Can someone help explain this to me? Cel Nav question

Hi guys,

Currently studying for CM/MA exams and need a little help with figuring out GMT from the given chronometer time for these Lat by upper/lower meridian transit problems. How do you know if it’s AM or PM? Is there something I’m missing on these? Here’s an example of one of the questions.

On 16 June in DR position LAT 50° 57.0’ S, LONG 53° 03.9’ W (ZD+4), you take an ex-
meridian observation of Acrux at lower transit. The chronometer time of the sight is 10h 08m
18s, and the chronometer error is 02m 12s fast. The sextant altitude (Hs) is 23° 49.0’. The index
error is 1.1’ off the arc, and your height of eye is 26 feet. What is the latitude at meridian

I know how to do the problems, but I am always getting the GMT part wrong. In this question, the correct GMT time was 10:06:06…how does one rule out that it’s not 22:06:06??? There’s no DR ZT given and that’s tripping me up.The rest of the problem I understand and can do.

Any help at all is highly appreciated!!!

When you were in Elementary school do you remember them going over the concept of ‘being in the ballpark’?

Not really making the connection here with what you’re saying man…the GHAs and Declinations are gonna be wrong if you do AM when it should have been PM and vice versa. Ballparking it doesn’t really do me any good, but thanks anyways. Of course I can find out if I was wrong with the GMT at the end when the answer choices are way different than what I calculated…but then I have to redo the entire problem and that sucks, plus it’s a waste of time and energy. I must be missing something.

Well, think about it for a minute and you might.

Not really looking for smart ass bull shit man. I am genuinely interested in figuring this out. In the example problem I provided, ZD+4, it would make sense that the star would be visible at GMT 10:06:06 (0606ZT) and possibly 22:06:06 (1806ZT).

Acrux is A. Dubhe is D. The problem on May 5 is C (get it? Cinco de Mayo)

Well at least they used to be to be until they changed all the answers around.

No shit, lol. Yes, the answer to the question was indeed A!
But with the money I’m shelling out for airfare, hotels, and rental car to take these damned tests I have to be sure, lol. That’s a very cool observation. Wish I would have known that when I took the 3M tests!
I am going to check the new question bank to see if that still holds true! :laughing:

And I’m not looking for your dumbass bullshit. I’m trying to help you. Think about it.


I think his point is you usually shoot stars during twilight.

Thanks man, that article seems to be just what I needed. This is much appreciated!

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Makes sense. Thank you guys for the replies!

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And it’s way more involved than your snide comment implies as you need to fully calculate out twilight for both possible times to get the answer. It’s not something simple like “gee, I’m taking a sun sight so use the one that’s daytime”.

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Nope. Whichever one you choose there’ll be an answer choice that matches it.

I’ve never met another industry so sincerely bound and determined to be so bad at communicating with one another.


No, it’s not. If you don’t learn to catch those tricks you’re going to have a difficult time with the exams. One is in the ballpark and one is not. Just like grade school.

If you don’t like the way I helped the guy, help him yourself. You’re just bitching and whining currently.

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That’s simply false. Try again.

No it’s not. Twilight is listed in the almanac. One time is during twilight, and one time isn’t.

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Try again at what? What the hell are you even babbling about?

The exact problem is in the link I provided. Go read it and learn a thing or two.


True. But BOTH are “in the ballpark”, which is my point.

Yes it is and it appears you didn’t even read your own link. Both possible times are “in the ballpark” and you actually have to calculate times of twilight to see which one is the correct time.