Why does the week have 7 days? (The Indian Astrology Series)

Astronomy essentially started off as a system of time-keeping. Ancient man used the cyclical movements of the large celestial bodies he observed in the skies to develop his system of time-keeping. Through his observations, he evolved the systems of a 7-day week, a 30-day month, a 12-month year and so on. But how did he arrive at these numbers, 7, 12, 30 and so on? This blogpost is an attempt to answer two key questions relating to numbers in Indian Jyotisha.

In Vedic astrology, each day of the week is referred to as a vara. The word Vara means ‘a turn’, from which incidentally comes the Hindi word baari, which also means turn. Here, the word vaara refers to the turn of a planet to rule a day.

Now, the seven distinct celestial bodies known to ancient man were the sun, the moon and the five planets including Jupiter, Saturn, Venus, Mars and Mercury. Although the visible sky is littered with several smaller, shining stars, these 7 large luminary bodies are the ones visible to the naked eye from the earth.

So each day or vara that lasted from one sun rise to another came to be dedicated to each of these 7 celestial bodies. That’s how we have somavara or Monday dedicated to the moon, the Mangalvara or Tuesday dedicated to Mars, Budhavara to Mercury and so on.

Although we share this idea of a seven-day week with Hellenistic or Greek astrology, not all cultures seem to have had a 7-day week. For example, ancient Chinese and Egyptian calendars had a 10day week whereas traditional Korean and Javenese calendars had a 5-day week.

But the real reason why the Indian system has a 7-day week is the moon. Yes! Remember the moon’s journey around the earth is the reason which led to the concept of months.

The moon’s phases or pakshas can be broadly classified into four quarters. The first quarter is the phase from the new moon to the half-way phase of the waxing moon, the second quarter is the phase from the half-moon to the full moon or purnima. Then comes the third quarter from the full moon to the half-way phase of the waning moon and the final phase from the half-moon to the new moon or amavasya. 

Four phases of the Moon

The time taken for a moon to go through these four phases is what we call a synodic month that lasts 29.5 days. But the time that the moon takes to complete a revolution around the earth is about 27.3 days. This system of accounting for the moon’s movement vis-à-vis certain fixed stars in the sky is called the sidereal system.

The average of the synodic and sidereal months works out to roughly 28 days. When these 28 days are divided across the four quarters of the moon’s phases that we talked about a while back, we get 7 days per quadrant. Each quarter thus becomes a week, made up of 7 days.

And that explains why we have 7 days in a week!

On Indian Calendars and Vedic Astrology

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