronomer has a planisphere that can be set for any date from 12,750 years ago to as far in the future, showing the stars as they can be/could have been seen then. If one is writing an historical novel of the Sea Peoples, about 1250 BCE, one might mention that Orion is much lower in the sky than now for a Northern Hemisphere observer. The sky of the autumn equinox in 6000 CE will contain many of what we now think of as spring constellations. To us it seems awesome (in the old sense of the word) that a little plastic contraption can tell us about things so far in the past and future.
But not exactly. Stars aren’t fastened to the sky; they have motions of their own in addition to precession. These “proper motions” (there don’t seem to be improper ones) are again small; for Arcturus, for instance, it amounts to slightly over six seconds of arc a year. But that’s enough to move it several degrees when looking thousands of years in the past or future, enough to show clearly on the planisphere. The Big Dipper will look noticeably different, the bowl flattening and the handle lengthening, some centuries hence.
Of course Shakespeare’s play is a masterpiece, and for it a bit of astronomical inaccuracy makes no real difference. But we think getting the details right in your stories is a worthwhile effort. And it might indeed lead to some inspiration. In Caesar’s time, the constellations of Ursa Major and Ursa Minor, the greater and lesser bears, circled on opposite sides of the true pole. Shakespeare could certainly have made much of the image of the steadfast point in the heavens being threatened by circling beasts.
