Anniversary Post: Halley’s Comet’s Very Close Approach

Halley's CometOn this day in 837, Halley’s Comet came the closest to the Earth in the entirety of human history. It came within almost three million miles (0.03 AU), which, by astronomical standards, is nothing. It is estimated that its tail spanned 60° across the sky. To see it today would be awe inspiring. To most of the people at that time, it was probably terrifying.

I’ve written before about going out to the beach in the early morning in 1986 to see it. The comet was at a distance of 0.42 AU and nothing but a smudge on the sky. It was the worst observational opportunity for at least 2,000 years. I remember seeing Carl Sagan speak on his book tour for Comet and he said that if anyone was alive in 2061, it would be a much greater show. And in 2134, it would be spectacular.

What did he mean? In 2061, we will at least be on the same side of the sun when Halley’s comet reaches perihelion. Unfortunately, I can’t find any data on just how close we will come. But in 2134, we will come within 0.09 AU. That should be quite a show. Of course, as time goes on, the comet gets smaller and smaller, so if we really want to dream, we should imagine being around in 837.

It’s these kinds of things that make me regret our short lifetimes. Usually, the changes of the human body, the degradation of the mind, make me welcome death — at least eventually. Not that one can reasonably complain. I mean, did we not just get to see the unmasking of Pluto in real time? And I still thrill to see the occasional shooting star. Every time has its advantages. And I’ve lived during the most exciting time in the history of astronomy.

9 thoughts on “Anniversary Post: Halley’s Comet’s Very Close Approach

  1. NASA has a great online page that tells you how close (and things like direction and magnitude) significant bodies come to Earth, for any date you choose.

    http://ssd.jpl.nasa.gov/horizons.cgi

    I can’t see how to give a link to the precise results for this, but if you use ‘1P/ Halley’ with the dates of July and August 2061, you find its closest approach (the result column is called ‘delta’) will be 0.48 AU at the end of July, and the expected “apparent visual total magnitude” will be about +2. So, not that impressive, I’d say. That distance matches, roughly, with the 50 million miles quoted here.

    And looking for the position of the Sun on those dates with the same tool, it seems to me they have the almost identical right ascension (roughly: comet: RA 0900 hours, Decl +37 degrees; Sun RA 0830 hours, Decl. +19 degrees). So I’d guess you’d only see it at twilight, and it’s not bright so you’d see very little (I may be misremembering my astronomy, though).

    • Well, then there’s no point in living until I’m 100. Actually the 0.48 AU sounds about right. The 0.42 AU quoted for 1986 seemed far too close. But I don’t have the time to go into it. Thanks for looking it up!

      • I have a hard time figuring out what 60 degrees means in total amount of coverage of the sky. 1/6th?

        Still, that would have been spectacular and a good reason to invent a time machine.

        • I thought about talking about that. It would be 1/3rd across the sky. But it seem more like half-way. In other words: something that would totally distort the sky as we normally see it. I’m not sure how wide it would be.

  2. However, it looks like “apparent visual total magnitude” means something a bit different from just ‘apparent magnitude’. Looking at the dates given here and magnitudes, the simple ‘apparent magnitude’ seems to be about 2 lower (ie, quite a lot brighter) than “apparent visual total magnitude”. An apparent magnitude of -0.3 is about the same as the maximum for Saturn. It’ll be better than 1986, but I still think you’ll only see it during twilight.

  3. I had that same feeling about regretting I would not live long enough back around 1980. I took an astronomy course taught by Virginia Trimble at the University of Maryland and she told us that one of the highly visible stars (I forget which one) was expected to go supernova around 2300 (IIRC). And I thought, “Wow! To be around to see that! If I could just live a couple hundred more years …” (Dr. Trimble was/is a well-known astronomer and her late husband, Joseph Weber, was famous and controversial for early attempts at detecting gravitational waves and neutrinos.)

    • I remember going on a field trip to Stanford and meeting this guy who was trying to detect gravitational waves and thinking, “Good look wasting your life!” This is why I’m not a famous scientist. (Well, one of the reasons.)

      It would be fun to see something that shows in a spectacular way that the universe is not constant — but in a way that wouldn’t kill me.

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