How Many Seconds Are in A Day?

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Seconds in a day calculation

One day has 24 hours, one hour has 60 minutes and one minute has 60 seconds, so 24 hours/day times 60 minutes/hour times 60 seconds/minute is equal to 86400 seconds/day:

1 day = 24 hours/day × 60 minutes/hour × 60 seconds/minute = 86400 seconds/day

What’s a Second?

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Before we get into how many seconds there are in a day, let’s talk about what a second is. A second is our base unit of time—base, but not smallest. A base unit is a unit defined on its own terms upon which other units are based. That means that all our other units, such as minutes, hours, nanoseconds, and so on, are all based on seconds. We talk about hours in terms of minutes, but minutes are based on seconds, bringing us back to the base unit.

A second used to be based on the Earth’s rotation cycle, with one second being 1/86,400 of the average solar day. Now that we know more about how the Earth rotates—and that the speed at which it rotates is slowing down—we now use a more accurate method. We base a second off of the radiation cycles of a caesium-133 atom, often called an “atomic clock.”

Because our astronomical year varies in length, we also sometimes add “leap seconds” to the clock to keep better time, or we’d eventually end up with time being way off of where it should be. Adding seconds is infrequent, and doesn’t actually change the time—all it does is keep us consistent. But those single leap seconds matter, keeping us on track so that a few missed seconds doesn’t become a missed minute or hour or day over time.

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What is a day?

A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. Days on other planets are defined similarly and vary in length due to differing rotation periods, that of Mars being slightly longer and sometimes called a sol.

The unit of measurement "day" (symbol d) is defined as 86,400 SI seconds. The second is designated the SI base unit of time. Previously, it was defined in terms of the orbital motion of the Earth in the year 1900, but since 1967 the second and so the day are defined by atomic electron transition. A civil day is usually 24 hours, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.

Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. However, its use depends on its context; for example, when people say 'day and night', 'day' will have a different meaning: the interval of light between two successive nights, the time between sunrise and sunset;the time of light between one night and the next. For clarity when meaning 'day' in that sense, the word "daytime" may be used instead, though context and phrasing often makes the meaning clear. The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns (circadian rhythms) of humans and many other species related to Earth's solar day and the day-night cycle.

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