I looked up the errata to see if this is a known error. It says there's an error in the figure, except that the error is L and that the correct version is 90 - L. So it would seem that my copy of BOB has been "corrected". Is there a different convention for latitude that I'm unaware of? - ThreadSky

astroalysa.bsky.social • 99 days ago

I looked up the errata to see if this is a known error. It says there's an error in the figure, except that the error is L and that the correct version is 90 - L. So it would seem that my copy of BOB has been "corrected".

Is there a different convention for latitude that I'm unaware of?

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jarron.bsky.social•99 days ago

If you replace L with 90-L, then it becomes 90-(90-L) which equals L?

astroalysa.bsky.social•99 days ago

Ha, that is indeed correct! It just seems like an odd way of phrasing it. I’d have written, “replace ‘90-L’ with L” because that’s a bit more clear (or at least I think so)

astroalysa.bsky.social•99 days ago

I’m not even sure if digital copies of textbooks have corrections applied to them or not. So perhaps my concern about the ambiguity is irrelevant.

jarron.bsky.social•99 days ago

Okay, just found my copy of C&O from 1996. There, it shows the angle as L, latitude. But that cannot be right, because when you are at the equator, L=0. But the angle between the equator and NCP is 90. So the equation must be 90-L.

jarron.bsky.social•99 days ago

Or am I misunderstanding what angle they’re referring to?

astroalysa.bsky.social•99 days ago

It’s the angle (altitude) between the NCP and the local horizon. So it should be L. If you’re at the equator, the NCP is at the horizon whereas it’s at zenith for someone at the north pole

dpthorngren.bsky.social•99 days ago

I guess they're doing azimuthal angle instead of declination? Weird, I don't recall seeing that before.

fringedoctor.bsky.social•99 days ago

Difference between zenith angle and elevation angle?

astroalysa.bsky.social•99 days ago

I can tell you that I’ve spent a grand total of maybe 5 hours of my entire astronomy education and career thinking about alt-az coordinates, so I hadn’t paid any attention to it until I was teaching it 😅

astrowright.bsky.social•99 days ago

Wow, yeah, that's weird. Someone got confused about the definition of latitude and they hypercorrected I guess? Strange no one has caught it and fixed it in all of these years…

Maybe we're somehow misinterpreting the diagram?

astroalysa.bsky.social•99 days ago

Someone else pointed out that replacing L with 90-L is indeed correct (since it’s then 90-(90-L)=L). Just seems an odd way of phrasing the correction and leaves room for ambiguity about whether you’re seeing a corrected version or not

astroalysa.bsky.social•99 days ago

I suspect more people aren’t familiar with the error because working with alt-az coordinates isn’t really a big thing in astronomy education?

astroalysa.bsky.social•99 days ago

You’d need to worry about it e.g. for determining if/when an object’s visible at a site, but I imagine most people just use pre-existing calculators for that.

eliothalley.bsky.social•99 days ago

Yeah, my guess is C&L is mostly used by intro classes, and those rarely cover details of how to find if some object is up. Observational astro classes *will* cover this, but they use more specialized textbooks. So the observ astro experts rarely have an excuse to check this diagram carefully.

astroalysa.bsky.social•99 days ago

Yeah, we touched on this in my observational astronomy exam. The textbook (which I still have) doesn’t cover it in great detail, though. It gives some coordinate conversion formulae, and basically just says “how you get this should be immediately obvious from these 2 diagrams, go!”

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