Sunrise Mystery in the Panchang: Upper Limb vs Central Disc

🌐 Read this in: हिन्दी · বাংলা

"Time is not a constant — it is a wondrous convergence of the Earth's rotation in space and the observer's mathematical standard. Every moment written on the leaf of a Panchang is, in truth, the silent witness of thousands of years of astronomical observation."

Have you ever wondered why, opening two different panchangs for the same city, you find a difference of three to four minutes in sunrise? The ordinary reader may think one of the panchangs must be wrong. But deeper into astronomy, we find that neither is wrong. At the root of this difference lie two distinct mathematical standards, known as Upper Limb and Central Disc.

Based on geographical position, cultural tradition, and the foundational texts (such as the Surya Siddhanta or Drik Ganita), different regions of India developed different conventions for time-reckoning. Between the North Indian Panchang and the Bengali Panjika there are structural differences in both month-reckoning (Purnimanta vs Amanta) and in the calculation of the solar day (Bengali Tarikh vs Pravishte). Today we shall take these up with a measured example.

The Astronomical Standards for Calculating Sunrise and Sunset

By "sunrise", the ordinary reader understands the arrival of the Sun in the eastern sky. But in mathematical astronomy, to fix "when the Sun rose", one needs a definite standard for the Sun's position relative to the horizon. That standard is mainly of two kinds.

1. Upper Limb (the upper or visible edge): In modern astronomy, and in many Hindi panchangs of the Drik tradition, the moment the Sun's upper edge becomes visible on the horizon is taken as sunrise — accounting for atmospheric refraction. Sunset is the reverse: when the upper edge sinks below the horizon.

2. Central Disc (the centre of the disc): In this convention, the moment the geometric centre of the Sun reaches the horizon is sunrise. The printed sunrise of the Bishuddha Siddhanta Panjika was compared with our Central Disc calculation, and hence MyAstrology's Bengali Panjika follows this convention. A few major Hindi panchangs also give times by the Central Disc.

The Science of the Difference of About Three and a Half Minutes

Two measurements are responsible for this difference: the Sun's angular semi-diameter and atmospheric refraction.

The Sun's angular diameter from the Earth is about 32 arc-minutes, that is, from centre to upper edge about 16 arc-minutes. And near the horizon, atmospheric refraction raises the Sun by about 34 arc-minutes. Adding the two gives about 50 arc-minutes (34 + 16 = 50).

The Earth rotates one degree in four minutes. At the equator, where the Sun rises vertically, this 50 arc-minutes is traversed in about 3.3 minutes. In northern cities the Sun rises at a slight slant on the horizon, so the time is a little longer — at Kolkata about three and a half minutes, at Delhi somewhat more.

"This is why the Upper Limb sunrise occurs a few minutes before the Central Disc sunrise, and the Upper Limb sunset a few minutes after."

A measured example: 11 October 2026, Kolkata. By Upper Limb the sunrise is at 5:31 AM, and by Central Disc at 5:35 AM. On the same day in Hyderabad, the two conventions give sunrise at 6:08 and 6:11 AM. In MyAstrology's Hindi and English Panchang, the reader may choose "Upper Limb" or "Central Disc" themselves; the Bengali Panjika follows the Central Disc, as the Bishuddha Siddhanta does.

Division of the Lunar Month: Purnimanta vs Amanta

The Indian Panchang is essentially a luni-solar calendar. Depending on the tithi on which the lunar month ends, the lunar month is of two kinds — Purnimanta and Amanta.

1. Purnimanta month: the lunar month that ends on the full-moon (Purnima) tithi. In Uttar Pradesh, Madhya Pradesh, Rajasthan, Bihar and much of the Hindi belt this convention prevails. Here the new moon (Amavasya) falls in the middle of the month, and the Krishna Paksha is named after the following month.

2. Amanta month: the lunar month that ends on Amavasya and begins with the Shukla Pratipada. In Maharashtra, Gujarat, Karnataka, Andhra Pradesh, Telangana — and in the Bengali Panjika too — lunar months are named by the Amanta reckoning.

Thus the same day is known by two different month names in two places. For example, Karwa Chauth is, in the Purnimanta reckoning, Kartik Krishna Chaturthi; in the Bengali Amanta reckoning it is Ashwin Krishna Chaturthi — yet the day is one. In the Shukla Paksha the month names are the same in both reckonings.

The Solar Day: Bengali Tarikh vs Pravishte

When the Sun enters a new zodiac sign (Sankranti), the solar month is counted from that moment. But which day is taken as the first of the month differs from region to region.

Bengali Tarikh: in the Bengali Panjika, the day of the Sankranti is the last day of the old month (month-end), and the following day is the 1st of the new month. If the Sankranti occurs after midnight but before sunrise, it shifts one day further back. Depending on the Sun's rate of change of sign, the Bengali month is 29 to 32 days long.

Pravishte (gate): many Hindi panchangs show the solar day under the name "Pravishte/gate", and there the day of the Sankranti itself is generally taken as 1. As a result the two numbers often differ by a day. Measured example: Kanya Sankranti falls on 17 September 2026 at 7:54 AM. On 11 October, the Bengali Panjika gives 24 Ashwin, while Pravishte gives 25. Both are right on their own rule.

Modern Technology and the Preservation of Ancient Conventions

These regional differences are not something to be erased by any one "correct" rule. In MyAstrology's Panchang, therefore, the reader's language and regional convention come first: in Bengali, the Central Disc sunrise, the Amanta month and the Bengali Tarikh; in Hindi and English, the Upper Limb sunrise and the Purnimanta month — and both can be changed by the reader with one touch. Planetary positions come from the JPL DE441 ephemeris and the VSOP87 theory, and are regularly verified against the printed Bishuddha Siddhanta Panjika.

"This diversity of the Panchang is not a mathematical error, but the wealth of our thousand-year astronomical practice. To preserve it undistorted by means of technology is our aim."

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