Last month, close to the full moon, I watched as the moon transited Jupiter. It got me to thinking. Is it safe to assume that when these two bodies are at their closest, their GHA's the same? Could one take sighting and determine a local time such that the two bodies are closest then compare to almanac to determine GMT? I have tried to find the lunar reduction tables but they seem to have disappeared off of the web.
celestial nav thought experiment
- Thread starter ChuckThomas
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I believe that what you discribe is/was part of the lunar distance method for determining longitude. http://www.usno.navy.mil/USNO/astronomical-applications
Hi Chuck,
It's been a while since I used them but if you're looking at two bodies at the same time, then it is the same time. Maybe I don't get the question. I saw a cell phone with an ap for celestial and it shows all the stars as you turn, even those below the horizon and during the day. Good bye sextant.
All U Get
i think i asked the question wrong. i used gha. i should have said sha. rephrased question: will the moon and jupiter be closest in the sky when their sideal time is the same. once i determine through observation that the two are as close as they are going to get, that is the moment when sha of the two are the same and therefore can determine gmt from almanac. is this assumption correct?
one more assumption is that we will be where we can observe this. i am thinking that one will have to be in the pacific somewhere to really be able to observe the exact moment that this will happen in a couple of weeks.
one more assumption is that we will be where we can observe this. i am thinking that one will have to be in the pacific somewhere to really be able to observe the exact moment that this will happen in a couple of weeks.
It's early in the morning here (0545) and I just looked up to see Orion's Belt. In my backyard, not out in the Pacific. When the moon's around in a coupla weeks or months, something will be in line. What are you trying to do with the conjunction? If it's finding the time, then you could be the next Donald Crowhurst!
Ross, I have spent many hours on the USNO site. It is great. I guess really what I am asking is more related to planet/moon measurements as opposed to star/moon measurements. When measuring star/moon distance, the assumption that closest point will be when SHA is the same is false. That is because the relative curve the moon follows in the sky. But since the planet/moon curves are almost the same can one assume that the closest point is where the SHA's are the same?
Stu, yes the goal is to determine GMT. (can you tell it is really boring here in middle MS?) Several years ago I was researching using lunar distances to determine time. It is an almost lost art. There were several web sites that gave great info. They seem to be gone to the big bit bucket in the sky. I would really like to ask this question on the starpath.com forum but I do not have an account. Any one have access to that site?
Stu, yes the goal is to determine GMT. (can you tell it is really boring here in middle MS?) Several years ago I was researching using lunar distances to determine time. It is an almost lost art. There were several web sites that gave great info. They seem to be gone to the big bit bucket in the sky. I would really like to ask this question on the starpath.com forum but I do not have an account. Any one have access to that site?
navigation assumptions
Chuck
You are assuming that the moon and planets move in courses parallel to the stars. if that were true then the time of conjunction (closest approach) would be lookupable in the charts. Since the planets and moon move in courses that are at an angle (the ecliptic generally) to the star courses you can only get a close approximation.
With that said I'm not at all convinced this is even a practical thought experiment since:
It does not happen often enough to do real navigation by.
My Timex if probably going to give me a more accurate time than this method.
Now if you consider measuring conjunctions with stars then you have a few more options but I'm still of the opinion that my Timex is a better source of time data. Also consider using GPS time. That is VERY accurate and available with only one sat visible.
Chuck
You are assuming that the moon and planets move in courses parallel to the stars. if that were true then the time of conjunction (closest approach) would be lookupable in the charts. Since the planets and moon move in courses that are at an angle (the ecliptic generally) to the star courses you can only get a close approximation.
With that said I'm not at all convinced this is even a practical thought experiment since:
It does not happen often enough to do real navigation by.
My Timex if probably going to give me a more accurate time than this method.
Now if you consider measuring conjunctions with stars then you have a few more options but I'm still of the opinion that my Timex is a better source of time data. Also consider using GPS time. That is VERY accurate and available with only one sat visible.
Re: navigation assumptions
Bill if you have to change the battery in your timex and have no other time scource then this method could have it place. Local apparent noon and knowing you position on earth can be worked back to determine GMT. Finding longitude is never easy and before Harrison it was very difficult. The cheapest quartz watch today is more accurate than Harrisons clock. The British were giving the award for being within 30 miles after a 1500 mile voyage.
Bill if you have to change the battery in your timex and have no other time scource then this method could have it place. Local apparent noon and knowing you position on earth can be worked back to determine GMT. Finding longitude is never easy and before Harrison it was very difficult. The cheapest quartz watch today is more accurate than Harrisons clock. The British were giving the award for being within 30 miles after a 1500 mile voyage.
well, yea! but...
you never go to sea with just one source of time data, right?
I use my timex
my ships clock (also quarts)
GPS (for ships clock drift rates determination)
and a sextant and almaniac
you never go to sea with just one source of time data, right?
I use my timex
my ships clock (also quarts)
GPS (for ships clock drift rates determination)
and a sextant and almaniac
" compare to almanac to determine GMT?"
If I am not mistaken, the almanac years ago used to have a table that showed transitions. It would show in GMT the day, hour, minute when a particular star or planet would disappear behind the moon and the time that it would reappear. Since it essentially appeared to happen at the same time all over the globe, it could be used to set your watch by. I don't whether or not it still exists, but it would save you a lot of figuring.
Ray
If I am not mistaken, the almanac years ago used to have a table that showed transitions. It would show in GMT the day, hour, minute when a particular star or planet would disappear behind the moon and the time that it would reappear. Since it essentially appeared to happen at the same time all over the globe, it could be used to set your watch by. I don't whether or not it still exists, but it would save you a lot of figuring.
Ray
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