At the equinoxes sunrise is around 6 a.m. (Latin: ante meridiem, before noon), and sunset around 6 p.m. (Latin: post meridiem, after noon). In the modern 24-hour clock, counting the hours starts at midnight, and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again differing according to the equation of time. At the equinoxes sunrise is around 06:00, and sunset around 18:00. History of timekeeping in other cultures
Egypt
The ancient Egyptians began dividing the night into wnwt at some time before the compilation of the Dynasty V Pyramid Texts in the 24th century BC. By 2150 BC (Dynasty IX), diagrams of stars inside Egyptian coffin lids—variously known as "diagonal calendars" or "star clocks"—attest that there were exactly 12 of these. Clagett writes that it is "certain" this duodecimal division of the night followed the adoption of the Egyptian civil calendar, usually placed c. 2800 BC on the basis of analyses of the Sothic cycle, but a lunar calendar presumably long predated this and also would have had twelve months in each of its years. The coffin diagrams show that the Egyptians took note of the heliacal risings of 36 stars or constellations (now known as "decans"), one for each of the ten-day "weeks" of their civil calendar. (12 sets of alternate "triangle decans" were used for the 5 epagomenal days between years.) Each night, the rising of eleven of these decans were noted, separating the night into twelve divisions whose middle terms would have lasted about 40 minutes each.
Derived measures
air changes per hour (ACH), a measure of the replacements of air within a defined space used for indoor air quality
ampere hour (Ah), a measure of electrical charge used in electrochemistry
BTU-hour, a measure of power used in the power industry and for air conditioners and heaters
credit hour, a measure of an academic courses contracted instructional time per week for a semester
horsepower-hour (hph), a measure of energy used in the railroad industry
hour angle, a measure of the angle between the meridian plane and the hour circle passing through a certain point used in the equatorial coordinate system
kilometres per hour (km/h), a measure of land speed
kilowatt-hour (kWh), a measure of energy commonly used as an electrical billing unit
knot (kn), a measure of nautical miles per hour, used for maritime and aerial speed
man-hour, the amount of work performed by the average worker in one hour, used in productivity analysis
metre per hour (m/h), a measure of slow speeds
mile per hour (mph), a measure of land speed
passengers per hour per direction (p/h/d), a measure of the capacity of public transportation systems
pound per hour (PPH), a measure of mass flow used for engines fuel flow
work or working hour, a measure of working time used in various regulations, such as those distinguishing part- and full-time employment and those limiting truck drivers working hours or hours of service

See also
Liturgy of the Hours
Horology
Horae, the deified hours of ancient Greece and Rome
Hexadecimal hour, a proposed unit lasting 1 h 30 min
Decimal hour or deciday, a French Revolutionary unit lasting 2 h 24 min
Golden Hour & Blue Hour in photography
Metric time

Notes
References
Bibliography
Further reading
Gerhard Dohrn-van Rossum (1996).