615. Bein Hashmashos

Shabbos 5:4

The time from sunset until the three middle-sized stars appear is called bein hashmashos (twilight). There is a doubt as to whether this period is considered part of the day or part of the night so the practice is to act stringently regarding it in all matters. Therefore, one may not light Shabbos candles at this time. One who performs an act of labor during the twilight period, whether on Friday leading into Shabbos or on Saturday leading out of Shabbos, would be required to bring a sin offering. The stars that indicate that night has fallen are not so large that they could be seen by day, nor so small that they are only seen at night; they are middle-sized stars. When three such stars are visible, it is definitely night.

Shabbos 5:5

The wick used for the Shabbos lights may not be made from anything that would cause the light to flicker. This includes wool, goat hair, silk, cedar fiber, flax that has not been carded, palm fibers and different types of soft trees, et al. Rather, we must use something that makes steady flame such as flax that has been carded, strips of linen clothes, cotton, etc. One who lights the Shabbos candles must ensure that the fire has caught on the majority of the exposed portion of the wick.