The term "second" has a number of different meanings in the context of math and numbers. Possibly the most important is that it is the base unit of time in the International System of Units (SI). Other measurements of time, such as minutes, hours, days, weeks, and so on, are defined based on the second.
The unit of a second has historically been defined as of a day. This is also the common understanding of the relationship between seconds and other measurements of time where:
- 1 minute = 60 seconds
- 1 hour = 60 minutes = 3,600 seconds
- 1 day = 24 hours = 86,400 seconds
- 1 week = 7 days = 604,800 seconds
- 1 month = ~30.436875 days = 2,629,746 seconds
- 1 year = ~365.2425 days = 31,556,952 seconds
These are the definitions used for our clocks based on the Gregorian calendar. Since the Gregorian calendar is based on the time it takes Earth to travel around the Sun, a year actually has an average of 365.2425 days, so the Gregorian calendar corrects this discrepancy with the use of leap years.
Under SI, the second has a highly precise definition that has changed over time. Briefly, since 2019, it is defined based on specific properties of the caesium-133 atom under certain conditions, and is a much more accurate way to keep time than the previous measure (the definition since 1960) based on the division of the Earth's rotation cycle.
Second as an ordinal number
"Second" can also be used as the ordinal number after first, where an ordinal number is a concept used to arrange natural numbers. For example, second grade comes after first grade; second place in a race comes after first place; the country with the second highest population comes after the country with the highest population, and so on.
Second as a measure of latitude and longitude
Seconds (and minutes) are also part of the measure degrees of latitude and longitude. In this context, the following symbols are used to denote seconds and minutes, respectively: " and '.
- Latitude is divided into 180°: 90° North ↔ 90° South
- Longitude is divided into 360°: 180° East ↔ 180° West
1° of latitude or longitude is equal to 60 minutes, or 3600 seconds.
Latitude and longitude are used to denote specific positions on Earth and can be expressed in terms of degrees, minutes, and seconds, or in a form that converts the minutes and seconds to a decimal. For example, the coordinates of the Empire State Building in New York City are:
40° 44' 54.388" N 73° 59' 8.39" W