NavList:
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Re: Star - Star Observations
From: Brad Morris
Date: 2010 Mar 13, 19:41 -0500
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From: Brad Morris
Date: 2010 Mar 13, 19:41 -0500
Hi Frank
You wrote:
Just out of curiosity, could you post some sample RAs and Decs for today from the Skyscout? Five or ten first magnitude stars would be enough to get a sense of the position quality.
You probably missed it. In a recent post, re-iterated here, I mused:
One disappointment yesterday was the conclusion that the Skyscout does not provide the degree of accuracy and that I must then retreat back to the
Nautical Almanac. This leaves me with 57 navigational stars (and 4 planets) distributed throughout the entire celestial sphere, or simply not enough.
After a good long think, it occurred to me that I could use the Astronomical Almanac Online,
Bright Stars table. In this table, about 4500 of the brightest
stars (magnitude greater than 4.5) are present, sorted in ascending order of Right Ascension. The thought was to use the Skyscout to point at some random
star. It will determine the most likely candidate, providing me with the scientific name, RA, declination and magnitude. Ah ha! Assuming the star does
have a magnitude greater than 4.5, then it will be in the table. In some experiments last night, it was quite clear that the concept is workable. That is,
I could readily pick out the star in the table, given the Skyscout inputs. The table provides right ascension in hours, minutes and seconds to the nearest
10th of a second. Ditto the declination. Converting 1/10 second of RA (time) yields 1.5 seconds of arc, about 4x better than the NA.
Question: I expect that the
Bright Stars table provides true (uncorrected) positional data. Will this star data require correction for aberration?
Kind of kills the point and shoot aspect that made the Skyscout tool so fun, but still, this simple method brings me back to a plethora
of stars, not just the handful available in my sky when looking at the Nautical Almanac. The Nautical Almanac is appropiate
for LOP altitude observations. How many lines are really required for a fix? About 3, which makes the NA perfect for LOPs.
However, for calibrating the arc, you need
many more pairings than the NA will provide for your particular place on the planet.
Using the Skyscout I can still point at random stars for
pairings. "Hey, that pair is just a little bigger angle, lets try!" In order to preset
the sextant, however, I now will have to enter into the Bright Stars table. These would be the stars most of us would use anyway.
After all, if you can't see the object easily, you are quite unlikely to use it for this purpose.
There really isn't a problem finding
the star in the Bright Stars table. The Skyscout provides the name and all scientific data.
For example, I powered up the Skyscout on this cold rainy night. I can't
see any stars, but as you know, that isn't how it works.
I pointed it up randomly at the sky (while sitting nice and dry inside) and
it says I pointed at 'Bayer: 5 UMi' and that the brightness
of this object is 4.25, the RA is 14h 27.53m and the Dec is 75.6960 degrees.
Page H19 of the Bright Stars table for 2010 states:
'Flamsteed / Bayer Designation: 5 UMi BS=HR no. 5430, RA = 14h 27m 30.9s
Dec = +75d 39m 14s and that the brightness is 4.25.
That sounds like a precise match to me and took far longer to type into
this email than to look up.
Now lets compare the data. From the Bright Stars table, the RA 14h 27m 30.9s, which I believe is 216d 52m 43.5s. From the
Skyscount, the RA is 14h 27.53m, which I believe works out to 216d 52m 57.0s. That works out to a difference of 13.5 seconds.
Pretty darned good if you are using the Skyscout for its intended purpose, but out by 2 tenths of a minute for what I want to use
it for. But with the Bright Stars table, the inaccuracy is avoided. And with 4500 stars to choose from, I think I may have a good
chance to find the pairings I want along the arc!
BTW: Thanks for confirming that I am not going at this problem completely cross-eyed. I try to reason things out in a way that
will make sense to me and also be somewhat practical for me. Its nice to know that sound advice by others confirms that I am
getting it right. Okay, maybe not all of the time but some of the time :-)
I will be attending Navigation Weekend this year and we can all play around with star to star sights, using the Skyscout.
Twilight is not required! Just a view of the stars and we can observe for as long as we can hold our sextants up.
Best Regards
Brad
"Confidentiality and Privilege Notice
The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you."
