Hello and good morning. As you probably know, it is Thursday. If you’re reading this later, however, I guess it might be useful for you to know that the 23rd of July in 2026 CE was a Thursday. Maybe you’re an anthropologist or an archaeologist, and it’s a thousand years from now, or more. If you’ve forgotten how to convert your year-numbering system to CE (AKA AD) then I am sorry, but I cannot help you.
Oh, wait, maybe I can: the last time comet Halley passed through the inner solar system was 1986 CE, so 40 years ago from the year in which I write this. Does that help?
Maybe you need a better description of comet Halley. Well, it’s a big ball of ice and some rock, and when it gets close to the sun, the heat causes a lot of the ice to sublimate and become vaporous, and the vapor, which scatters light well, turns into a sort of aura and a “tail”, trailing away from the sun in a direction caused by the solar wind…
Oh, so you know what a comet is and looks like, you just want to know about this specific comet. You want to know things to help you determine which comet it is so that you can get a clear guide to what I mean by “1986 CE/AD”.
Well, I can only tell you a little more than I already have. It’s a short period comet, orbiting eccentrically (as comets tend to do) from well inside the Oort Cloud or even the Kuiper Belt*. It is quite regular, as things in orbits that are not frequently significantly perturbed tend to be. The period of its orbit is, if memory serves, about 76 years. That ought to help you a lot, at least if you have astronomical records and keep track of such things at any level.
Years, of course, are a natural time increment, being the length of time it takes the Earth to go around the sun once “exactly”, returning to the precise spot in its orbit relative to the sun. Mind you, the solar system, and the galaxy, and the local group, and all that other stuff are constantly moving, so in fact neither Earth nor comet Halley ever truly returns to where it was before. But the solar system moves as a whole, more or less consistent in its internal motions despite the fact that its all orbiting the center of the Milky Way** about once every quarter billion years.
Of course, far enough in the future, the period of comet Halley will change, and eventually it will basically evaporate. Also, the Earth’s orbital period will change over time (though much more slowly, since it is much more massive and its orbit less open to perturbation by other quite massive objects).
And, of course, in about five billion years‒about twenty turns ‘round the Milky Way‒the sun will become a red giant. It won’t be like flipping a switch or something, where the sun is just the sun one day, 4,999,999,999 years and 364 days (or whatever) from now, then the next day, boom, it’ll swell up and devour the inner solar system.
No, it will be a pretty gradual process, for as the sun fuses more and more hydrogen, there will be proportionally less available, and the outward pressure caused by fusion will decrease, and so the the sun’s core will contract and it will get hotter so that helium will begin to fuse, but that won’t last nearly as long as the hydrogen did.
Also, as the core of the sun contracts and gets hotter, the outer portions of the sun will swell due to the increased heat within. Well before the sun is a full-on red giant, its increasing temperature will have made conditions impossible on Earth for life as we know it.
But if you’re still around, five billion years from now, I hope your technological advances will make my help utterly irrelevant. You might even have made the natural life cycle*** of the sun as much a matter of historical trivia as will presumably be the natural lifespan of an average modern human.
Anyway, by five billion years from now, you’ll be seeing the close-encounter/merger/collision of the Andromeda galaxy with the Milky Way, and that’s likely to be quite a spectacle. That is, unless you’ve developed technology to intervene in that scenario. It seems implausible, but five billion years is a long time for scientific and technological advancement.
In closing‒just in case you’re even farther in the future even than that and cosmic expansion has caused all but the local galaxy to pass over the cosmic horizon‒I want to tell you that this is not the only galaxy in the universe. At the time of this writing, within the visible universe, there were between a hundred billion and a trillion extant galaxies.
But the universe, which “began” much more compactly, has been expanding since nearly 14 billion years ago, and if that expansion is continuously accelerating, as currently seems to be the case, the distance to all other galaxies will have eventually become too great and the recession too rapid for light ever to reach the Milky Way from any of them again. But they did and do exist, even if there’s no trace left of them that you can see.
I thought you might like to know.
TTFN

*Do you need me to explain what these are? Let me know. Oh, wait, I’m in your past. You can’t let me know without time travel (unless I’m still alive in your time), and if you had that, you wouldn’t need me to try to give you clues to the date system used in my time.
**Other candy bars are available.
***Other personal exercise devices are available.



