Why the Day is Divided This Way: How Planetary Hours are Calculated

Planetary hours seem like a mysterious system, but they are based on simple arithmetic and three astronomical moments. Let's break down why the day is divided into 12 and 12, where the hour boundaries come from, and how the entire scheme is assembled into a single table.

Basic Rule

The rule of planetary hours in one sentence: daylight is divided into 12 equal parts, and the night is divided into 12 equal parts.

This leads to three consequences:

1. There are always 24 planetary hours in a day — 12 daytime and 12 nighttime.
2. The hours are "unequal": their length depends on the duration of the day and night.
3. Hour boundaries are tied to the sunrise and sunset, not to midnight or noon.

This scheme differs from the 60-minute hours we are accustomed to, which is precisely why they are called "seasonal" or "unequal" hours.

Why Exactly 12

The number 12 was not chosen by chance — it is a legacy of ancient counting systems.

• Divisibility. 12 is divisible by 2, 3, 4, and 6, which makes calculations convenient. Twelve is also easy to link to the Babylonian sexagesimal system: 5 × 12 = 60.
• Egyptian influence. The Egyptians divided daylight into 12 hours and the night into 12 hours; for nighttime observations, they used 36 "decans" — three dozen star groups.
• Babylonian influence. The Babylonians worked with sexagesimal arithmetic and divided the day into 12 "double hours." The numbers 6, 12, and 60 were working tools of astronomy.
• Celestial landmarks. An observer could easily see divisions on the sky that were multiples of 6 and 12: a lunar month of about 30 days, the zodiac of 12 constellations, and a year of 12 months.

As a result, the division of the day into 12 + 12 was established in tradition and transferred to planetary hours without changes.

Reference Points: Sunrise, Sunset, Next Sunrise

To calculate planetary hours for a specific city and date, you need three moments:

Important: "night" is the segment from sunset to the next sunrise, so for calculation, you need to know the sunrise of the next day. This also means that planetary hours "do not know midnight": they do not use midnight as a boundary at all.

What is Considered Sunrise

In astronomy, there are several definitions of sunrise. For planetary hours, the moment when the upper edge of the solar disk appears above the mathematical horizon is traditionally used, taking into account atmospheric refraction (on average about 0.5 degrees). It is precisely this definition that is used in modern services, including lunar.day.

Different services may use:

• Geometric sunrise — the moment when the center of the disk crosses the horizon, without taking refraction and the disk's size into account.
• Civil sunrise — the moment the upper edge of the disk appears, taking refraction into account (used for everyday calculations).
• Nautical and astronomical dawn — when the Sun is 6, 12, or 18 degrees below the horizon.

The difference between definitions is only a few minutes, but for an accurate planetary hour table, it is important, which is why services indicate which definition they use.

Unequal Hours in Practice

The length of a planetary hour is calculated as follows:

• Daytime hour = (sunset - sunrise) / 12
• Nighttime hour = (next sunrise - sunset) / 12

Let's look at the example of Minsk (53.9° N) for two dates:

A pattern is visible: in summer, daytime hours are longer than nighttime ones; in winter, it's the opposite; and at the equinox, they are almost equal. The closer to the pole, the stronger the spread; at the equator, hours are almost always close to 60 minutes.

How the Planetary Sequence is Built

Now for the most interesting part — how planets are linked to numbers.

1. The first daytime hour of each day belongs to the planet that rules the day of the week. For example, on Monday, the Moon opens the first hour.
2. Then, each subsequent hour transitions to the next planet in the Chaldean order: Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon → then Saturn again, and so on.
3. The nighttime sequence does not start over: the 13th hour of the day (the first nighttime hour) receives the planet following the planet of the 12th daytime hour.
4. After 7 hours, the planet repeats: the planet that ruled the first hour will return in the 8th and 15th hours.

Since 24 = 7 × 3 + 3, three planets rule for four hours a day, and four planets rule for three hours. Because the week is also seven days long, the ruler of the day and the first hour coincide, and the entire sequence of days repeats every week without interruption.

Day and Night Rulers

On the planetary hours page of lunar.day, two rulers are shown:

• Day ruler — the planet of the first daytime hour, which is also the planet of the day of the week.
• Night ruler — the planet of the first nighttime hour (the 13th hour of the day).

The night ruler is calculated simply: it is the planet that stands 12 positions ahead of the day ruler in the Chaldean order (taking into account the repetition of the circle). In practice, this means that the night ruler changes every day and does not coincide with the day ruler.

Polar Day and Polar Night

At high latitudes during part of the year, the Sun does not set (polar day) or does not rise (polar night). In such conditions, the usual division into 12 daytime and 12 nighttime hours loses its meaning.

Tradition offers special regimes:

• Polar day. The day is divided into 24 hours from sunrise to sunrise; the usual sequence of planets is used, starting with the planet of the day of the week. The upper "daytime" hours and "nighttime" hours are essentially equal in length.
• Polar night. The Sun does not rise, but the boundaries of the day are still tied to the astronomical moment of the "upper culmination" of the Sun or the calculated time of sunrise if the Sun were not below the horizon.

On lunar.day, the polar day or night regime is determined automatically based on the city's coordinates and the date, and this is clearly displayed in the hour table.

Step-by-Step Example: Minsk, August 20, 2026

Let's break down a full calculation with a real example.

Step 1. Determine the three moments. For Minsk on August 20, 2026: sunrise 05:56, sunset 20:28, next sunrise 05:58 (local time, UTC+3).

Step 2. Calculate the hour lengths.

• Day: 20:28 - 05:56 = 14h 32m = 872 min. Daytime hour: 872 / 12 ≈ 72.7 min.
• Night: 24h - 14h 32m = 9h 28m = 568 min. Nighttime hour: 568 / 12 ≈ 47.3 min.

Step 3. Determine the day ruler. August 20, 2026 is a Thursday, its ruler is Jupiter. The first daytime hour is Jupiter.

Step 4. Build the sequence. The hours go: 1st — Jupiter, 2nd — Mars, 3rd — Sun, 4th — Venus, 5th — Mercury, 6th — Moon, 7th — Saturn, 8th — Jupiter, and so on. At night, the sequence continues from the 13th hour.

Step 5. Verify. At 22:00 local time, which is 66 minutes after sunset, the second nighttime hour (the 13th hour of the day) is occurring: the 12th daytime hour was occupied by Mercury, so the 13th is the Moon, and the 14th is Saturn. At 22:00, Saturn rules — the second planet of the nighttime chain.

The same result is instantly given by the table on lunar.day — you don't need to calculate it manually, but understanding the mechanics is useful.

Day and Night Rulers: Reference Table

To avoid recalculating every time, it's convenient to have a ready-made table of rulers for the whole week. The night ruler is the planet that stands 12 positions ahead of the day ruler in the Chaldean order (with a transition through the beginning of the circle).

Let's check Tuesday: the day ruler is Mars (position 3 in the order: Saturn=1, Jupiter=2, Mars=3, Sun=4, Venus=5, Mercury=6, Moon=7). Counting 12 steps from Mars: 3 + 12 = 15; 15 - 7 = 8; 8 - 7 = 1 → Saturn. It matches the table: the night ruler of Tuesday is Saturn.

This table is the key to the entire system: knowing the day of the week, you get both planets that open the daytime and nighttime chains, and then everything follows the Chaldean order.

How Planetary Hours Were Calculated Without a Computer

Before the advent of programs, planetary hours were calculated in three ways.

First method — tables. Ready-made tables for each day of the week listed the planets for all 24 hours. It was enough to find the day and hour — and the ruler was in the column. Such tables are found in manuscripts and in the first printed calendars.

Second method — "counting from sunrise." Knowing the moment of sunrise and the length of the hour, they calculated how many minutes had passed since sunrise, divided by the length of the hour, and obtained the number of the current hour. Then, using the number and the Chaldean order, they found the planet.

Third method — astrolabe. On the back of an astrolabe, the scales of planetary hours were depicted: based on the position of the Sun on the instrument's scale, the ruler of the current hour was read without calculations. Astrolabes were actively used by Arab astronomers, and later by European ones.

To check tables, a simple technique is convenient: the current hour number needs to be adjusted to the Chaldean order. If the first daytime hour is Jupiter (position 2), and it is currently the 7th hour, then the planet is: 2 + 6 = 8; 8 - 7 = 1 → Saturn. The same thing that any modern planetary hour calculator does, just without the formulas for sunrise and sunset.

Example for a Winter City: How the Picture Changes

Let's look at the same Minsk, but on December 21 — the shortest day. Sunrise is about 09:25, sunset is about 16:45, next sunrise 09:26.

• Day: 16:45 - 09:25 = 7h 20m = 440 min. Daytime hour: 440 / 12 ≈ 36.7 min.
• Night: 24h - 7h 20m = 16h 40m = 1000 min. Nighttime hour: 1000 / 12 ≈ 83.3 min.

The difference is impressive: a daytime hour in December is almost half as short as a summer nighttime hour. If in June there was enough "daytime" left at sunset for tasks, then in December all activity fits into short 36-minute segments.

In practice, this means that the same "hour of Venus" lasts 85 minutes in summer, and 37 minutes in winter, and from the point of view of tradition, these hours are considered equal in meaning, but not in duration. That is why the planetary hour tables always show the real start and end time of each hour, not an abstract number.

Polar Example: Norilsk

At the latitude of Norilsk (69.3° N), in mid-June the Sun does not set at all, and in December it does not rise.

During the polar day, the system counts hours from the moment of sunrise (or, during a period of complete lack of sunset, from a conditional point of the day) and divides them into 24 equal segments. The planetary sequence is maintained: the first hour is the planet of the day of the week, then the Chaldean order follows. The only difference is that the "daytime" and "nighttime" hours become equal in length.

In the polar night, the moment when the Sun is closest to the horizon, or the calculated sunrise, is taken as the boundary for the beginning of the day. lunar.day automatically switches the polar day and polar night mode based on the city's coordinates and the date, so the table is always correct.

Features to Keep in Mind

• Hours are local. In different cities at the same moment, the planetary hour may differ because the sunrise and sunset are different.
• Local time. All hour boundaries are specified in the local time of the city, taking the time zone into account.
• Hours "catch up" to sunrise. Planetary hours reset every day at the moment of sunrise, not at midnight, so the "planetary morning" does not coincide with the calendar one.
• Small differences in services. Different services may use different definitions of sunrise and account for refraction differently, so the hour boundaries may diverge by a few minutes.

How lunar.day Calculates Hours

On lunar.day, planetary hours are calculated based on the city's coordinates (latitude, longitude, altitude) and the date:

• Sunrise, sunset, and the next sunrise are calculated using standard astronomical algorithms, taking refraction into account.
• Daytime and nighttime hours are calculated by dividing by 12, the planetary sequence is built from the ruler of the day of the week.
• Time is adjusted to the local time zone of the city.
• The polar day and polar night regime is determined automatically.

The result is a table of all 24 hours with exact start and end times, the day and night rulers, and the current hour, which updates in real-time. For more on what to plan for each hour, read the article on choosing the right time for tasks.

Understanding these rules allows you to consciously use planetary hours — as a cultural tradition and a curious tool, not as a rigid prescription.