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+--- Thread: USCG Crazy Exam question: Sidereal vs Tropical Year (/showthread.php?tid=252)
USCG Crazy Exam question: Sidereal vs Tropical Year - PeterB - 05-02-2023
USCG Exam Question Sidereal vs Solar (Tropical) Year
I was poking about and came across a USCG practice exam here:
https://www.dco.uscg.mil/Portals/9/NMC/pdfs/examinations/q154_nav_problems-oceans.pd
Question 8 on page 3 asks:
8. The Tropical year differs from which year by 20 minutes?
A) astronomical
B) sidereal
C) equinoctial
D) Natural
The correct answer is B) sidereal
To start off with I'm not even sure why they would ask this question. I don't know of any case where such knowledge would be useful to a navigator or required for his daily execution of his duties. I also do not recall it being taken up in any of the many texts on celestial navigation I have studied. If anyone else knows of a good example of when such knowledge would be useful or required please let me know.
What I do know is that the sidereal day is 4 min shorter than the mean solar day and this is information that is useful to celestial navigators. Particularly for planning meridian sights in advance or for general knowledge of what is happening in his sky day-by-day.
So it seemed to me that after 365 1/4 days of a mean solar year that the sidereal time reckoning should be 24h 21 min ahead of the solar time reckoning. So what happened to the extra day?
As it turns out the sidereal year is 20 min longer than the mean solar year. Some of that has to do with precession. I have not gotten into that part and I do not intend to, but I was still scratching my head on the whole missing day.
I think I have it worked out now, at least schematically. See the attached illustration of a planet that circles its sun in 4 solar days and how, during those 4 solar days, it actually experiences 5 sidereal days.
Still I wonder where is this curious bit of trivia useful to a navigator? Any ideas?
PeterB
RE: USCG Crazy Exam question: Sideral vs Tropical Year - c_davidson - 05-04-2023
Peter,
I'm working on it. Say, that test is very interesting.
Clen
RE: USCG Crazy Exam question: Sidereal vs Tropical Year - PeterB - 05-04-2023
Here are two other links you might like. One is to another practice exam, the other is to the "Training Almanac for 1981" that you need to use for all of the practice exam celestail problems and which also includes things like the Polaris Tables. (TheNauticalAlmanac.com sight offers a 1981 almanac, but it doesn't have the Polaris Tables.) They are all down-loadable to your local device.
https://www.dco.uscg.mil/Portals/9/NMC/pdfs/examinations/q127_nav_problems-oceans.pdf
https://www.dco.uscg.mil/Portals/9/NMC/pdfs/examinations/10_1981_nautical_almanac.pdf
UPDATE: CelNav57 has informed me that the 1981 Almanac on this site has been updated to include the Polaris Tables.
Peter
RE: USCG Crazy Exam question: Sidereal vs Tropical Year - Fred_B - 05-05-2023
Peter,
Not knowing if this'll help. Consider this, Aries moves 15.041° per hour.
Just an idea.
Fred