EXPLANATIONS

02/19/2024

 

EPHEMERIDES

Provided here are files intended to assist amateur astronomers looking for observational data for the Sun, the Moon, the planets and Pluto.  The data listed differ from those typically found in the "Astronomical Almanac for the Year xxxx" in the following ways:

(1) All of the data reference the equator and equinox of date, unless noted otherwise, e.g. distances always have light time factored in rather than being "true" distances as if the speed of light were infinite. Thus, distance, parallax, and visual sizes as listed here should be more useful than the corresponding data listed in the AA.  Where the size of an object is needed in a computation, the size used here is always that of the equatorial diameter or radius (it appears the AA may use the mean radius or diameter).

(2) Through at least 2023, the AA is based on DE430/LE430, but the source for solar system objects used here is DE440/LE440.  However, the right ascensions and declinations listed here are in very close agreement with the AA.  Because DE440 is based on about seven more year's worth of data for the outer planets, these two data items, as listed here, may be better than their counterparts in the AA (until the AA switches to a more recent DE version).

(3) In all cases where the Sun shines on an object to be viewed from our planet, the tabulated time refers to the dynamical time of the observation of the referenced object at its location at time (TT - light time1), as illuminated by the Sun at its location at time (TT - light time1 - light time2).  The methodology used to produce the AA is not totally clear.

(4) The methodology used to calculate apparent right-ascension and declination for the planets, here and in the AA, takes into account relativity, and the results are in very close agreement with MICA and AA2023.

(5) The other items listed here such as phase angles, percent illuminated, light path lengths, mostly do not take relativity into account.  However, all coordinates used here in computing locations are based on DE440 data, and the visual diameters listed here are in close agreement with the AA.  Because the visual diameters listed here are based on apparent distance, they might be more useful than if they were based on "true" distance.

(6) The methodology used for phase angle in the AA is not known in detail.  The phase angles listed here use apparent positions and distances, not "true" distances or locations, and are rigorous, other than for relativistic effects being ignored. Also, phase angles are sometimes calculated based on ecliptic longitude, but here the phase angle is the actual separation angle between the lines planet-to-sun versus planet-to-Earth, and might well be more accurate than those listed in the AA.  Note that the phase angle is a major factor in calculating the visual magnitude.  The old tradition of calculating phase angles based on ecliptic coordinates is probably not used for the AA listings, but certainly not used here.

(7) The visual magnitudes listed in the AA take into account that the Sun is not a point light source - this can be very important for Mercury, somewhat important for Venus, and of minor importance for the outer planets.  In any case, the magnitudes listed here are close to those in the AA for the outer planets, and often for Mercury and Venus as well.

Each year's listing contains 80 pages of data for the Sun (starting on page 1), the Moon (page 9), Mercury (page 17), Venus (page 25), Mars (page 33), Jupiter (page 41), Saturn (page 49), Uranus (page 57), Neptune (page 65), and Pluto (page 73).

Link to ephemerides for 2023.

Link to ephemerides for 2024.

Link to ephemerides for 2025.

 

EPHEMERIS TRANSITS

Provided here is a list of the times of transit times for the Sun, the Moon, and the planets over the ephemeris meridian.  The positions of these bodies is accurately computed in Barycentric Dynamical Time, but the orientation of the Earth, and therefore the time of transit for a specific meridian, is dependent on UT1 - which cannot be predicted with guaranteed accuracy.  However, by consulting these IERS websites the latest data on the relationships between UTC, UT1, and TT can be obtained.  When reliable data for UT1-UTC become available they may be used to determine transit times in terms of UTC, it should not be necessary to adjust the times in UT1.

These listings are similar to those on pages E46-E53 of the Astronomical Almanac for the Year 2023, except that the times in the AA listings are proclaimed to be in TT - which they are not, they appear to be in UT1, rather than in UTC, for certain they are not in TT.  Also, in the listings here, the Sun and the Moon are included, and the times of the seasonal solstices and equinoxes, as well of the lunar quarters are included.  The times listed here are in UT1 rounded to the nearest tenth of a second.

Listings for 2023, and 2024-2025, are available.