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Why is the statement “Venus is the morning star” informative even though both expressions refer to Venus?

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Austrian postage stamp featuring a portrait of philosopher Ludwig Wittgenstein, with German text above and below, and a denomination of 5 schillings. — Philosophy of language An Austrian stamp depicting Ludwig Wittgenstein.

In the philosophy of language, the statement "Venus is the morning star" is informative because, while both expressions share the same referent (the planet Venus), they possess different senses.21

This distinction was famously articulated by the logician Gottlob Frege. According to Frege, the sense (Sinn) of an expression serves as its "mode of presentation"—the specific way in which the referent is presented to the mind. Because "the morning star" and "Venus" present the planet through different descriptions or cognitive paths, they contribute differently to the thought expressed by a sentence.23

Consequently, an identity statement like "Venus is the morning star" provides new knowledge by revealing that two distinct modes of presentation actually pick out the same object in the world. This explains why such a statement can represent a genuine astronomical discovery, whereas a trivial identity statement like "Venus is Venus" does not.23