Nearly five centuries before Johannes Werner, the Egyptian astronomer Ibn Yunus used a trigonometric identity equivalent to {𝟮𝗰𝗼𝘀(𝗮)𝗰𝗼𝘀(𝗯)=𝗰𝗼𝘀(𝗮+𝗯)+𝗰𝗼𝘀(𝗮-𝗯)} in astronomical calculations for determining time from the altitude of the Sun and stars.
Ibn Yunus (1009 CE) was an Egyptian Muslim mathematician. His great-grandfather was a hadith scholar and an associate of Imam Shafi'i.
His methods for determining the time from stellar altitude were equivalent to:
{𝟮𝗰𝗼𝘀(𝗮)𝗰𝗼𝘀(𝗯)=𝗰𝗼𝘀(𝗮+𝗯)+𝗰𝗼𝘀(𝗮-𝗯)}.
That same formula, {𝟮𝗰𝗼𝘀(𝗮)𝗰𝗼𝘀(𝗯)=𝗰𝗼𝘀(𝗮+𝗯)+𝗰𝗼𝘀(𝗮-𝗯)}, was later named Werner's formula, so the credit went to someone else, even though Ibn Yunus had already worked with it centuries earlier.
🧵[THREAD]
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