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Hello everyone!

On February 25 in the afternoon the sky was more or less clear for several hours. I looked in Stellarium to find out, is there any other object than Sun visible during the day time. The second brightest object was Jupiter. I decided to try to take Sun-Jupiter lunar distance. Sorry, no! Not a lunar distance! I meant Jupiter Solar distance! : ))

I was not sure of ability to find Jupiter during daytime, but it was an interesting challange. So I made some preperation: I measured in Stellarium distance between Jupiter and the Sun, memorised an approximate altitude, azimuth of Jupiter and also inclination angle with horizon if looking from the Sun to Jupiter and from the Jupiter to Sun. 

With these data I was ready for the experiment. I went out, measured index error by Sun (6 times). Then I preset sextant to expected angle and looked to the Sun through my sextant’s telescope. I inclined sextant  to the angle I found in Stellarium (it was about 45° to the left) and started very slowly rotate the instrument around the axis of the telescope untill finally I saw Jupiter as very week spot next to the Sun (Jupiter was slightly dimmer than if the sky were completely clear, because of little haze in the sky). Now I was sure that it is possible to see Jupiter under the existing weather conditions.

To get a brighter image of Jupiter I had to change the direct and reflected objects. Namely — Jupiter had to be visible as direct (not reflected) image. So I turned sextant to the azimuth angle where Jupiter was expected to be, and elevated the sextant until I found the Sun’s reflected image in the field of view. Then very slow adjustments of sextant position enabeled me to find Jupiter. And I took 6 distances.

It is obvious that these distances can be used to check the accuracy of the observations, if we know where we are. But then I started to think: can Jupiter/Sun distances provide any useful information for navigation? I know that Sun moves about 1°  during 24 hours along ecliptic. Very roughly it means the Sun moves 0,16’ in 4 minutes of time. 0,16’??? Hey! But this is an accuracy that can be achieved with good metal sextant! I know that my observation skills with the instument I used allows me to get 0’-0,2’ accuracy in 66% cases, 0’-0,25’ accuracy in 84% cases and 0’-0,3’ in 90% cases. Only 4% of observations are in error range 0,35’-0,4’.

To get true distance angles I used Paul Hirose’s Lunar 4.4 program. I found that the mean error from the true value for this set of distances was 0,15’.

Then I used the same program to get out geocentric values of Jupiter/Sun distances. There it could be found that rate of change of the geocentric distance was 0,14’ in 4 minutes of time. This means that I was able to get my longitude with accuracy about 1° (because this is the angle the Earth rotates in 4 minutes). Of course, there is some element of luck in this. It was possible to get an error in distance, for example, 0,3’ and then my longitude would be about 2° off. 

For modern standarts it is a bad result, but if we remember the first trials of lunar distances in the 18. century, the accuracy what was expected then (at time when Maskelyne lived) was also only 1…1.5 degrees of longitude.

Jupiter is not the most convenient object for Solar distances. Of, course Venus is much advantageous, because of brightness. I have made Venus/Sun distances for several times, but never thought of possibility to get longitude out of them.

Maybe someone has already experimented with this, but I have not come across this kind of information (if you have some information, please, share with it). It is something new for me. Of, course, it is clear why such a method is not very practical, but I still find it being an interesting aspect of navigation. For example, I think in historic context it should have been easier to predict the position of planets and the Sun in the sky than that of the Moon. But on the other hand requirements for observation accuracy are much higher than for lunars (the Sun is moving more than 10 times slower than the Moon). Besides the visibility is more limited than in case with lunars. There is much more difficult to get altitudes of Jupiter or Venus during daytime (except when they are calculated). Yes, indeed there is a huge amount of flaws : ))

Modris Fersters