A solar or lunar eclipse:

The Talmud[1] states that an eclipse is a bad omen for the entire world.[2] More particularly, a solar eclipse is a bad omen for the gentile nations, while a lunar eclipse is a bad omen for “The enemies of Israel[3].”[4] [Although an eclipse is set in nature as part of the orbit of the sun and moon, and is foreseen many years in advance, nonetheless, the Sages learned that its occurrence is a bad omen for the world due to the four sins found amongst people, as will be explained. Several explanations for this have been offered: 1) It refers to G-d arranging visibility of the eclipse, which is weather dependent.[5] 2) The occurrence itself is not caused by the sins, but rather the occurrence represents a time of bad omen in which the below mentioned sins are punished for, if applicable in that area. It is a time of severity in which the above sins are more likely to get punished.[6] Furthermore, this bad omen causes people to have feelings for the sins to be mentioned, and create within them a bad nature. Nonetheless, man has ability to control these instincts and not give into his inclination.[7]] 3) The Talmud is not referring to an eclipse, but rather to sunspots, which are black dots found by the sun.[8]   

The location of the bad omen: The Talmud[9] states that the bad omen given by an eclipse applies to the location in which the eclipse was seen. If it was seen in the eastern hemisphere than it is a bad omen for those who live in the eastern hemisphere. If it was seen in the western hemisphere than it is a bad omen for those who live in the western hemisphere. If it was seen in the middle of the sky, then it is a bad omen for the entire world.

Who receives retribution? The Talmud[10] states that aside for the nations who live in the area of the eclipse receiving Divine retribution, so too their deity [which refers to the angelic minister who oversees that nation[11]] receives punishment.

Avoiding punishment:[12] At times that the Jewish people fulfill the will of G-d they do not need to fear the eclipse, as the verse[13] states “Don’t learn from the ways of the gentiles and from the signs in Heaven do not fear.”

What sins cause G-d to bring an eclipse and its bad omen:[14] There are four sins which case the bad omen of the solar eclipse to take place.

1. The sin of not respecting the leader of the Beis Din through giving a proper eulogy at the time of his funeral.
2. The sin of rape of a young married woman, which the community ignores.[15]
3. Homosexuality.
4. Simultaneous murder of siblings.

There are four sins which case the bad omen of the lunar eclipse to take place.

1. Forgeries, such as forged documents and papers which slander an individual.
2. False testimony.
3. Raising domestic livestock in Eretz Yisrael and letting them eat from others fields.
4. Cutting down good trees [who can bear fruit].

Fasting: Some Poskim[16] record that one is to fast when a lunar eclipse occurs.[17]

Teshuvah: The sighting of an eclipse should bring a person to perform Teshuvah and pray to Hashem to nullify the upcoming evil decree.[18]  The sighting of the natural phenomenon is to bring a Jew to a renewed recognition of G-d’s managing of the world.[19]

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[1] Sukkah 29a

[2] The reason: This is similar to a king who made a feast for his servants and then got angry at them and asked his servant to remove the torch [i.e. turn off the lights]. [Sukkah ibid] Accordingly, the concealment of the light of the sun or the moon is a sign that Hashem is angry and is hiding his Chesed from us.

[3] This is a euphemism, referring to the Jewish people.

[4] The reason: As the gentile follow a solar calendar while the Jews follow a lunar calendar. [Sukkah ibid]

[5] 1^st explanation in Likkutei Sichos 15 p. 7; Igros Kodesh 15:260

[6] Likkutei Sichos ibid; Aruch Laneir Sukkah ibid

[7] Likkutei Sichos ibid

[8] Yearos Devash 2 Drashos 3 of Rebbe Yonason Eibashits

[9] Sukkah ibid

[10] Sukkah ibid

[11] Rashi ibid

[12] Sukkah ibid

[13] Yermiyahu 10:2

[14] Sukkah ibid

[15] The Talmud ibid refers to a Naarah Meurasa

[16] M”A 580:1; Sefer Chassidim 230; M”B 580:2; See Mamar Mordechai 580:5; Iyun Yaakov Sukkah ibid; Yifei Laleiv 2:12; Kaf Hachaim 580:3

[17] This is due to the fact that it is a bad omen for Jewry. [ibid]

[18] Igros Kodesh 15:260 [printed in Shulchan Menachem 1:355]

[19] Toras Menachem 5751 p. 308-309

[20] Igros Kodesh 15:260 [printed in Shulchan Menachem 1:355]; Orchos Rabbeinu p. 95

[21] The reason: The Sages never established a blessing for an eclipse. No Poskim record that a blessing should be recited for an eclipse. Seemingly, the reason the Sages did not establish a blessing for an eclipse is because it is a bad omen, and requires prayer to nullify it and not a blessing. [Igros Kodesh ibid]