The hour is a unit of time and is a multiple of an SI base unit with the symbol hr.
There are 60 seconds in a minute and 60 minutes in an hour. Consequently, there are 3600 seconds in an hour.
In a mean solar day, there are 24 hours.
Originally in the Middle East, the hour was defined as 1/12 of the day or night time. However, seasonal factors clearly affected all of these definitions and an average is used today to normalise and allow calculations.
The tropical year is a unit of time that is a multiple of an SI unit. We have used the symbol at.
The average tropical year up to the year 2000 was 365.2422 days of 86400 seconds.
The phrase “tropical” comes from the Greek for “turn”; tropikos. Following this, and where the “turn” referred to is the same as that defining the tropics of Capricorn and Cancer (the extreme latitudes North and South where the sun can still appear directly overhead or perpendicular to the Earth’s surface); a tropical year is known as the time it takes the sun to complete a full cycle of seasons and return to the same relative position to the observer on Earth.
In 1627, Johannes Kepler used the findings of Tycho Brahe and Waltherus (the Rudolphine Tables) to establish the mean tropical year as 365 solar days, 5 hours, 48 minutes and 45 seconds.