From 0001 CE through 2018 CE the Molad of Tishrei is compared with the Conjunction. Here we do not use Dehiyyot so Rosh HaShanah begins after 6:00 PM at the beginning of the Hebrew Day containing the Molad.
Actual sighting of the Crescent New Moon is approximated at the first 6:00 PM with the moon being at least 24 hours old (from the conjunction). It is true that the Crescent New Moon of Tishrei can be seen in Israel sometimes after 20 hours but on average it is seen only 32 hours after the conjunction
At the right is a table based on this approximation. It must be recognized that age of the moon is not the only criterion that needs to be used to determine when the moon could be seen. The orbit of the moon is critical as well. This is why the Global Visibility Maps of Robert van Gent and the data from Sky View Cafe are so valuable.
The meaning of Skew is that the parameters of the Fixed Hebrew Calendar push the day we effectively use as Rosh HaShanah forward. 70% of the time 2 days forward. To find a reason for this was a major focus of my research.
From this study it is apparent that occasionally (3.86% of the time) we skew 3 days forward. In that case the Old Moon will be very visible Erev Rosh HaShanah which gives keen insight to the Talmud Rosh HaShanah 25a. I develop this in my book.
Here is part of the Molad of Tishrei Study.
