Planetary hours

Posted 2023-07-19. Last updated 2023-10-23.

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Long ago, people used to wake up at dawn and stop working or go to bed at sunset. Some civilizations, like the Romans, divided daytime (from dawn to sunset, the natural day) into twelve equal parts and nighttime (sunset to dawn) into another twelve equal parts, building a twenty-four hour day. Let's call these hours natural hours. The natural hours were numbered from one to twelve, indicating whether they were daytime or nighttime hours. For example, the first hour after sunrise was called hora diei prima by the Romans, while the second hour after sunset was hora nocti seconda. Sometimes it's convenient to number them from one to twenty-four.

Natural hours define a different calendar than the usual one: not only do the days begin at different times (dawn vs. midnight), but also the daytime hours have a different duration than nighttime hours, seldom lasting exactly sixty minutes. This is because the day and night vary in length due to the changing seasons and depending on the latitude. They are only equal during the spring and autumn equinoxes, something well known to the ancients. As spring (or autumn) progresses, the day becomes longer (or shorter), and the night becomes shorter (or longer). Thus, the longest day of the year occurs at the summer solstice, while the longest night occurs at the winter solstice.

Now, in the astrological tradition of the past, the planets influenced every aspect of life. Pay close attention to the names of the days of the week and where they come from:

MondayMoon's dayDies LūnaeDay of the Moon
TuesdayTiw's dayDies MartisDay of Mars
WednesdayWoden's dayDies MercuriīDay of Mercury
ThursdayThor's dayDies IovisDay of Jupiter
FridayFrigg's dayDies VenerisDay of Venus
SaturdaySaturn's dayDies SaturnīDay of Saturn
SundaySun's dayDies SōlisDay of the Sun

The names were taken from Roman tradition and the names of the Roman deities converted to equivalent Germanic gods, except for Saturn.

So Monday is the "day of the Moon," Tuesday the "day of Mars," Wednesday the "day of Mercury," etc.; each day dedicated to a different god or celestial body. So in English, all the days maintain their mythological-planetary roots. In languages such as Spanish, Italian and Greek, Saturday and Sunday are named after the Sabbath and God.

However, the names of the days did not come out of nowhere; they originated from the tradition of planetary hours. The idea is that each natural hour has a planet that rules it and influences it with its own nature (for example, among others, the Moon is related to emotions, the Sun to wealth, Mercury to intellectual matters, Venus to fun and love, etc.). We can define the hour ruled by the Sun as the first natural hour of Sunday. The rulers of the rest of the hours are determined by following the Chaldean order, which repeats: Saturn, Jupiter, Mars, Sun, Venus, Mercury, and the Moon.

These seven are the bodies of the solar system observable with the naked eye; the classical planets. They are arranged in decreasing order according to their apparent speeds in the sky relative to the background of fixed stars; Saturn moves very slowly, while the Moon moves rapidly. It is, of course, well known that the Sun is, in fact, a star, that the Moon is the natural satellite of the Earth, and that the stars do change their positions, albeit extremely slowly.

Thus, the second hour of Sunday belongs to Venus, the thirteenth (the first of the night) belongs to Jupiter, and so on. As 24 is not divisible by 7, the first hour of Monday has a ruler other than the Sun, which is the Moon, giving its name to the day. If you calculate the rest of the hours, you will notice that Mars rules the first hour of Tuesday, Mercury that of Wednesday, and so on. As the least common multiple of 24 and 7 is exactly 24 times 7, the cycle repeats after exactly seven days, determining the length of a week.

Some sources give different rulers for the day and the night, instead of assigning the ruler determined at dawn to the whole day. In this case, the ruler of the night is the ruler of the first hour of the night. For example, the ruler of Sunday night would be Jupiter. Interestingly, this means that you can determine different names for the days of the week based on the nocturnal rulers, which has caused some confusion, as it implies that there are two concurrent systems of days. However, in the contemporary use of planetary hours, it is undisputed that the ruler of the day is the one of the first daytime hour.

Unfortunately, it is not known when the planetary hours began to be tracked, although the history of the seven-day week dates back to nearly the third millennium BC. The first date with a known day of the week is February 6, 60 AD, which was a Wednesday.

If you calculate the day of the week for that date using any good algorithm, you will find that it was indeed a Wednesday, although the people of that time might not have agreed with you. In fact, a graffiti carved in Pompeii that day purports to have been made on a Sunday. This can be explained by the two systems of days mentioned earlier, where Wednesday night could be called Sunday (because the Sun rules the 13th hour on Wednesdays). If we rely on this, then we can deduce that we have had the same days of the week at least since that date.

Notice that to define the rulers of the hours, I had to start with a known day: Sunday. This is common in all sources that talk about the hours, but it is a benefit we only have now that we haven't lost track of the date; if the world were to end and we forgot the date, I'm not sure if we could easily recover it. We could probably deduce it cleverly from the study of the solstices and equinoxes, but I'm not too sure.

It's worth mentioning that planetary hours can't replace conventional hours. Think about it! Planetary hours are not precise. However, according to this system, "Sunday night" ends at dawn, which seems very intuitive to me, as I have never felt that the early morning truly counts as the next day.

The heptagram of the days.
The planets are in the order of the hours in the outer circle and the order of the weekdays in the inner heptagram.



Planetary hours can be used in astrology to choose the best moments for certain events. For example, since Venus rules love, the best time to propose to your girlfriend might be on a Friday, Venus's day, during the first, eighth, fifteenth, or twenty-second hour, that are of Venus, depending on whether the planets are in favorable positions or if you think it's appropriate to propose at four in the morning. This all requires a rigorous analysis of the astrological charts of the possible moments. Of course, if you spent all day playing League of Legends and haven't taken a shower in weeks, it doesn't matter if you choose Venus's day and hour—your girlfriend will have to love you a lot to say yes, even if the stars are on your side!

Planetary hours can also be used in horary astrology, which seeks to answer questions by reading astrological charts of the present moment, to decide if the question or chart makes sense. However, in his book Horary Textbook, John Frawley, a contemporary traditional astrologer, completely rejects planetary hours, indicating that they have no value after wasting a lot of time finding uses for them. I emailed him some time ago to ask about this and other things, and he replied with the following:

There is no meaning whatever in the idea of planetary hours or days. It is just the ancient version of sunsign astrology. The internet does not work better on Wednesdays and the girls do not become prettier on Fridays. If this did happen, people would have noticed - Apple would be telling us that 'Now every day can be like Wednesday' and Christian Dior would be saying 'Hey girls - you can get that Friday look!'

However, many others would disagree.


Planetary hours are used in planetary magic, which seeks to use the energies of the planets to achieve a specific end. At what time are the energies of the planets most accessible and powerful? Of course, when they rule the hour, the day or both. Many classic grimoires indicate that certain spells and experiments should be performed at the most appropriate planetary hours and days according to their rulers.

Chapter II of The Key of Solomon discusses which magical operations should be performed on which days and at what hours. For example, "the hours of Mercury are appropriate for scientific projects, divination, eloquence, intelligence, quick business affairs, apparitions, and answers to questions about future things" because Mercury is associated with these matters. The Key also lists a number of "sacred pentacles" consecrated to each planet. For example, the Fourth Pentacle of Jupiter (pictured) uses the energy of this planet to bestow wealth and honor (key characteristics of Jupiter) upon the holder, provided it is engraved in silver on the day and hour of Jupiter when Jupiter is in the sign of Cancer, in which it is highly dignified (by exaltation).

The fourth pentacle of Jupiter

The Arbatel de magia veterum mentions a series of seven Olympic spirits "which do inhabit in the firmament, and in the stars of the firmament: and the office of these spirits is to declare Destinies, and to administer fatal Charms, so far forth as God pleaseth to permit them." Each one is associated to a different planet in the Chaldean order and are "called simply, in that time, day and hour wherein they rule visibly or invisibly, by their Names and Offices which God hath given unto them; and by proposing their Character which they have given or confirmed." For example, the spirit of Saturn is Aratron, who, should you need his services, must be called in an hour or day of Saturn.

In the Picatrix, there are numerous planetary spells and talismans with much more elaborate consecration rituals than those of the Key. Let's look at an example from Chapter V of the first treatise. To prevent a man from marrying a woman, the Picatrix instructs you to make two talismans: one when Leo is rising (i.e., coming up in the east) in the hour of Jupiter, and another when Cancer is rising in the hour of the Moon when the Moon is moving fast, crescent, and connected to the ascendant. If you combine and then bury them in the hour of Venus, then they will never marry. Quite curious!


I have read at least one source that related the "witching hour" of yore to the last nocturnal planetary hour with the same ruler as the day, that is, the 22nd hour. The witching hour occurs, depending on the location, between 3 and 6 in the morning. While it doesn't mention planetary hours, the legend says that the "witching hour" is the part of the night when magical activity is at its highest, and powerful demons, ghosts, and witches can appear.

Personally, I have no practical use for planetary hours, but I find it more romantic to think it's the hour of Jupiter rather than, let's say, 3 in the afternoon, so I look at the planetary hour from time to time and think about it. It certainly adds a bit of flavor to the present.

Avigi says:
"So mysteries are more easily solved on Wednesdays at sunrise, huh?"


Given any time XX:YY, you can easily calculate the corresponding planetary hour.

  1. If the time is not sufficient to determine it, open your window and find out if it's daytime or nighttime. Note: it's daytime if the Sun is in the sky.
  2. If it's daytime, calculate the sunrise and sunset for your geographical location using an algorithm or ephemeris and determine the duration of the day. If it's nighttime, consider the most recent sunset and the upcoming sunrise to calculate the duration of the night.
  3. Divide this duration by twelve to get the duration of the daytime or nighttime natural hour.
  4. Now you need to calculate the number of the planetary hour. There are two ways to do this:
    1. If it's daytime, start at sunrise and count how many times you need to add the duration of the natural hour as many times as needed to exceed the time XX:YY; how many times you did this indicates the number of the hour. If it's night, start at sunset and use the duration of the nocturnal natural hour.
    2. If it's daytime, determine how much time has passed since sunrise and XX:YY. Divide this by the duration of the daytime natural hour, take the integer part, and add one. If it's nighttime, do the same but with sunset and the nighttime natural hour.
  5. If it's daytime, find the ruler of the day (indicated by the name of the day of the week) and count forward according to the Chaldean order as many times as the number you got in the previous step indicates. Take the starting planet as the first planetary hour. If it's nighttime, do the same, but twelve more times. Remember that the usual calendar doesn't matter; the day starts at sunrise and ends at sunrise, so you shouldn't change the ruler of the day past midnight.

Let's work through a very inauthentic example. Let's say it's Tuesday and it's daytime; specifically, it's 14:20. You calculated that the sunrise was at 7:00 and that sunset will occur at 17:00. Therefore, the day lasts ten hours. If you divide ten hours by twelve, the daytime natural hour lasts 0.8333 hours or 50 minutes. Now you can calculate the number of the planetary hour in two equivalent ways:

  1. Starting at 7:00 (sunrise), add 50 minutes (the duration of the daytime natural hour) until you pass 14:20. It turns out you need to add 50 minutes nine times, so 14:20 falls within the ninth hour.
  2. Calculate how much time has passed since sunrise and 14:20: 7 hours and 20 minutes. Divide this by the duration of the daytime natural hour, which is 50 minutes long, take the integer part, and add one. In this case, 7 hours and 20 minutes are 440 minutes. Dividing 440 by 50 gives 8.8, so 14:20 falls within the ninth hour.

Since it's Tuesday, the first hour is ruled by Mars. Following the Chaldean order, the ninth hour is ruled by the Sun.

Or if you don't want to think, you can use my program:


hora.c is a C program I wrote to calculate the planetary hours using the Almanac for Computers sunrise/sunset calculation algorithm without reliance on online services. It's not perfect, but it's enough for my needs (checking the planetary hour from time to time), and it was an interesting exercise.

Without arguments, the program calculates the current planetary hour and the remaining hours of the day using the latitude and longitude set as macros in the source code and the system's timezone. The program accepts four optional arguments: option, latitude, longitude, and timezone (in that order). If you provide the latitude, you must provide the longitude; in this case, the program will show the planetary hours for that location but in the system's timezone, unless you also provide a specific timezone as an argument.

You don't have to include an option but the program supports a few of them: For example, -r makes hora.c display only the symbols of the current rulers of the day and the hour. Run hora -h for more.

For example, ./hora 40.9 -74.3 shows the planetary hours for Wayne, New Jersey, in the system's timezone. ./hora 40.9 -74.3 -5 does the same but displays the hours in UTC-5, New Jersey's timezone. ./hora -r 40.9 -74.3 -5 does the same but only the current rulers are shown.

The program assumes that sunset occurs before midnight (and conversely, that sunrise occurs after midnight), so it doesn't calculate planetary hours correctly for locations within the polar circles. Sometimes, the hours don't make sense if you don't provide the correct timezone.

Download hora.c and the English patch. Compile the program with cc -o hora hora.c -lm, patch it with patch -u hora.c -i hora.patch and run it as ./hora.

By the way, this article was finished on the hour of the Moon.

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