Is possible to identify the Lagrange L2 point in the sky from the earth?

Question

I'm reading now that the James Webb observatory will be at L2 point. Is possible in any way to look at this point from here in earth?

I mean... given my LAT and LON here what is the best time to have a sight line? Is near what constelation?

I read it is at "straight behind earth from sun point of view" but I have no idea what it means from my rotating POV.

If you can give me an example, I'm at -22.8885366,-43.104668.

Thanks.

If it can help (https://www.spaceweather.com/):

... so I tried the site and ...

*******************************************************************************
 Revised: Aug 29, 2013                SEMB-L2                                32
                      (Sun & Earth-Moon Barycenter Lagrange 2)
#1) The Sun & Earth-Moon Barycenter Lagrange-2 (L2) point is a location 
       where the Earth's gravitational field partially counters that of the 
       Sun.
#2) This L2 point is about 1.5 million km (~900,000 miles) away from the 
       Earth, opposite the direction of the Sun, or slightly less than one 
       percent of the Earth-Sun distance (four times the distance from Earth 
       to the Moon)
#3) L2 has been selected as the location of the next generation James Webb 
       Space Telescope, was used by the Genesis spacecraft on the return to 
       Earth, and is (or will be) used for WMAP, Herschel, and Planck 
       spacecraft.
Time-span available: 1900-Jan-01 to 2151-Jan-01


Ephemeris / WWW_USER Wed Dec 29 18:42:20 2021 Pasadena, USA      / Horizons

Target body name: SEMB-L2 (32)                    {source: Sun-EMB_L2}
Center body name: Earth (399)                     {source: DE441}
Center-site name: (user defined site below)

Start time      : A.D. 2021-Dec-30 00:00:00.0000 UT

Stop  time      : A.D. 2021-Dec-31 00:00:00.0000 UT

Step-size       : 1440 minutes

Target pole/equ : No model available
Target radii    : (unavailable)

Center geodetic : 316.895332,-22.888537,0.0100000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 316.895332,5878.92988,-2465.356 {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  : (Undefined)
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.211228.p220323

EOP coverage    : DATA-BASED 1962-JAN-20 TO 2021-DEC-28. PREDICTS-> 2022-MAR-22
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

$$SOE
 2021-Dec-30 00:00     06 36 55.95 +23 19 55.7     n.a.    n.a.  0.00987312593538  -0.2824505  179.6621 /L    0.3347   0.8202458   352.89131   -70.88475         n.a.     n.a.
 2021-Dec-31 00:00     06 41 13.66 +23 15 44.7     n.a.    n.a.  0.00986809386682  -0.2810705  179.6899 /L    0.3071   0.8042847   351.62780   -71.15373         n.a.     n.a.
$$EOE

... but I have no idea how to read these informations ...

I think the answer is here:

$$SOE
 2021-Dec-30 00:00     06 36 55.95 +23 19 55.7     n.a.    n.a.  0.00987312593538  -0.2824505  179.6621 /L    0.3347   0.8202458   352.89131   -70.88475         n.a.     n.a.
 2021-Dec-31 00:00     06 41 13.66 +23 15 44.7     n.a.    n.a.  0.00986809386682  -0.2810705  179.6899 /L    0.3071   0.8042847   351.62780   -71.15373         n.a.     n.a.
$$EOE

and I need to take R.A. and DEC. May I use this post ( from myself ) to "project" the L2 on earth surface...

Can someone correct me? What is "RA" and "DEC" from the data above?

You do realize it’s an immaterial point, right? There’s nothing there to look at… — Pierre Paquette, Dec 30 '21 at 22:35
@PierrePaquette there is, in fact, something to look at: https://en.wikipedia.org/wiki/Gegenschein — fraxinus, Dec 31 '21 at 07:18
@fraxinus: It is not located at the L2 point, however; only in the direction of the L2 point… And very faint, to add… — Pierre Paquette, Dec 31 '21 at 07:26
It is the direction that is important. The sky is 2D. On the other hand, the Gegenschein is pretty much related to the L2 point properties. — fraxinus, Dec 31 '21 at 09:36
@TonyK wait, is that right? With Earth's 23° tilt and the Sun being different places in the summer and winter... — uhoh, Dec 31 '21 at 18:48
@uhoh: the tilt of the earth doesn't matter. The eccentricity of the earth's orbit does matter, but it is small enough to be ignored here. — TonyK, Dec 31 '21 at 19:46
@TonyK If it's December in the southern hemisphere at 45° S latitude then at local noon the Sun will be at 68° above the horizon. Twelve hours later the L2 point will be at... Oh I see :-) not 68° but 22° Yes, got it! — uhoh, Dec 31 '21 at 19:49
@PierrePaquette yes bro. I may say look to that direction. If I found something in the sight line, good. — Magno C, Jan 05 '22 at 11:04
WOW guys!! Thanks so much for so many answers and comments... the folks here are the best! — Magno C, Jan 05 '22 at 11:15
As a layman I left here a comment to you that came here for the same question to consider read ALL these answers and comments. All of them are very useful and contains valuable informations. — Magno C, Jan 05 '22 at 11:34

score 12 · Answer 1 · answered Dec 31 '21 at 00:06

Stargazer's rough, quick method:

Look at the sky at local solar midnight, halfway between sunset and sunrise.

Locate the ecliptic. If you're a stargazer, you should know where that is: it's the line the zodiac constellations are strung along.

Visualize the local meridian: it's just the line running north-south through the zenith.

L2 is approximately where the meridian intercepts the ecliptic at local solar midnight. The direction opposite the sun.

PM 2Ring · Answer 2 · 2021-12-31T08:00:15.343

To get the L2 position from Horizons, you need to ask it for "Apparent AZ & EL", which is quantity 4 in the Observer table custom settings.

When setting the time span, you need to use UTC time, which is 3 hours ahead of Rio time.

Here's some hourly data for 2021-Dec-30 at the Rio de Janeiro Observatory, from 21:00 UTC to 9:00 UTC, 6 PM to 6 AM Rio time.

*******************************************************************************
Ephemeris / WWW_USER Wed Dec 29 20:44:55 2021 Pasadena, USA      / Horizons    
*******************************************************************************
Target body name: SEMB-L2 (32)                    {source: Sun-EMB_L2}
Center body name: Earth (399)                     {source: DE441}
Center-site name: Rio de Janeiro
*******************************************************************************
Start time      : A.D. 2021-Dec-30 21:00:00.0000 UT      
Stop  time      : A.D. 2021-Dec-31 09:00:00.0000 UT      
Step-size       : 60 minutes
*******************************************************************************
Target pole/equ : No model available
Target radii    : (unavailable)                                                
Center geodetic : 316.777100,-22.895181,0.0415471 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 316.777100,5878.67273,-2466.046 {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  : (Undefined)
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: YES (Earth refraction model)
RA format       : HMS
Time format     : BOTH
EOP file        : eop.211228.p220323                                           
EOP coverage    : DATA-BASED 1962-JAN-20 TO 2021-DEC-28. PREDICTS-> 2022-MAR-22
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 Date_________JDUT     Azi____(r-app)___Elev     S-T-O
************************************************************************
$$SOE
 2021-Dec-30 21:00 2459579.375000000 *    68.226098  -7.556782    0.3538
 2021-Dec-30 22:00 2459579.416666667 C    62.535351   4.538617    0.3504
 2021-Dec-30 23:00 2459579.458333333 A    55.344521  16.272960    0.3342
 2021-Dec-31 00:00 2459579.500000000      45.963828  26.973052    0.3073
 2021-Dec-31 01:00 2459579.541666667      33.537379  35.855740    0.2728
 2021-Dec-31 02:00 2459579.583333333      17.511014  41.867692    0.2349
 2021-Dec-31 03:00 2459579.625000000     358.801758  43.856383    0.1992
 2021-Dec-31 04:00 2459579.666666667     340.297002  41.321775    0.1717
 2021-Dec-31 05:00 2459579.708333333     324.707466  34.889141    0.1568
 2021-Dec-31 06:00 2459579.750000000  m  312.699873  25.730820    0.1538
 2021-Dec-31 07:00 2459579.791666667 Am  303.631549  14.862374    0.1562
 2021-Dec-31 08:00 2459579.833333333 Cm  296.654543   3.072328    0.1566
 2021-Dec-31 09:00 2459579.875000000 *m  291.106149  -9.148816    0.1495
$$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.
SOLAR PRESENCE (OBSERVING SITE)
  Time tag is followed by a blank, then a solar-presence symbol:
   '*'  Daylight (refracted solar upper-limb on or above apparent horizon)
   'C'  Civil twilight/dawn
   'N'  Nautical twilight/dawn
   'A'  Astronomical twilight/dawn
   ' '  Night OR geocentric ephemeris


LUNAR PRESENCE (OBSERVING SITE)
  The solar-presence symbol is immediately followed by a lunar-presence symbol:
   'm'  Refracted upper-limb of Moon on or above apparent horizon
   ' '  Refracted upper-limb of Moon below apparent horizon OR geocentric
        ephemeris


'Azi____(r-app)___Elev' =
  Refracted apparent azimuth and elevation of target center. Compensated
for light-time, the gravitational deflection of light, stellar aberration,
approximate atmospheric yellow-light refraction, precession and nutation.
Azimuth is measured clockwise from north:
North(0) -> East(90) -> South(180) -> West(270) -> North (360)
Elevation angle is with respect to a plane perpendicular to the reference
surface local zenith direction. TOPOCENTRIC ONLY.  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
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

So on that night the L2 is at its highest elevation around midnight (3:00 UTC), when it's 43.856383° above the horizon. Its almost exactly due North at that moment, with an azimuth of 358.801758°.

FWIW, here's the batch-file for that query:

!$$SOF
MAKE_EPHEM=YES
COMMAND=32
EPHEM_TYPE=OBSERVER
CENTER='880@399'
START_TIME='2021-12-30 21:00'
STOP_TIME='2021-12-31 9:00'
STEP_SIZE='1 HOURS'
QUANTITIES='4,24'
REF_SYSTEM='ICRF'
CAL_FORMAT='BOTH'
TIME_DIGITS='MINUTES'
ANG_FORMAT='HMS'
APPARENT='REFRACTED'
RANGE_UNITS='AU'
SUPPRESS_RANGE_RATE='NO'
SKIP_DAYLT='NO'
SOLAR_ELONG='0,180'
EXTRA_PREC='NO'
RTS_ONLY='NO'
CSV_FORMAT='NO'
OBJ_DATA='YES'

And you can run the query using this Sage / Python script

this is a complete answer. it would be good for me, because AZ/EL is the easiest way to get a rough sense of where to look in the sky at a given time. but it appears the OP updated their question to include the constellation, and posted an answer to their own question discussing RA/DEC, so i added a separate answer focusing on those. — giardia, Dec 30 '21 at 07:37
The example of using the Sun at 10 am is accurate only at the equinoxe when the Sun's declination is near 0. At other times of the year, the L2 point could be up tp 56 degrees away from where the Sun was since the declination of L2 is opposite of the Sun! — JohnHoltz, Dec 30 '21 at 13:42
Also, the JWT won't settle into an orbit around L2 until late next month: its taking a slight arc to get there. So pointing at L2 now might miss it significantly. When it gets there, it will orbit the L2 point at a distance, but I don't know if the distance will be significant or not to your telescope. (See https://youtu.be/6cUe4oMk69E for a depiction.) — kjpires, Dec 30 '21 at 16:26
@kjpires Sure, but the OP is specifically asking how to locate the L2 point itself, not the JWST. And of course, it will be rather hard to see the JWST from Rio de Janeiro. ;) — PM 2Ring, Dec 30 '21 at 19:07
This is blatantly false. As JohnHoltz notes above, the declination of L2 is (almost) opposite of the Sun. Almost, because there are minor differences stemming from the fact that L2 is significantly closer to the Earth than the Sun is, and because the Earth does not rotate around its own 'center', but the barycenter of the Earth-Moon system. See giardia's answer. — , Dec 30 '21 at 21:05
@JohnHoltz as the Earth's orbit around the Sun is not circular, the Lagrange points don't even exist; their definition requires a circular orbit. So anything written about Sun-Earth L2 is "blatantly false" in some sense. — uhoh, Dec 30 '21 at 21:42
@user44842 The Earth does rotate around its centre, with a period of 1 sidereal day. The Earth & Moon also revolve around the Earth-Moon barycentre, with a mean sidereal period of ~27.32166 days. — PM 2Ring, Jan 01 '22 at 01:47
Seems perfect to me. But I'm not a pro and I can't evaluate all these coments to know if it is 100% correct. — Magno C, Jan 05 '22 at 11:54
@MagnoC The critical comments here refer to an earlier version of my answer which did have some flaws, but I've removed those parts. You can see the earlier versions in the edit history. giardia's answer is fine, I upvoted it myself. — PM 2Ring, Jan 05 '22 at 12:30
@MagnoC BTW, in a similar vein to TonyK's comment on your question, the L2 point is fairly close to the Moon when it's a Full Moon. There's a small difference of up to 5° because the Moon's orbit is tilted by ~5° to the ecliptic. — PM 2Ring, Jan 05 '22 at 12:37
@PM2Ring great to know! But this may be a problem. The moon can be a lighthouse. Anyway... at least I know where to look at. — Magno C, Jan 05 '22 at 13:20
@giardia actually I don't care very much about constellations. Its more because I'm not a professional and often got myself lost in the sky. Constellations are a good way to me to take some reference. But PM 2Ring gave me the moon ( not in THAT sense! ). — Magno C, Jan 05 '22 at 13:26
@MagnoC Yes, the Full Moon's just a quick way to roughly locate L2. — PM 2Ring, Jan 05 '22 at 13:38

score 7 · Accepted Answer · answered Dec 30 '21 at 07:33

7

Running the JPL Horizons calculation as in the original question gives the RA and DEC of the L2 point:

$$SOE
 2021-Dec-30 00:00     06 36 55.95 +23 19 55.7     n.a.    n.a.  0.00987312593538  -0.2824505  179.6621 /L    0.3347   0.8202458   352.89131   -70.88475         n.a.     n.a.
 2021-Dec-31 00:00     06 41 13.66 +23 15 44.7     n.a.    n.a.  0.00986809386682  -0.2810705  179.6899 /L    0.3071   0.8042847   351.62780   -71.15373         n.a.     n.a.
$$EOE

The RA is the right ascension, the longitude on the celestial sphere divided into 24 hours. So the L2 RA is 06:41:14 for the Dec 31 date. This makes sense because it should be directly opposite the Sun on the celestial sphere, and the Sun (according to a separate Horizons calculation) is 18:40:05 at that time (about +12h or 180° away from L2 in the sky). It's not exactly 12h, but that could be due to Earth's motion around the Earth-Moon barycenter.

The DEC is the declination, the latitude on the celestial sphere. Again, L2 is opposite the Sun on the celestial sphere, so the DEC of +23:19:56 for L2 is on the other side of the celestial equator from the Sun. Horizons says the Sun is at DEC -23:07:01 for the specified location and date. A back of the envelope calculation shows that a few arcmin difference is due to how close L2 is to the Earth, compared with the celestial sphere which is infinitely distant.

Consulting a sky map shows that RA=06:36:56 DEC=+23:19:56 is in Gemini. Horizons also has an output field you can select for "Constellation ID," which says L2 is in Gemini.

answered Dec 30 '21 at 07:33

giardia

2,088
3
17

1

that could be due to Earth's motion around the Earth-Moon barycenter. Correct. My query also requests the Sun-Target-Observer angle, which is <0.5°. Of course, when the JWST is near L2 it will also be perturbed by the Moon. – PM 2Ring Dec 30 '21 at 08:15
1

But as seen from Earth, L2 does a comlete circuit of the ecliptic every year. So it won't be in Gemini for long. In fact, it will be in the sign of the Zodiac (or the house that will be current in six months' time; this implies that it has already left the house of Gemini, and is now (as of 22nd December) in the house of Cancer. (Houses are not the same as constellations, though, so you may be right about Gemini.) – TonyK Dec 31 '21 at 12:17
@TonyK - Yes, all true. The OP's Horizons query used a very short date range, so Gemini is the answer for that range. The RA/DEC for L2 in this date range is securely in Gemini based on a star map I found on the Internet. The constellations are more gerrymandered than the "houses," but that is probably a topic for astrology.stackexchange.com, not here. =) – giardia Dec 31 '21 at 18:53
@giardia this date range I gave is because I think ( I'm not a pro ) this is the best way to know an instant position, not a future position. But this may be not important anyway. By "position" I mean "direction to look at" ok ? – Magno C Jan 05 '22 at 11:18
@MagnoC - Yes, that works. So did the RA/DEC + constellation name work for you? RA/DEC will change throughout the year as the Earth goes around the Sun. – giardia Jan 05 '22 at 16:51
@giardia didn't work on this yet. My boss throws me another "do-it-now" task ... Tell you ASAP. – Magno C Jan 06 '22 at 11:42

Is possible to identify the Lagrange L2 point in the sky from the earth?

3 Answers3