Tropical year

Tropical year

Time Measurement

Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017

The ephemeris second was defined in 1956 and designated as the original SI second in 1960. It was defined by dividing the tropical year or the interval between the annual vernal equinoxes that occur on or about March 21. The tropical year was defined as 31,556,925.9747 ephemeris seconds. Determining the precise instant of the equinox is difficult, and this limited the uncertainty of ephemeris time (ET) to ±50 ms over a 9 year interval. It also made the ephemeris second nearly impossible to realize in a laboratory and of little or no use to metrologists or engineers. Thus, its tenure was understandably short. It remained part of the SI for just 7 years before being replaced by the much more accessible atomic definition [5,6,13].

The mathematics of leap years: sophistication versus content in mathematics education

View Article

Journal Information

Published in International Journal of Mathematical Education in Science and Technology, 2022

Yip Cheung Chan, Kam Moon Pang, Kenneth Young

Operationally, the day is the (mean) interval between successive noons (when the Sun reaches the highest point in its apparent east–west motion). The tropical year is the (mean) repetition interval for the seasons, or the apparent north–south motion of the Sun. (In modern terms, it is the time interval for the ecliptic longitude of the Sun to increase by 360°) The ratio between the tropical year and the day, r = 365.24219 (Meeus & Savoie, 1992), in particular its fractional part, is at the heart of the problem. For the present purpose, this empirical value is taken to be exact and constant.

Explore chapters and articles related to this topic

Time Measurement

The mathematics of leap years: sophistication versus content in mathematics education