The story of AM and PM

A.M. and P.M., meaning ante-meridian and post-meridian, were never established by law or scientific decision, they simply evolved in the course of time. The measuring of time is linked to the instruments used to measure it, so it is linked to clocks. The meridian is related to Greenwich meantime, that was once the international standard. Learn more in this article.

The measurement of time.

Telling the time At no point did any law-making body, decide that a.m and p.m had any official status as time telling terms. People began using them as the measurement as time developed. The word meridian has no connection with morning, or indeed noon. It refers to the international line of measurement at zero longitude running through Greenwich observatory. Primitive time telling In ancient times, the terms were not used. People knew that there was noon, and used the terms morning and afternoon, but then the instruments used to measure time were not subtle, and so there was only minimum mathematics used. Sundials, water clocks and candle clocks were the only instruments available. In practice, the day started with sunrise. Clocks Things changed when clocks were invented. We already had twenty four hours in the day, but rather than having the expense of designing a twenty-four hour clock, the cheaper option of twelve hours was taken, making the dial go round twice. There was still the problem of deciding when the day started. Sunrise varies day by day, so hours of daylight are longer in summer and vice versa. A time to start the day was set at an arbitrary point in the night, midnight, at which the sun would never be rising.

Problems still

Yet, there used to be lots of different local times, even into the nineteenth century. An official time was needed. The one that became popular was Greenwich meantime, which was the time taken at the royal observatory at Greenwich. This was connected with the establishment of longitude. Greenwich Greenwich established the meridian, the line of zero longitude, from its own location, which is 180 degrees from the international date line. They set clocks with noon being measured by noon at Greenwich. Hence, morning was ante-meridian, before noon at Greenwich. Afternoon, was post-meridian, after noon at Greenwich. This enabled scientists and administrators to have a standard time measurement that became the official time and created the language of official time.