I was in Safed a few years and made it a point to be near the grave of Rav Isaac Luria around sunset.
And pretty much every day I saw something that confirmed the approach of Rabbainu Tam concerning the time the night starts.
That is for the first 59 minutes after sunset, nothing dramatic changes in the sky except for it slowly getting darker. Then right at 59 minutes something dramatic happens. A kind of dome forms over the area where the sun went down. Then that dome itself begins to sink until at exactly 72 minutes it sink below the horizon and the sky is consistently dark. [You can see how this corresponds to the Gemara in Shabat.]
There is also something about what you call "medium." The word medium has no meaning except as compared to something else. This the number 5 is medium between 0 an 10 but not medium compared to 100 and 1000000. So to be able to gauge what is a medium star you need to first get an idea of what all stars look like in the middle of the night. Once you see all the stars that can be seen by the naked eye, then you pick three medium stars. Then you learn how to identify them by learning thoroughly the map of the sky. That is to learn to identify the constellations and the place of each star in a constellation. Then after you know what is a medium star, you go out and see on some night when it becomes visible. Three are visible at 72 minutes.
However stars that are seen a half hour after sunset when you compare them with other stars in the middle of the night are not medium stars. They are giants compared to all the others. They are what the Gemara calls large stars. Large stars do not tell you when the night begins. Only three medium stars.
In terms of stars I also saw something there and also in desert regions in Israel. No stars are seen at sunset. None. So if twilight begins at sunset, where are the two medium stars? According to the Gemara twilight begins when one medium star is seen, not large stars which can be seen before then. So it is curious that at sunset no large large stars, nor medium stars are visible. That seems to knock the idea of twilight beginning at that time.
There is also something about what you call "medium." The word medium has no meaning except as compared to something else. This the number 5 is medium between 0 an 10 but not medium compared to 100 and 1000000. So to be able to gauge what is a medium star you need to first get an idea of what all stars look like in the middle of the night. Once you see all the stars that can be seen by the naked eye, then you pick three medium stars. Then you learn how to identify them by learning thoroughly the map of the sky. That is to learn to identify the constellations and the place of each star in a constellation. Then after you know what is a medium star, you go out and see on some night when it becomes visible. Three are visible at 72 minutes.
However stars that are seen a half hour after sunset when you compare them with other stars in the middle of the night are not medium stars. They are giants compared to all the others. They are what the Gemara calls large stars. Large stars do not tell you when the night begins. Only three medium stars.
In terms of stars I also saw something there and also in desert regions in Israel. No stars are seen at sunset. None. So if twilight begins at sunset, where are the two medium stars? According to the Gemara twilight begins when one medium star is seen, not large stars which can be seen before then. So it is curious that at sunset no large large stars, nor medium stars are visible. That seems to knock the idea of twilight beginning at that time.
