What is a solar day?

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What is a sidereal day? One full turn relative to the stars, not the Sun. On Earth that is 23 hours 56 minutes. A star rises about four minutes earlier each night because of that difference — which is the orbit showing up in the clock.

What is the equation of time? The difference between clock noon and sundial noon. The Earth’s orbit is a slight ellipse and its axis is tilted, so the true solar day wanders by up to about 16 minutes through the year. Clocks run a mean 24 hours and ignore it.

What is a leap year? A year with 29 February. The Earth takes about 365.242 days to go around the Sun, so a 365-day calendar drifts. Adding a day most years that divide by 4 — but not most centuries — keeps the seasons from walking off the months.

Who invented time? Nobody invented time — people read it off the sky. A day is one spin of the Earth. A year is one trip around the Sun, announced by the returning seasons. A month began as one cycle of the Moon. Every clock and calendar since is bookkeeping for those three motions, which is why time and space are not two subjects but one.