It takes the Moon about a month or 29.5 days to complete a cycle of phases. In astrology, this was rounded to 30 degrees and it’s where we get 12 divisions (12.37 cycles of the Moon) in a 360 degree zodiac based on the Sun cycle of 365.24 days. Considering that there is about 4 weeks in a month, every 7 days marks approximately one quarter phase of the Moon. Notably, the word “month” and “moon” share a common root in many languages, including English. The Old English “mona” for “moon” and “monað” for “month”.
Unlike the year and month, the ordering of the days in a week is constructed without any astronomical reference. Instead, it was developed with the ‘Chaldean order’ dating back to ancient Babylonia. This order is determined by the speed of a planet and geocentric proximity:
Moon → Mercury → Venus → Sun → Mars → Jupiter → Saturn
Using this system, each hour of the day was assigned a planetary ruler and the cycle would repeat every 7 hours.
Sunrise was considered to be the first hour and the planet of this hour was determined the ruler of that day. Since there are 24 hours in the day and 7 repeating planetary hours, the first hour would have a different planetary ruler each day of the week. For instance, the Sun rises at the hour of the Sun on Sunday, Moon on Monday, Mars on Tuesday, and so on.
The day is divided into 12 hours from sunrise to sunset and 12 hours from sunset to sunrise but the naming of the hours is not the same as we’d recognize on a clock today. As mentioned, the day would begin with the rising Sun and end when it sets. The first hour of a day as we know it is midnight when the Sun reaches the lowest point below the horizon and begins to rise upward.
In the table below, the ruler of each day is shown at the first hour:
From this system arises a new sequence for the days of a week:
Sun → Moon → Mars → Jupiter → Venus → Saturn
When setting the Chaldean order clockwise as a repeating cycle, a heptagon is formed. The new sequence was remembered by connecting the resulting angles to construct a heptagram inside.
The 7-day week was officially established by Constantine of the Roman Empire in 321 AD but it originated centuries earlier from the Akkadians and Babylonians. In contrast, Egyptians divided their 30-day month based on ‘decan’ which were 10 degree sections of each zodiac also used for timekeeping by the hour.
Each day of the week was named after the ruling deity associated with each planet and this varied based on translation and culture. Latin is a Indo-European language sharing root with Persian, Indian, Celtic, Germanic, among others. English is a form of Old English that arose from Germanic tribes. These tribes were influenced by the spread of Roman tradition and religious syncretism. This carried the idea that people worshiped the same major deities under different names and stories. For instance, Zeus is a Greek god identified as being semantically related to Thor in Norse/Germanic mythology since they’re both considered to be the god of thunder among various other associations. While some deities were translated from Rome, the rest came to be replaced by their native equivalent.
This association with planets and deities is shown in the table below but a comprehensive list can be found here: