What is an ephemeris?
An ephemeris is a table or calculation giving the positions of the planets, Sun and Moon at regular times.
What it is
Printed ephemerides list longitudes for each day. Modern ones are computed from numerical models of planetary motion. The site uses the astronomy-engine library, which implements models used in professional almanacs.
How it is worked out or read
The tools ask the ephemeris for a position at the birth instant, convert it to the sidereal zodiac and report the result. Nothing is looked up from a stored table of predicted values.
Where it appears on this site
The method page names the library and its licence. See how the ephemeris works for more.
Reading a printed ephemeris
A printed ephemeris lists the position of each planet for each day, often at noon, in a table of degrees, minutes and sometimes the daily motion. To find a planet's position at a given time, you read the entries for the surrounding days and interpolate. The Moon, which moves quickly and irregularly, needs finer steps than the outer planets, which change little in a day. Older Jyotish almanacs used tables of this kind, and some still do. The method is slow but transparent, and it gives a useful check on a computer result.
Why a computed ephemeris is preferred
A numerical model can give a position for any instant, not only for the times in a table. This matters for tithi and nakshatra end times, which occur at irregular moments. The model also gives speeds, which are needed for retrograde detection. The trade-off is that the model depends on its assumptions. A good practice is to compare a computed value with a printed one for the same date. Where they agree within a few arcminutes, the model is reliable for chart work.
Where ephemerides are used beyond Jyotish
Ephemerides are used far beyond astrology. Navigators, surveyors and astronomers have relied on them for centuries, and modern spacecraft missions use very detailed ephemerides to plan their routes. The same mathematics that gives a planet's position for an astrology chart gives the position needed for an observatory to point a telescope. This shared foundation is one reason the positions in Jyotish can be checked against astronomy, and why the site treats the calculation of positions as a matter that can be verified independently.
Checking an ephemeris by hand
A reader can check an ephemeris by hand for a few known dates, such as a new moon or a full moon, where the Sun and Moon are close together or opposite. If the computed longitudes place the Moon near the Sun at those times, the model is behaving as expected.
Ephemeris and the astronomer's tools
An ephemeris is one of the basic tools of astronomy, used long before computers. Understanding that the Jyotish positions rest on the same astronomical tradition gives the reader confidence that the calculations are part of a long and checkable practice.