In order to represent seasonal change, the points of light are modified to become full height characters. The symbol for light is the same as that for a track or route. Hence, the symbols might be thought of as representing the path of the sun.
The modified signs are combined into trigrams, that can be likened to the Chinese notation known as the bāguà 八卦. The broken and unbroken tracks are combined to give the relative levels of light in each season. They should be interpreted by reference to the Chinese sequence known as the "Earlier Heaven" sequence attributed to Fuxi.
For image credits, please see below.
This Indus sign is equivalent to the Chinese symbol qián ☰. The symbol has three yang lines, in other words the maximum possible light. It is an ideograph for summer.
Mahadevan's sign number 89 also has a second possible reading of sky or heaven, in the same way that qián can be read as sky or heaven in Chinese.
This Indus sign is equivalent to the Chinese symbol kūn ☷. The symbol has three yin lines, in other words the maximum possible darkness. It is an ideograph for winter.
This Indus sign is equivalent to the Chinese symbol lí ☲. The symbol has two yang lines and one yin line. It represents the transition from darker to lighter days, and is an ideograph for spring.
Illustrative Text References:
Summer: Harappa: Tablet: H-942 B: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 340: Collections in Pakistan: Suomalainen Tiedeakatemia.
Sky or Heaven: Mohenjo-daro: Seal: M-1688 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 16: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Winter: Harappa: Tablet: H-942 A: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 340: Collections in Pakistan: Suomalainen Tiedeakatemia.
Spring: Mohenjo-daro: Seal: M-1688 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 16: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Repetition:
In Chinese, points in the solar year can be identified more precisely by combining two of the bāguà into a hexagram.
Hence, in Indus script, when the ideograph for summer is repeated, we get the maximum possible light of the period of maximum possible light, in other words midsummer.
This is equivalent to the hexagram qián ䷀ from the Yìjīng.
Illustrative Text Reference:
Midsummer: Mohenjo-daro: Seal: M-20 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 13: Collections in India: Suomalainen Tiedeakatemia.
Image Credits:
Indus Script Signs with Numbers: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.
Indus Script Sign without a Number and Seasonal Change: Lynn Fawcett, 2019.