Christmas Star

It has been doubly depressingly cloudy for a while now. “Doubly” because I like sunshine and because I’ve been wanting to see the Great Conjunction.

Tuesday evening it was finally clear enough that I could. (I missed the date of closest approach, Monday (12/21), by only one day, so I was very happy.)

A conjunction is when two or more moving astronomical objects momentarily share the same longitudinal (vertical) coordinate — they “line up” relative to the coordinate system. (The Wiki article has a beautiful picture of a triple conjunction.)

Note that “momentarily” is very broadly defined (weeks or months can be a moment astronomically). Note also that a conjunction assumes a point of view. Commonly it’s simply “from Earth”. Especially note that the apparent closeness is strictly visual — the objects are typically very far away in space.

There are two coordinate systems that can apply. In the general case there is the equatorial coordinate system, a spherical mapping based on the Earth. It uses right ascension (RA or α) and declination (dec or δ) as axes. The former matches our lines of longitude, the latter lines of latitude. Objects are in conjunction in this system if they share a right ascension (which is generally a vertical line visually).

For planets, another system is the ecliptic coordinate system, which fixes on the ecliptic plane (and is therefore inclined, in a complicated way, relative to the Earth). Complicating it further, the system can center on either the Earth or the Sun. Objects in this system are in conjunction if they share an ecliptic longitude (which is usually not a vertical line to us visually).

As in the current great conjunction:

[Digital zoom. Ugh.] Jupiter left, more distant Saturn right.

The thing about planets is, since they already share the line of the ecliptic (i.e. they have roughly the same ecliptic latitude), sharing an ecliptic longitude means they’re very close in the sky.

So a conjunction doesn’t always mean objects are close together in the sky (note the picture on the Wiki page), but with planets considered relative to the ecliptic coordinate system, it does.

[The Wiki picture shows three objects sharing a right ascension. The line of the ecliptic passes through the Moon, Venus, and Mercury. (The ecliptic is actually vertical in that picture! The ecliptic lines of longitude are horizontal.) In terms of the ecliptic, those object are not in conjunction, but Venus and Mercury are getting close.]


A great conjunction is a conjunction of Jupiter and Saturn.

(So don’t call it the great conjunction of Jupiter and Saturn, because that means the conjunction of Jupiter and Saturn of Jupiter and Saturn.)

For this great conjunction, we’re looking across the system. [WWT]

A great conjunction is great because it’s rare (not because Jupiter and Saturn are huge). The inner planets move faster and fall into conjunctions more often. Uranus (however you care to pronounce it) and Neptune are slower, but we can’t see them with the naked eye.

§ §

I went through an astronomy phase in grade school, got really into it for a while, and have been generally interested ever since. Given other interests, though, it’s not something I’ve actively pursued.

Back in the late 1980s I used an astronomy software, called RedShift, that I really liked, but that ancient version is long gone. (I probably still have the 3.25″ diskettes somewhere, but nothing that can read them.)

They look close together to us, but they’re not close at all! [RS]

When I got my iPad four years ago, I bought the RedShift iOS app, and have enjoyed that a lot. I’ve even transferred screen grabs from my iPad to my PC for blog posts. Thinking about doing that for this post, the light bulb finally went on. Hey, dummy! Why not get RedShift for the PC?

Now here’s the thing. Back in the day, RedShift was a hot ticket, but these days it feels more like an also ran. Its Wiki article is pathetic, and it isn’t mentioned much, or only in passing, in reviews of astronomy software. (Yet you can get it for the PC, Mac, iPad, iPhone, and Android, so they’re still a player. To spoil the punchline, the PC version is essentially the iOS version, and definitely a win. Lovely to have RedShift on my PC again.)

One thing I like about RedShift is that I’ve always found it easy to use and, crucially, never frustrating.


Not so Starry Night, a more expensive app (with two even more expensive versions available). It had good reviews, but I spent a very frustrating hour+ with this software once I installed it. It does a lot, but the user interface is, to my eye, atrocious.

I got the “Enthusiast” version ($80). There’s a “Pro” version ($150) and a “Pro Plus” version ($250). There’s also a $50 mini version. In contrast, the RedShift “Premium” was only $60.

[I just now tried to get a Solar system view out of Starry Night, I know it’s possible, I’ve done it before, but once again all I got is frustration. I’m coming to dislike this software. I did use it to make the following image.]

Starry Night view of 12/21/2020. Jupiter and Saturn are upper left.

The red line is Jupiter’s orbit, the brown line is Saturn’s orbit, and the green line is the ecliptic itself. On the other hand, the grid shows the equatorial coordinate system; note how the lines of right ascension are vertical. The ecliptic lines of longitude would be 90° relative to the green line.

Ironically, one of the better choices is free. Microsoft has a freely available Windows PC app, called World-Wide Telescope. Those without a Windows PC can access the app on the website. That first Solar system diagram above is from WWT. (The second one is from RedShift.)

I may do a post about these apps in the future as I get to know them better (one certainly can’t go wrong with the free WWT; it has a lot of features).

§ §

I was curious how special this great conjunction was compared to past years. I’ve been hearing people make a big deal of it. The wiki page has a table of great conjunctions, so I scrapped the data and made this chart:

Great conjunctions over 1200 years. Separation is in arcminutes.

Okay, yeah, pretty special. The last one was in 1563 (6.8).

There will be another one ever so slightly better (6.0 versus our 6.1) in 2080, so stay tuned. It’ll be a while until the next one.

§ §

We had a blizzard Wednesday (as I wrote most of this), and there was doubly no chance of a sighting (clouds and a snowstorm). I’m really glad I got to see the “Christmas Star” while the planets were still close!

It looks like it might be clear tonight, but since it’s -1° at 9:00 AM it may be a bit chilly (that does tend to make the sky crystal clear, though). Unfortunately, by tonight they’ll be three days past closest separation:

They won’t be so close tonight!

And they’re lower in the sky (closer to the setting Sun) each evening. As it is, I have to walk about 15 minutes to the spot that gave me a low enough horizon. Not sure I’m up for that in near zero dusk.

Stay redshifted, my friends! Go forth and spread beauty and light.

About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he imagines are large thoughts. View all posts by Wyrd Smythe

10 responses to “Christmas Star

  • Wyrd Smythe

    All those clouds the past week-and-a-half are a consequence of the unusually warm December weather we’ve been having. No snow until yesterday, but it hit us good. Major temp change!

    And very cold, very clear, skies. 😀

    Here’s what I woke up to this morning:

  • Anonymole

    Humans can find meaning in the most obscure, coincidental events. Not you, per se, humans, in general. As if we’re still hunter/gatherers trying to make sense of the mystical cosmos above and around us.

    “What does it mean, Magra?”
    “The spirits are mating, they come together and the brighter one has his way with the othern’. Later, her offspring will litter the sky.”
    “Ah, just as Durn and Leeta are doing now?”
    “Yes, but more quietly.”
    “Ha, ja, they make the most noise of all of the peoples during conjunction.”

    • Wyrd Smythe

      Our minds are powerful pattern matchers — we see patterns in everything (real or not).

      I think about what it must have been like in our early days, when there wasn’t any light pollution. People knew the night sky, how the dome of stars rotated above them. (I dunno if you’ve ever wilderness camped or been able to see the night sky away from light pollution, but it is spectacular. Especially when you wake up at 3 AM and go out for a piss with dark-adapted eyes. So stunning you feel like you could fall up into it.)

      So those especially bright stars that “wandered” around… that was weird! Must mean something, right? Has to! (And, as it turned out, it did.)

      Conjunctions are fun, but, as you imply, do have more value in astrology than astronomy. The Solstices and Equinoxes, on the other hand; those were the first version of the Farmer’s Almanac! What those things tell us is entirely real.

  • Brian

    “I went through an astronomy phase in grade school, got really into it for a while, and have been generally interested ever since. Given other interests, though, it’s not something I’ve actively pursued.” I can totally relate to that paragraph.

    I have dabbled with a variety of astroglobe softwares although can’t remember the past ones I used. There was one I particularly liked when I was participating in SETI@home as it could access that data there and show me the area of sky it was currently processing radio telescope data from. These days I use Stellarium but I have just checked out that you mention and will certainly take a closer look.

    I was woken up around 3am last night, thanks to my cat, and caught a great view of the Pleiades from my bedroom window.

    • Wyrd Smythe

      Ah, the Sisters, yes! They seem to be almost what Orion is shooting at. The two shoulder stars, and then Aldebaran draw a line right to them! (The O-Man is my winter buddy. 🙂 )

      I had a friend who participated in that SETI@home program. I found the idea of so many personal computers joining in a processing network the most interesting aspect of it. A early version of crowd sourcing!

      What do you think of Stellarium? I saw it mentioned in a few lists.

      • Brian

        I quite like Stellarium. I found it somewhat quirky and awkward to use as first. I use is simply for browsing round the sky, to discover what I’m looking at in the real sky, or I fast-forward or back to see what what will be/was where when. The sky looks pretty, rather present things in a more raw format than I was familiar with, but I’m sure it has more capable features that I’ve yet to delve into.

      • Wyrd Smythe

        Just this morning I was wondering how much I’ll use the three astronomy programs I now have on my PC. I have two versions on my iPad, and don’t use them very often (as you say, to check what I’m seeing, or could be seeing, in the sky).

        (I have yet another piece of software for tracking the ISS and setting up viewing notifications. Used it a bunch at first; haven’t touched it in quite a while.)

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