Solar Math: Variables and Expressions from Around the Cosmos

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There are many simple mathematical formulae that astronomers use to describe different aspects of the universe and the physical world. (The above photo of Saturn was taken by NASA's Cassini spacecraft from behind Saturn looking back towards the Sun.) 

Problem 1: 

Find, P, the length of Earth's day 500 million years in the future if $P=24$ hours $+0.004Y$, where $Y$ is the number of millions of years that have elapsed.