Ephemeris April 2025

What an ephemeris is, and why a table has rows

An ephemeris is a table of where the planets are, computed for a regular instant — traditionally midnight or noon Greenwich, one row per day. Astrologers used printed ones for centuries, and the format has shaped how the subject talks: a chart was built by reading the row for your birth date and interpolating to your hour.

The interpolation was the work. A daily table gives you two positions a day apart, and getting the position at, say, 14:20 meant estimating between them. For most bodies that is fine — the Sun and Mars move so little in a day that a straight-line estimate is accurate to a fraction of an arcminute.

The Moon is the exception, and it is worth putting a number on. Comparing linear interpolation between midnight rows against the true position, the worst error across a year of samples is 3.5 arcminutes for the Moon and 1.5 for Mercury, against 0.1 for Venus and effectively zero for Mars and the Sun. Small, but the Moon is also the body most likely to be near a sign boundary.

This one is computed, not tabulated

The ephemeris on this site is not a stored table being looked up. Positions are computed from the Swiss Ephemeris for the exact moment requested, which removes the interpolation step entirely — the value you see for 14:20 is the value at 14:20, not an estimate between two midnights.

That matters most for the Moon, which covers about thirteen degrees a day and changes sign roughly every two and a quarter days. A birth an hour either side of an ingress gets a different Moon sign, and an interpolated table is exactly where that decision goes wrong.

It also means the table is not limited to a printed range. Any date the underlying ephemeris covers can be computed on demand, including dates far outside the span a book would carry.

Delta T, and why historical charts need it

There is a correction most people never encounter that becomes important for old charts. Planetary positions are computed in a uniform timescale, but civil time is tied to the Earth's rotation, which is not uniform — it varies with tides, mass distribution and other effects. The difference between the two is called Delta T.

It is not a small or steady number. Measured for a few reference years: about 90 seconds in 1600, 17 seconds in 1750, roughly minus two seconds around 1900, 29 seconds in 1950, 64 seconds in 2000 and 69 seconds now.

The value going negative around 1900 surprises people and is the clearest sign that this is a measured quantity rather than a formula. For a modern chart a minute of Delta T moves the Ascendant by about a quarter of a degree — usually irrelevant. For a seventeenth-century chart it is a minute and a half, and combined with uncertain local time it is one reason historical charts carry more doubt than their neat presentation suggests.

Reading a position properly

A longitude is given as a sign, a degree, and minutes of arc — 14° Leo 27' means fourteen degrees and twenty-seven minutes into Leo, which is 134.45 degrees measured from 0° Aries. Sixty minutes make a degree, so the notation is sexagesimal rather than decimal, and 14° 30' is 14.5 degrees and not 14.3.

The R that appears beside a position means retrograde: the planet is apparently moving backwards against the zodiac, which happens because the Earth is overtaking it or, for Mercury and Venus, because they are passing between us and the Sun.

Daily motion is the other column worth reading. It tells you how fast a body is travelling, which is what decides whether a transit is exact for a day or a season — and a planet slowing towards a station is one that will sit on the same degree for weeks.

What this table is and is not for

It is the raw material. Every chart on this site is built from these positions, and having them visible means a reader can check a claim rather than take it — which is the main argument for publishing an ephemeris at all.

It is not a forecast. A position is a fact about a moment; what turns it into a statement about a person is a contact with a birth chart, and that requires the birth chart.

And it is geocentric, which is the convention in Western astrology and worth stating rather than assuming. These are positions as seen from the Earth, which is why the Sun appears to move and the outer planets appear to reverse — neither of which is true of the solar system, and both of which are true of the view.