DAVID PARKER / SCIENCE PHOTO LIBRARY DAVID PARKER / SCIENCE PHOTO LIBRARY
Eta is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8. The lower-case letter is used as a symbol in many sciences including thermodynamics, chemistry, optics, physics and mathematics. The Greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles.
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