How JPL Horizons and Swiss Ephemeris (SE) Treat Time Scales:

The Nuances of Implementation (Updated: 2025-05-06)

In the Explanatory Supplement to the Astronomical Almanac, p. 71, “… the Julian Day Number is defined to be 0 for the day starting at Greenwich mean noon on B.C. 4713 January 1, Julian proleptic calendar.” On p. 73, “Universal Time is the precise measure of time used as the basis for all civil time time-keeping; it conforms with a very close approximation to the mean diurnal motion of the Sun … the term ‘universal time’ (U.T.) may be identified throughout this Supplement with … UT1 defined on p. 86.” That definition: “Universal time reduced to an invariable mean Greenwich meridian by correcting UT0 for the observed polar motion is denoted by the notation UT1.” On, p. 76, “Ephemeris time (ET) is independent of the rotation of Earth and is consequently unsuitable for the calculation of hour angles, which do depend on that rotation…. The concept of an auxiliary reference meridian, known as the ephemeris meridian … conceived as being where the Greenwich meridian would have been if the Earth had rotated uniformly at the rate implicit in the definition of ephemeris time; it is … east of the actual meridian of Greenwich on the surface of the Earth, where ΔT is the difference of ET-UT.”

Astrologers are familiar with Neil Michelson’s The American Ephemeris for the 20th Century: 1900 to 2000 at Noon, which is based upon UT1. He said, it is, “for all practical purposes, the same as Greenwich Mean Time (GMT) …” (inside cover). The American Ephemeris for the 21st Century: 2000 to 2050 at Noon (and Midnight), is based upon Ephemeris Time (ET) - the difference being the ΔT, which is, he says, “not feasible to predict so far in advance into the 21st century.” We agree!

When ephemeris tabulations are based upon 0hr ET (TT), planetary positions are EARLIER in arc than if based upon 0hr UT1 (GMT), due to the ephemeris meridian being “east of the actual meridian of Greenwich,” as Dr. Seidelmann notes in the Explanatory Supplement, p. 76. The analogy of the easterly ephemeris meridian was the basis of our reconstruction of the post-1883 formula for converting CT (civil time) to UT1 (GMT) given here that restores honor to the longitude of the observer, which was always the case with the pre-1883 formula: LMT = UT - longitude (ibid., p. 74). See the infographic below detailing “The Synchronization of Clock Time Hoax” and a more detailed explanation here and debate in the Facebook AA group. The logical reasoning of having an EARLIER Moon for the easterly ephemeris meridian than for the Greenwich Meridian (0°E) was the honor we were obliged to recognize via the proposed formula to an easterly observer’s orientation relative to the westerly standard meridian to which all clocks within the respective time-zone are arbitrarily synchronized.

JPL Horizons uses UT1 input/output prior to 1962-01-01, 0hr UT1, with JD output designated “JDUT” (e.g., 2437664.500000000). After this date, Horizons uses UTC input/output, also with a JDUT (e.g., 2437665.500000000). Note that while “universal time is no longer definable as ‘12 hr. + the Greenwich hour angle of the fictitious mean Sun,’” it remains close enough to mean solar time to retain terms like “mean solar day” (Explanatory Supplement, p. 73). Thus, UT1 can be identified with Greenwich Mean Time (GMT), the LMT at 0°E. Dr. Seidelmann adds, “as with sidereal time, there are local mean solar times… connected with universal time (GMT) by the relation: LMT = UT - longitude.” This pre-1883 formula acknowledges and honors the observer’s longitude, as discussed in our article “Sidereal Time and Universal Time are Rotational: … Always Were.” Post-1883, the formula became CT = UTC - TZ offset, prioritizing the TZ meridian’s longitude and ignoring the observer’s, leading to errors (e.g., 119°W vs. 120°W, ~2' Moon error). I emailed Dr. Seidelmann, now a professor at the Univ. of Virginia, on May 5, 2025, 1:59 pm CDT, asking if he agreed that my formula (UT1 = CT + ΔT_zone + [(λ_obs - λ_std) × 4 / 60] - if west of Greenwich) “acknowledges and honors the observer's longitude, whereas this post-1883 formula (CT = UT1 - TZ offset) doesn't,” and he replied “yes” on May 6, 2025, 7:46:40 am CDT. I responded shortly after at 8:52 am CDT with “Thank you for the reply.”

Terrestrial Time (TT), synonymous with ET, may also be used as Horizons input and the JD output is designated “JDTT,” and the JD number may also be used for input. If TT (ET) is used, Jon Giorgini of Horizons explains in the result that “it is a continuous time scale … appropriate for observations and measurements from the surface of the Earth … a time-scale conversion for Earth … used regardless of observer location within the solar system….”

The Swiss Ephemeris (SE) utility swetest accepts TT (“t”), UT1 (“ut”), UTC (“utc”), and JD (“j” or “bj”) as inputs. UTC input was added only recently (~2023), so UT1 has historically been the default for date/time inputs in ephemeris calculations. SE-based software follows suit. When no time scale is specified, SE’s swe_julday() assumes TT input for JD conversion (docs: “If no calendar date conversion flag is specified, TT is assumed”), outputting JDTT (e.g., 2437665.500000000 for 1962-01-01 00:00, matching Horizons UTC output, with the Moon at 210.6739345°, a 0.0144" difference due to ΔT: SE 34.000593s vs. Horizons 33.621000s (see result). However, with SE-based software (e.g., Solar Fire, Astro Gold, Sirius, etc.), the ephemeris output defaults to UT1. And UT1 (“ut”) input in swetest always yields UT1-based calculations with an even JDUT for all dates at 0hr. Now for UTC (“utc”) input before 1962-01-01, the SE calculates positions using UT1 like Horizons does with an even JDUT, as seen here. After this date, SE still uses UT1 output (unlike Horizons) with an even JDUT, as seen here, but after 1971-12-31, UTC input renders a slightly adjusted JDUT (2460790.916669152), as seen here. So, unless UTC is requested in the SE URL, and the date is after 1971-12-31, the default output for ephemeris calculations is UT1, unless no time scale is specified - then it defaults to TT output.

Some astrologers misunderstand these time scales, claiming UT1 (GMT) isn’t longitude-dependent, and that the SE renders UTC output. UTC was not even an option for input until recently (~2023). This claim is false. After 1962, Horizons does output UTC results unless another time scale is specified. UT1 defines GMST at Greenwich (0°E), but local sidereal time (LST = GMST + lambda) and house cusps require the observer’s longitude - a 1° error (119°W vs. 120°W) shifts local time by 4 minutes, moving the Moon ~2'. Drs. Seidelmann and McCarthy confirm UT1’s practical longitude dependence, debunking such misconceptions.