The first Mishna in Massekhet Pesaḥim teaches about the law that requires bedikah – a search for leaven – on the night of erev Pesaḥ, the 14th of Nisan. The expression used by the Mishna in teaching this law is, “Ohr le-arba-ah asar bodkin et he-ḥametz le-ohr ha-ner“, which is translated as, “On the night of the 14th we search for leaven by the light of a candle.” The difficulty with this is that the first word – ohr – appears to mean “the evening of” even though it usually means “light” (as, in fact, it does at the end of the Mishna’s teaching – le-ohr ha-ner, “the light of a candle”).
Explaining how the word or comes to mean “the evening of” is such a difficult question that the entire first daf of Massekhet Pesaḥim is dedicated to this one issue. So many different explanations are given that the rishonim ask why the Gemara continues to quote other proofs that or really can mean “the evening before,” even after convincing arguments have been brought. The Rashba is quoted as saying that as an obvious question, the issue was raised in all of the yeshivot where this Mishna was studied. When the Gemara was edited, the various answers were all collected and put together.
In his commentary on the Mishna, Maimonides points out that Rabbi Yehuda haNasi who edited the mishnayot chose an obscure word to open the massekhet because he preferred to make use of a positive word at the beginning of the tractate, rather than a word that carries with it connotations of darkness. As we will see, this explanation is given by the Gemara later on.
As an example of a proof offered by the Gemara that the word or can legitimately be used to mean “evening,” the Gemara quotes a passage from Tehillim (148:3) that describes how the kokhavei or – the stars of ohr – offer praise to God. Since stars are in the sky at night, clearly this means “night stars” and the word or can mean “night.” The Gemara responds that this is not a proof, since it may simply mean “stars that give light.” This argument is rejected by the Gemara, since that would imply that only stars that give light praise God, while stars that do not give light do not.
Some commentaries understand the Gemara’s reference to stars which do not give light as referring to comets or the moon, whose light is reflected from the sun, as they do not have internal light sources. This does not seem to be the Gemara’s intent, and, in fact, there are certainly dark stars in the heavens which do not give off light, such as the gravitational forces of black holes in space, whose presence and location can be found through other means.