brandon-rhodes
commented
2 years ago When two astronomy sources disagree, it's helpful to bring in a third to break the tie. Here's the HORIZONS service from the JPL:
https://ssd.jpl.nasa.gov/horizons/app.html#/
Putting in the circumstances of your script, I get this back:
Revised: July 31, 2013 Moon / (Earth) 301
GEOPHYSICAL DATA (updated 2018-Aug-15):
Vol. mean radius, km = 1737.53+-0.03 Mass, x10^22 kg = 7.349
Radius (gravity), km = 1738.0 Surface emissivity = 0.92
Radius (IAU), km = 1737.4 GM, km^3/s^2 = 4902.800066
Density, g/cm^3 = 3.3437 GM 1-sigma, km^3/s^2 = +-0.0001
V(1,0) = +0.21 Surface accel., m/s^2 = 1.62
Earth/Moon mass ratio = 81.3005690769 Farside crust. thick. = ~80 - 90 km
Mean crustal density = 2.97+-.07 g/cm^3 Nearside crust. thick.= 58+-8 km
Heat flow, Apollo 15 = 3.1+-.6 mW/m^2 Mean angular diameter = 31'05.2"
Heat flow, Apollo 17 = 2.2+-.5 mW/m^2 Sid. rot. rate, rad/s = 0.0000026617
Geometric Albedo = 0.12 Mean solar day = 29.5306 d
Obliquity to orbit = 6.67 deg Orbit period = 27.321582 d
Semi-major axis, a = 384400 km Eccentricity = 0.05490
Mean motion, rad/s = 2.6616995x10^-6 Inclination = 5.145 deg
Apsidal period = 3231.50 d Nodal period = 6798.38 d
Perihelion Aphelion Mean
Solar Constant (W/m^2) 1414+-7 1323+-7 1368+-7
Maximum Planetary IR (W/m^2) 1314 1226 1268
Minimum Planetary IR (W/m^2) 5.2 5.2 5.2
********************************************************************************
*******************************************************************************
Ephemeris / WWW_USER Sun Apr 3 20:20:44 2022 Pasadena, USA / Horizons
*******************************************************************************
Target body name: Moon (301) {source: DE441}
Center body name: Earth (399) {source: DE441}
Center-site name: GEOCENTRIC
*******************************************************************************
Start time : A.D. 1920-Jan-01 00:00:00.0000 UT
Stop time : A.D. 1920-Jan-03 00:00:00.0000 UT
Step-size : 720 minutes
*******************************************************************************
Target pole/equ : MOON_ME {East-longitude positive}
Target radii : 1737.4 x 1737.4 x 1737.4 km {Equator, meridian, pole}
Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}
Center pole/equ : ITRF93 {East-longitude positive}
Center radii : 6378.1 x 6378.1 x 6356.8 km {Equator, meridian, pole}
Target primary : Earth
Vis. interferer : MOON (R_eq= 1737.400) km {source: DE441}
Rel. light bend : Sun, EARTH {source: DE441}
Rel. lght bnd GM: 1.3271E+11, 3.9860E+05 km^3/s^2
Atmos refraction: NO (AIRLESS)
RA format : HMS
Time format : CAL
EOP file : eop.220331.p220624
EOP coverage : DATA-BASED 1962-JAN-20 TO 2022-MAR-31. PREDICTS-> 2022-JUN-23
Units conversion: 1 au= 149597870.700 km, c= 299792.458 km/s, 1 day= 86400.0 s
Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (>38.000=NO), Daylight (NO )
Table cut-offs 2: Solar elongation ( 0.0,180.0=NO ),Local Hour Angle( 0.0=NO )
Table cut-offs 3: RA/DEC angular rate ( 0.0=NO )
******************************************************************************************************************************************************************************
Date__(UT)__HR:MN R.A._____(ICRF)_____DEC APmag S-brt delta deldot S-O-T /r S-T-O Sky_motion Sky_mot_PA RelVel-ANG Lun_Sky_Brt sky_SNR
******************************************************************************************************************************************************************************
$$SOE
1920-Jan-01 00:00 02 01 10.30 +14 23 30.8 -11.018 4.765 0.00246765439775 -0.0531506 112.7865 /T 67.0810 35.549496 74.463976 -2.867256 n.a. n.a.
1920-Jan-01 12:00 02 29 55.02 +16 10 10.4 -11.230 4.644 0.00245283756825 -0.0492540 119.4153 /T 60.4603 36.006521 76.307414 -2.638729 n.a. n.a.
1920-Jan-02 00:00 02 59 30.77 +17 43 12.2 -11.436 4.519 0.00243932478970 -0.0441181 126.1296 /T 53.7556 36.436697 78.415421 -2.348077 n.a. n.a.
1920-Jan-02 12:00 03 29 55.17 +19 00 09.0 -11.636 4.390 0.00242747346736 -0.0377550 132.9212 /T 46.9752 36.825767 80.765241 -1.997167 n.a. n.a.
1920-Jan-03 00:00 04 01 02.73 +19 58 44.2 -11.832 4.255 0.00241762969461 -0.0302318 139.7778 /T 40.1312 37.159124 83.322183 -1.590267 n.a. n.a.
$$EOE
******************************************************************************************************************************************************************************
Column meaning:
TIME
Times PRIOR to 1962 are UT1, a mean-solar time closely related to the
prior but now-deprecated GMT. Times AFTER 1962 are in UTC, the current
civil or "wall-clock" time-scale. UTC is kept within 0.9 seconds of UT1
using integer leap-seconds for 1972 and later years.
Conversion from the internal Barycentric Dynamical Time (TDB) of solar
system dynamics to the non-uniform civil UT time-scale requested for output
has not been determined for UTC times after the next July or January 1st.
Therefore, the last known leap-second is used as a constant over future
intervals.
Time tags refer to the UT time-scale conversion from TDB on Earth
regardless of observer location within the solar system, although clock
rates may differ due to the local gravity field and no analog to "UT"
may be defined for that location.
Any 'b' symbol in the 1st-column denotes a B.C. date. First-column blank
(" ") denotes an A.D. date. Calendar dates prior to 1582-Oct-15 are in the
Julian calendar system. Later calendar dates are in the Gregorian system.
NOTE: "n.a." in output means quantity "not available" at the print-time.
'R.A._____(ICRF)_____DEC' =
Astrometric right ascension and declination of the target center with
respect to the observing site (coordinate origin) in the reference frame of
the planetary ephemeris (ICRF). Compensated for down-leg light-time delay
aberration.
Units: RA in hours-minutes-seconds of time, HH MM SS.ff{ffff}
DEC in degrees-minutes-seconds of arc, sDD MN SC.f{ffff}
'APmag S-brt' =
Moon's approximate apparent visual magnitude and surface brightness. When
phase angle < 7 deg (within ~1 day of full Moon), computed magnitude tends to
be about 0.12 too small.
For Earth-based observers, the estimated dimming due to atmospheric
absorption (extinction) is available as a separate, requestable quantity.
Surface brightness is the average airless visual magnitude of a
square-arcsecond of the illuminated portion of the apparent disk.
Units: MAGNITUDES & MAGNITUDES PER SQUARE ARCSECOND
'delta deldot' =
Apparent range ("delta", light-time aberrated) and range-rate ("delta-dot")
of the target center relative to the observer. A positive "deldot" means the
target center is moving away from the observer, negative indicates movement
toward the observer. Units: AU and KM/S
'S-O-T /r' =
Sun-Observer-Target apparent SOLAR ELONGATION ANGLE seen from the observers'
location at print-time.
The '/r' column provides a code indicating the targets' apparent position
relative to the Sun in the observers' sky, as described below:
Case A: For an observing location on the surface of a rotating body, that
body rotational sense is considered:
/T indicates target TRAILS Sun (evening sky: rises and sets AFTER Sun)
/L indicates target LEADS Sun (morning sky: rises and sets BEFORE Sun)
Case B: For an observing point that does not have a rotational model (such
as a spacecraft), the "leading" and "trailing" condition is defined by the
observers' heliocentric ORBITAL motion:
* If continuing in the observers' current direction of heliocentric
motion would encounter the targets' apparent longitude first, followed
by the Sun's, the target LEADS the Sun as seen by the observer.
* If the Sun's apparent longitude would be encountered first, followed
by the targets', the target TRAILS the Sun.
Two other codes can be output:
/* indicates observer is Sun-centered (undefined)
/? Target is aligned with Sun center (no lead or trail)
The S-O-T solar elongation angle is numerically the minimum separation
angle of the Sun and target in the sky in any direction. It does NOT indicate
the amount of separation in the leading or trailing directions, which would
be defined along the equator of a spherical coordinate system.
Units: DEGREES
'S-T-O' =
The Sun-Target-Observer angle; the interior vertex angle at target center
formed by a vector from the target to the apparent center of the Sun (at
reflection time on the target) and the apparent vector from target to the
observer at print-time. Slightly different from true PHASE ANGLE (requestable
separately) at the few arcsecond level in that it includes stellar aberration
on the down-leg from target to observer. Units: DEGREES
'Sky_motion Sky_mot_PA RelVel-ANG' =
Total apparent angular rate of the target in the plane-of-sky. "Sky_mot_PA"
is the position angle of the target's direction of motion in the plane-of-sky,
measured counter-clockwise from the apparent of-date north pole direction.
"RelVel-ANG" is the flight path angle of the target's relative motion with
respect to the observer's line-of-sight, in the range [-90,+90], where positive
values indicate motion away from the observer, negative values are toward the
observer:
-90 = target is moving directly toward the observer
0 = target is moving at right angles to the observer's line-of-sight
+90 = target is moving directly away from the observer
UNITS: ARCSECONDS/MINUTE, DEGREES, DEGREES
'Lun_Sky_Brt sky_SNR' =
Sky brightness due to moonlight scattered by Earth's atmosphere at the
target's position in the sky. "sky_SNR" is the visual signal-to-noise ratio
(SNR) of the target's surface brightness relative to background sky. Output
only for topocentric Earth observers when both the Moon and target are above
the local horizon and the Sun is in astronomical twilight (or further) below
the horizon, and the target is not the Moon or Sun. If all conditions are
not met, "n.a." is output. Galactic brightness, local sky light-pollution
and weather are NOT considered. Lunar opposition surge is considered. The
value returned is accurate under ideal conditions at the approximately 8-23%
level, so is a useful but not definitive value.
If the target-body radius is also known, "sky_SNR" is output. This is the
approximate visual signal-to-noise ratio of the target's brightness divided
by lunar sky brightness. When sky_SNR < 1, the target is dimmer than the
ideal moonlight-scattering background sky, so unlikely to be detectable at
visual wavelengths. In practice, visibility requires sky_SNR > 1 and a
detector sensitive enough to reach the target's magnitude, even if it isn't
washed out by moonlight. When relating magnitudes and brightness values,
keep in mind their logarithmic relationship m2-m1 = -2.5*log_10(b2/b1).
UNITS: VISUAL MAGNITUDES / ARCSECOND^2, and unitless ratio
Computations by ...
Solar System Dynamics Group, Horizons On-Line Ephemeris System
4800 Oak Grove Drive, Jet Propulsion Laboratory
Pasadena, CA 91109 USA
General site: https://ssd.jpl.nasa.gov/
Mailing list: https://ssd.jpl.nasa.gov/email_list.html
System news : https://ssd.jpl.nasa.gov/horizons/news.html
User Guide : https://ssd.jpl.nasa.gov/horizons/manual.html
Connect : browser https://ssd.jpl.nasa.gov/horizons/app.html#/x
API https://ssd-api.jpl.nasa.gov/doc/horizons.html
command-line telnet ssd.jpl.nasa.gov 6775
e-mail/batch https://ssd.jpl.nasa.gov/ftp/ssd/hrzn_batch.txt
scripts https://ssd.jpl.nasa.gov/ftp/ssd/SCRIPTS
Author : Jon.D.Giorgini@jpl.nasa.gov
(Yes, it's verbose, but I always quote it at length because sometimes even the extra information is valuable.)
If I take your script and adjust it so that it uses UT1 rather than UTC, like HORIZONS, and so that it uses the J2000 epoch, then I get near perfect agreement with HORIZONS:
from skyfield.api import load
ts = load.timescale()
planets = load('de421.bsp')
earth=planets['earth']
moon=planets['moon']
epoch=2422324.5 # julian date for Jan 1st 1920 00:00 GMT
t_string = ["1920-01-01 12:00:00+00:00","1920-01-02 00:00:00+00:00","1920-01-02 12:00:00+00:00"]
tlist = [(1920, 1, 1, 12), (1920, 1, 2, 0), (1920, 1, 2, 12)]
for tup in tlist:
t = ts.ut1(*tup)
print(t.utc_jpl())
ra, dec, distance = earth.at(t).observe(moon).apparent().radec() #epoch=2422324.5)
print (ra, dec,"\n")Output:
A.D. 1920-Jan-01 11:59:39.4330 UTC
02h 29m 55.69s +16deg 10' 14.0"
A.D. 1920-Jan-01 23:59:39.4338 UTC
02h 59m 31.59s +17deg 43' 15.8"
A.D. 1920-Jan-02 11:59:39.4347 UTC
03h 29m 56.14s +19deg 00' 12.5" Given that NASA and Skyfield agree, my guess would be that your old 1920 ephemeris might be off by 12 hours? Could you share an image of the page you're looking at?
EHofbeck
JoshPaterson
The American Ephemeris and Nautical Almanac for the year 1920 lists the moon's right ascension and declination as follows (for a few selected dates and times): Jan 1st, 1920 00:00 GMT RA= 2h 25m 31.43s DEC=15 deg 48’ 47.8’’ Jan 1st, 1920 12:00 GMT RA= 2h 55m 2.10s DEC= 17 deg 24’ 3.5’’ Jan 2nd, 1920 00:00 GMT RA= 3h 25m 21.53s DEC= 18 deg 43’ 37.6’’
I’ve attempted to reproduce the almanac data with the following
the output is:
in each case, the output almost exactly reproduces the almanac data, but for 12 hours early than the sought time. What coding changes do I need to make?