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
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