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Heber D. Curtis on the nature of nebulae and galaxies and the size of the Universe. Debate took place on April 26, 1920, in the hall of the United States National Academy of Sciences in Washington DC. Shapley was of the opinion that spiral nebulae (what are now called galaxies) are inside our Milky Way, while Curtis was of the opinion that the spiral nebulae are 'island universes' far outside our own Milky Way and comparable in size and nature to our own Milky Way. ️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️