Education Article #3: For the Joy of Sharing

Weekday Sequence

By Dr. Ravi Shankar Iyer
CEO of Accenture Center of Excellence at IIT Madras, Chennai, India

Foreword by Venkat
A smile around brings a smile within and peace within has the potency to bring peace around. Intense emotions and intricate logic as well seem to carry the power to recreate themselves in their vibrations. Such a deeply thought design for the weekdays in our ancient texts on astronomy is not only intricately logical but beautiful as well. It touches and stirs our mind into the realms of the capacities we are endowed with and not yet realized. Dr. Ravi Shankar brings out the structure and method behind the sequence of the naming of the weekdays based on his study from various quoted sources. The tables make it easy to absorb the logic and the method.

Explore and be inspired by the beautiful ‘algorithm’ behind the naming of our weekdays in Dr. Shankar’s easy-to-read writing.

Dr. Shankar is the CEO of the Accenture Center of Excellence at IIT, Madras (in Chennai, India), where he is setting up multiple laboratories and research initiatives in Industrial IoT and robotics.

Dr. Shankar has a Bachelor’s degree from IIT Madras, and Master’s and PhD degrees, from IISc, Bangalore. He loves travelling having visited over 40 countries and lived in 7. He is an avid reader, and loves teaching.

Earlier, Dr. Shankar was the Senior Director at Conexant, heading the Embedded Wireless Networking Group in Asia. He has also had stints with Motorola and Siemens in the UK, US and Singapore. At Motorola, his team won TMC magazine’s Internet Telephony Product of the Year award for 2002. He has founded four startups – GrayCells, TeleSilikon, Bubble Motion and VoiceBell – and mentored over 30 startups, primarily in healthcare and blockchain. He has over 20 patents and more than 30 publications in prestigious technical journals.

Weekday Sequence

Wondered why the sequence of days in a week seems arbitrary? What is the logic behind, say, Saturday following Friday? This is explained in the astronomical treatise Surya Siddhant [1].

मन्दादधः क्रमेण स्युश्चतुर्था दिवसाधिपाः ॥

वर्पाधिपतयस्तदूत्तृतीयाश्च प्रकीर्त्तिताः ॥७८॥

ऊर्ध्वक्रमेण शशिनो मासानामधिपाः स्मृताः ॥

होरेशाः सूर्यतनयादधोऽधः क्रमतस्तथा ॥७९॥

                                    –Surya Siddhant 12:78-79

सप्तैते होरेशा: शनैश्चराद्या यथाक्रमं शीघ्रा:|

शीघ्रक्रमाच्च्तुर्था भवन्ति सूर्योदयाद् दिनपा: ||

                                    – Aryabhatiya, KalaKriya Pada, Verse 16

Table 1: Planetary speeds as observed from the Earth (in ascending order).

PlanetDay of WeekDuration
SaturnSaturday 30 years
JupiterThursday 18 years
MarsTuesday 1.5 years
SunSunday 1 year
VenusFriday Approx 1 year
MercuryWednesday Approx 1 year
MoonMonday 1 month

The first shloka (from Surya Siddhant) says consider the speed of the “planets” (includes the Sun and Moon), as seen from Earth. Table 1 shows the time taken by the planets to return to the same spot in the sky (nakshatra). The longer the duration, the slower the planet is.

According to the Surya Siddhant, and as restated by Aryabhatta II in his Aryabhatia, every hour (hora, or ‘होरा’ in Sanskrit – see the reference to hora — होरेशा: — in the two shlokas) of the day is assigned a planet, in the sequence (top to bottom) given in Table 1. Let us assume that the first hour of a day is assigned the “planet” Saturn. Every hour after that is assigned to the next planet in the sequence, which rolls over after the 7th planet (Moon), i.e., Jupiter is assigned the hour after Saturn, the next hour is assigned to Mars, and so on. The 7th hour of the day is assigned the Moon, the 8th hour is Saturn, the 9th hour is Jupiter, and thus the pattern repeats ad infinitum. It can easily be seen (see Table 2) that the planet associated with the first hour of the next day will be the Sun, the day after that, the Moon, and so on, and this pattern repeats every 7 days (a week).

Each day is given the name of the “planet” which is assigned to the first hour of that day. Thus, the first day is called शनिवासरः (the day of Saturn), the next (see Table 2) is भानुवासरः or रविवासरः (the day of Sun), and so on.

Table 2: The sequence of the days of the week emerges from the pattern described in the section above

PlanetWeekday (Sanskrit)Weekday (English)HoraHoraHoraHoraHoraHoraHora

A Note on the Antiquity of the Surya Siddhant

Surya Siddhant is a reference book on astronomy, written in Sanskrit. Although Wikipedia says that it was composed around 800 CE, it has been shown by many including Dr. Anil Narayan, formerly of ISRO, who used information in the Surya Siddhant on the latitudes of nakshatras, that it had been updated circa 7500 BCE [2]. John Playfair (who lived in the 18th century) has this to say about the Surya Siddhant – “The observations on which the astronomy of India is founded, were made more than three thousand years before the Christian era; and in particular, the places of the sun and the moon, at the beginning of the Kali-yoga/Calyougham (i.e., 17/18 February 3102 B.C.), were determined by actual observation.” [3]. Nilesh Oak and Rupa Bhaty used chapter 12 of the Surya Siddhant to show that the only time the following conditions prevailed:

1. 2 pole stars (north and south)

2. A 24° incline of the polar axis, and

3. Periapsis during the peak of Grishma Ritu (summer)

was around 12000 BCE [4]. The two pole stars in 12000 BCE were Abhijit/Brahmarashi (Vega, in the north) and Agastya (Canopus, in the south). Today, we only have the northern pole star, and that is Dhruv (Polaris). The change in pole stars is due to the precession of the Earth – the polar axis moves in a circular motion with a period of about 25,771.5 years. The tilt of the Earth’s axis (called its obliquity) oscillates between 22.1° and 24.5° in a 41,000-year cycle. Today, the Earth’s obliquity is 23°26’11.6” (or 23.43655∘) and decreasing [5].


[1] Surya Siddhanta, Accessible at:

[2] Anil Narayanan, “Dating the Surya Siddhanta using Computer simulation of Proper Motions and Ecliptic variations”, Indian Journal of History of Science, Volume 45, issue 4, p20-21, 23 March 2010. Accessible at:


[4] Nilesh Oak and Rupa Bhatty, “Ancient updates to Sūrya-siddhānta”, “India Facts”, 19 March 2019, Accessible at:

[5] Alan Buis, ”Milankovitch (Orbital) Cycles and Their Role in Earth’s Climate”, February 27, 2020, NASA Jet Propulsion Laboratory

By Dr. Ravi Shankar Iyer
CEO of Accenture Center of Excellence at IIT Madras, Chennai, India

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