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Planetary hours are an ancient theory that each of the 24*7 hours of the week is "ruled" by a certain celestial object in the solar system. Among other consequences, this theory gave rise to the names of the weekdays in most European languages.

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Is it completely arbitrary or could it have some basis in what actually happens in the sky? Like certain planets being more/less prominent in the certain hour of the night.

After all, other astrological concepts are based on what can or cannot be seen in the night sky at a certain time, say, you are a Taurus if the constellation of Taurus could be seen in the sky when you were born.

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  • $\begingroup$ "you are a Taurus if the constellation of Taurus could be seen in the sky when you were born." No, in Western astrology you are a Taurus if the Sun was in the Zodiac sign of Taurus when you were born. That corresponds to an ecliptic longitude of 30° to 60°, which is no longer near the constellation of Taurus, due to the precession of the equinoxes. $\endgroup$
    – PM 2Ring
    Oct 18, 2022 at 18:48
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    $\begingroup$ Astrology questions are off-topic here except for questions that "concern devices (such as symbols) that are in use in astronomy where the answer lies in the history of astrology". This question is on the borderline. $\endgroup$
    – David Hammen
    Oct 18, 2022 at 20:10
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    $\begingroup$ @DavidHammen "Does X have any real astronomical basis?" + "Is it completely arbitrary or could it have some basis in what actually happens in the sky?" seems to me to be squarely on-topic as a question as written. When one posts such a question one (usually) doesn't yet know the answer; there's no way to know ahead of time how much real astronomy is in it until answers can be posted. $\endgroup$
    – uhoh
    Oct 18, 2022 at 20:39
  • $\begingroup$ @David Very borderline. But it is connected with the order of the weekdays used in many calendars, which is why I felt it was ok to answer. $\endgroup$
    – PM 2Ring
    Oct 18, 2022 at 20:40
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    $\begingroup$ The reason why I posted this question in this community is that I am specifically interested in the astronomical perspective. The cultural/religious/superstituous aspects of planetary hours don't interest me in the slightest. $\endgroup$
    – 0x60
    Oct 19, 2022 at 11:45

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Not really, it's just simple modular arithmetic. Write the "planets" in order of their speed:

Index Planet
0 Moon
1 Mercury
2 Venus
3 Sun
4 Mars
5 Jupiter
6 Saturn

Now step through the list cyclically, with a step size of 4:

Index Planet
0 Moon
4 Mars
1 Mercury
5 Jupiter
2 Venus
6 Saturn
3 Sun

That gives us the order of the weekdays.

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    $\begingroup$ To address “why 4?”, it is notable that if you reverse the order, Saturn first Moon last, and assign one planet to each hour rather than each day, then a day which begins with Saturn will end with Mars, so the next day will run from Sun to Mercury, the next from Moon to Jupiter, the next from Mars… thus giving the right pattern without having to pull an arbitrary “4” out of thin air. Of course the 4 is still there, but in the more natural form of $24\mod 28=-4$. $\endgroup$
    – Martin Kochanski
    Oct 18, 2022 at 19:56
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    $\begingroup$ Whether this is the true origin of the pattern, a Babylonian expert will know. $\endgroup$
    – Martin Kochanski
    Oct 18, 2022 at 19:56
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    $\begingroup$ @Martin, sure. Also, $24\equiv -4\pmod 7$ $\endgroup$
    – PM 2Ring
    Oct 18, 2022 at 20:09

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