The American Ephemeris "For The 21st Century ( 2000 - 2050 )" (254 pagini)

"The American Ephemeris for the 21st Century is a midnight ephemeris, based on Ephemeris Time (ET). This differs from the earlier volume in this series, The American Ephemeris for the 20th Century which is based on Universal Time (UT).

The calculation of planetary positions requires a uniform measurement of time. The Earth's rotation is too irregular to be used for this purpose even though our clocks are synchronized to that rotation. Earthquakes, tidal coupling with the Moon and other disturbances cause the Earth either to speed up or slow down. We adjust our clocks to this changing speed of the Earth by adding a "leap second", such as was done on June 30, 1982. We are now adding leap seconds rather than subtracting, since the Earth's rotation is slowing slightly; hence, the civil day is becoming ever so slightly longer.

The difference between ET and UT is called "Delta T". In order to calculate the most accurate horoscopes, mainly for precise solar and lunar returns, one must adjust the time of the chart by adding the Delta T to the UT of the chart, before interpolating to find the planet positions. It is not feasible to predict so far in advance into the 21st century what the Delta T values will be; so this ephemeris is based, as are practically all ephemerides used by astrologers, on ET.

The planetary positions in this ephemeris differ slightly from the earlier editions, because of more accurate data available from JPL (Jet Propulsion Laboratories). Since 1984, JPL data has been used in The Astronomical Almanac, a joint publication of the US Naval Observatory and the Royal Greenwich Observatory. These differences are so small, that they will show up mainly in the times of aspects, sign ingresses, 0 declinations and stations which appear in the phenomena section at the bottom of each page.

The most dramatic change will be in a few void-of-course Moon times, where an aspect time previously used to start the void period shifts to just after the Moon enters a new sign, so that an earlier aspect becomes the determining time for the begining of the void Moon."

Neil F. Michelsen