Axial Tilt, Latitude and Other Planet Details

It’s been a while since I did a worldbuilding post.  This one will be thin, because I just have a few points of data rather than anything particularly visual or interesting, but my friend Chris and I spent most of today batting planetary details back and forth, trying to figure out some of the specs of my world.

I already had a lot of the details, because that’s what I do when I’m bored.  Year length, day length, sunrise/sunset times, lunar calendar, planetary dimensions, globe map…

But I’d had an astronomy question for him, related to the ending of Book 3, and after solving that, he’d had a few questions too.  Like what the planet’s axial tilt was, which I didn’t recall calculating.

So we looked at some of the data I already had, like my remade globe map.

Globe Map with Tropics and Polar Circles

Globe Map with Tropics and Polar Circles

I lost the original GIMP2 file, but did mark the tropics and the equator on the exported images.  After some tweaking, we decided that the axial tilt is 28.5 degrees — meaning the planet is slightly more tilted than Earth is, giving it slightly more extreme seasons.  Not that it matters much right now, since most of the central continent where the story takes place is in the northern tropics.

An axial tilt of 28.5 also means that the polar circles — the areas where there is at least one whole day where the sun never sets, and one where the sun never rises  — are bigger.  Earth’s polar zones start at 66 degrees latitude, more or less, while Halci’s start at 61.5 degrees.

Figuring out the tilt and actually taking a look at the latitudes of some of my cities made me revise my sunrise/sunset times from my original figures.  I think I made them way back before I had a globe map, when the continent was much larger in my mind because I hadn’t yet run mile-markers across it.  Books 1-3 have involved a lot of cross-country travel though, which I wanted to make sure happened at a reasonable pace and within a reasonable time-frame.  Thus, when I started matching the text to the maps, the continent shrank.

So while I’d been thinking of the ‘northern’ cities, like Daecia City, as having a fairly extreme summer/winter daylight difference, most of them are at the edge of the tropics.  (Daecia is just above the orange zone, about 31 degrees N latitude, so technically in the subtropics.)  This doesn’t mean they’re balmy; Cairo is at 31 N, but so are Shanghai and Nagasaki, and the world itself is several degrees cooler on average than Earth is.

And by ‘several’, I mean probably 52F/11C, as opposed to Earth’s current average temperature of 58.3F/14.6C.  This is actually a warm period for the world, which is almost always in an ice age (which just means it has polar ice at all times) but is currently not undergoing glaciation.

There are also certain regular climate features that drive temperatures down in the not-so-north.

Climate Distribution

Climate Distribution

Air Currents and Seasonal Weather

Air Currents and Seasonal Weather

There are very large ice-caps on both poles, and past ice ages brought glaciation all the way down to the continent’s northern mountains.

Anyway, actually doing the calculations helped me fix the sunrise/sunsets, which shouldn’t require much tweaking in the text since I don’t recall stating many times — hurrah for clock-indifferent cultures.  It also helped me figure out solar noon zeniths throughout the year, which concerns me because I like details, okay?  And details of the sun’s movements become more important as the story goes on, so I’d like to be correct with them.

It’s been too long since I took an astronomy class, so yay for Chris!

We used some calculation tools while we tried to figure the days out, so I’ll share the few Chris linked me:

Length of Day (maaaaaath, maaaaaaaaaath!)

ESRL Global Monitoring Division — Global Radiation Group (spreadsheets!)

Sunrise Sunset Times (calculator)


About H. Anthe Davis

Worldbuilder. Self-published writer.
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6 Responses to Axial Tilt, Latitude and Other Planet Details

  1. Jay Dee says:

    Nice! I love this kind of post. I need to work on worldbuilding posts, too. In fact, I need to get to work on my geography posts, which will help me with my country profiles for Ariadne. And that will include things that you wrote about, such as latitude, climate, and so on.

    • I need to get back to my country/culture profiles myself. Alas, I lost most of the images I was using for reference when my hard drive crashed, so I’m having to reassemble those — and also I’m torn on how much detail to go into on certain countries, since there may be spoilers. But hopefully I’ll be doing regular worldbuilding posts again soon.

      • Jay Dee says:

        I’m planning on simply doing physical geography at first. Anything cultural or any demographics will come later as I write the books. They’ll also come with a spoiler warning.

  2. Jay Dee says:

    Reblogged this on I Read Encyclopedias for Fun and commented:
    I’ve already done posts about this before, but it’s nice to see another point of view. And this one has some really nice maps!

  3. Erica Dakin says:

    How big is Halci compared to Earth?

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