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This rate is known as the mean tropical year.

This would indeed be the case if the mean value of the civil year of the Julian calendar was exactly equal to the tropical year.

Accurate observations with the elaborate apparatus used by professional astronomers show that this period, which is called a tropical year, is 365 days 5 hours 48 minutes 46 seconds.

The tropical year being thus ascertained, their tables showed the exact time of the equinox or sun's transit across the equatorial, and of the solstice.

The scheme itself denotes, not only a very certain mode of keeping the record of time, but a very exact knowledge of the tropical year.