The observable changes in the diurnal pattern of light and dark can be represented by combinations of the basic single line for light and the double line for dark. The Chinese call the resulting combinations the sīxiăng 四象, or four phenomena.
For image credits, please see below.
Four Phenomena: Indus Signs from Mahadevan:
Four Phenomena: Additional Sign found at Dholavira:
The format of the Indus notation differs from that in Chinese. However, the purpose is the same. One thing that the two systems of notation have in common is that the symbols for dawn and dusk are mirror images of one another. In the absence of an indicator of the correct orientation for a symbol, it is impossible to distinguish between the two.
Illustrative Text References:
Dawn: Dholavira: Seal: SI. Number: 127: Acc. Number: 25871 (NB. The photograph is of the seal. Therefore, the inscription would be the mirrored image of that seen in the photograph.): R.S. Bisht, 2015: 8.2.3 Details of Individual Seals: Excavations at Dholavira (1989-90 to 2004-2005), p. 288: https://www.scribd.com/document/262316120/Excavations-at-Dholavifra-1989-2005-RS-Bisht-2015: Accessed: 12 November, 2018.
Day (Daylight): Mohenjo-daro: Graffiti on pottery: M-2069 A¹: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 118: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Dusk: Allahdino: Seal: Ad-6 A (NB. The inscription would be the mirrored image of that seen in photograph Ad-6 A): Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 388: Collections in Pakistan: Suomalainen Tiedeakatemia.
Night (Darkness): Harappa: Tablet: H-2108 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 285: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Comments:
The concept of the four phenomena is attributed to a mythological ruler of China, Fuxi, and is recorded in the Yìjīng.
In the Yìjīng, the basic yīn and yáng symbols of the liǎngyí progress through a further five levels of notation to eventually give rise to 64 hexagrams.
All of the levels are derived by creating a yīn and yáng version for each of the symbols in the previous level.
The Indus notation was used within inscriptions to convey information about the day-to-day scheduling of activities, but its origin was most probably observational astronomy. I have therefore added a diagram below, by way of example, of how the notation could be used in an astronomical context.
In the diagram, the symbols for the four phenomena are used to plot the relative positions of the sun in the sky. The diagram represents the perceived path of the sun over the celestial sphere, in the Northern Hemisphere, through the course of a day.
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.
All Other Images: Lynn Fawcett, 2017 - 2020.
Note:
1. There is no photograph of M-2069 in Parpola. The source of Parpola's drawing was Marshall, where the graffiti is presented with an incorrect orientation. Sir John Marshall, 1931: Mohenjo-daro and the Indus Civilization, Vol. III, Pl. XC, Inscribed and Painted Pottery, Pot Mark Number 5: Arthur Probsthain.