Calendars – keeping track of time
Glossary
BCE. Before the Common Era. There are different ways of designating dates. BCE is one way of indicating dates on the Gregorian calendar that occurred before the date traditionally regarded as the year 1.
The numbering of the Gregorian calendar was instituted by Dionysius Exiguus in 532. He investigated the date of the birth of Jesus Christ and set that as the start of the year 1. Thus, the year 1999 is referred to as 1999 CE (Common Era) or 1999 AD (anno Domini – ‘in the year of our Lord’). Years before the birth of Christ are designated as BCE – ‘before the Common Era’, or BC – ‘before Christ’.
Dates can also be designated as the number of years before the present (BP).
equinox. The times of the year when the sun crosses the celestial equator (the projection of the Earth's equator onto the sky) making the length of day and night nearly equal at all latitudes. There are two equinoxes each year, one in March, known in the northern hemisphere as the 'vernal' equinox, and one in September, known in the northern hemisphere as the 'autumnal' equinox.
The dates of the equinoxes do not occur precisely when the lengths of the day and night are equal, but are out of step by a few days. This discrepancy is because of the finite size of the sun and the bending of sunlight by the atmosphere.
More information can be found at FAQ–Equinoxes (US Naval Observatory).
gnomon. A column that can indicate the time of day by the shadow that it casts on a marked surface. On a sundial, the pin or vertical triangular plate that casts the shadow is called a gnomon. More information can be found at Design of the Richard D. Swensen sundial (University of Wisconsin-River Falls, USA)
interpolation. To estimate a value for a function between values that are already known or determined. This is in contrast to extrapolation, which is estimating the value of a function lying outside the range of known values. More information can be found at Linear interpolation – how it works (The Greenhouse, National Science Foundation's Graphics and Visualization Center, USA).
solstice. The time of year when the sun is furthest from the celestial equator (the projection of the Earth's equator onto the sky). The summer solstice occurs in mid-summer and the winter solstice in mid-winter. For more information see The equinoxes and solstices (National Maritime Museum, Greenwich, UK).
Page updated August 2006.