Quote:
Originally Posted by derstosstrupp
I have noticed that sometimes the NewSextant mod thinks a body is in a different spot than it appears to be. Typically for a noon latitude sight I don’t see UT show up after putting the object in. If I drop down a degree or so, sure enough, there it is. Now of course I don’t need time for a latitude sight, but it’s just odd. I have run into the same trying to shoot some stars, not very many, but sometimes I run into an issue where UT doesn’t show.
Regarding the sun, when I am doing a running fix for example, if I take, say, and afternoon sun line, for which I get UT displayed, when I use real almanac declination I get very close. However, when I use real declinations for a noon latitude sight (where UT doesn’t show up as mentioned before), I get very close using Alpheratz’s sun declinations. So there is definitely some disconnect perhaps in NewSextant. Not a big deal, because we can use “two sets of books”. But definitely something to investigate.
In short, if I can see UT show up in the display, I get very close using real declinations, but if UT doesn’t show, Alpheratz’s data seems to be closer. I have had a friend test this as well in parallel.
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Thank you, you have raised a very important issue. I know what's the matter here. This problem you are talking about is not a bug in the mod. The reason lies in celestial mechanics. There is only one possible calculation method for time correction, and this method is implemented in the New sextant mod:
There is a formula that describes the change in altitude within 10 seconds of time:
Delta h per 10 sec = 2.5 arc min * cos (Latitude) * sin (Azimuth)
The New sextant mod calculation algorithm calculates the altitude for a celestial body and compares it with the Measured altitude.
Delta h = |Calculated altitude - Measured altitude|
Time correction is performed based on the above two formulas.
Now about celestial mechanics. After the rise of a celestial body, it increases its altitude relatively quickly. The rate of increase in altitude decreases as you approach the upper culmination. Immediately before upper culmination, at the moment of upper culmination, and immediately after upper culmination, there is practically no change in altitude. After upper culmination, altitude decreases very slowly at first, then it happens faster, the rate of altitude decrease is maximum before the celestial body sets over the horizon. The most accurate time correction is possible when the Azimuth of the celestial body = 90 or 270: this corresponds to the largest altitude increment per unit time. The possibilities for time correction are almost exhausted when you observe the upper or lower culmination of a celestial body, when there are practically no changes in altitude (Azimuth = 0 or 180).
And now the most interesting thing is why UT disappears. Even if you follow all the recommendations for working with the sextant, which I talked about in my videos, you can achieve an accuracy in measuring altitude at the level of + -0.5 ... 1.0 arc minutes. The upper culmination moment corresponds to the maximum altitude that a celestial body can reach at a given latitude. When at the moment of upper culmination you measure Altitude with an error towards overestimation of the result, let it be at least the smallest error + 0.1 arc minutes, then for the New sextant mod calculation algorithm you get an altitude that a celestial body can never reach - based on the measured altitude, the time cannot be corrected, so UT does not appear.
Everything I talked about above concerns both the measurement of stars and the Sun.
Now, for the Sun alone. I have to manually correct over 2500 lines of the Sun.txt file in the New sextant mod in order for this mod to be usable for time-corrected measurements of the Sun. When taking measurements of the sun, you need to make sure that the SEXTANTSUNDIAMETER (String # 53 of "page layout.py") matches the size of the sun in the mod being used. For example, the new test build TWoS 2.2.23 should have SEXTANTSUNDIAMETER = 2.18666666667, but the Sun.txt file has not been redesigned yet, it is premature to talk about it. When measuring the Sun, when in the sextant interface field you enter the value "0" to indicate the celestial body being measured, the sextant "knows" that you are measuring the lower limb of the solar disk and the sextant reading is the altitude of the solar disc center, taking into account the influence of dip + refraction. that is, you don't need to add sun semidiameter to the measured value.
If you want to check the relevance of the declination of a celestial body (remember the maximum accuracy of a single measurement is + -0.5 ... 1.0 arc minutes, and in the case of measuring the Sun due to blurred contours of the solar disk, the error can be 2 - 3 arc minutes), regardless work of New sextant mod and time correction, then you should take measurements without entering the number of the celestial body in the sextant interface field. The sextant reading in this case will represent the actual altitude (without the influence of dip + refraction), which you can use directly to calculate declination when measuring the altitude of upper culmination at a latitude known to you. When checking the declination of the Sun, you should also not enter "0" in the sextant interface field. Add Sun semidiameter to the sextant readings (1 deg 5.6 arc minutes in new test build 2.2.23) and calculate the declination.