C/2019 S4 (Lemmon)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	24 Nov 2019	19.0	3.660 AU	3.010 AU	01h37m	+72°24'	124.4°	12.9°	139°
Perihelion	9 Apr 2020	19.3	3.438 AU	3.820 AU	23h40m	+66°04'	60.4°	14.7°	333°
Today	13 Dec 2025	28.8	15.054 AU	15.957 AU	15h54m	-34°44'	22.6°	1.4°	245°

The interactive orbit chart above shows the comet's path through the solar system and its position at the given date. Green and blue lines are shown perpendicular to the ecliptic plane: Green if the path is above the ecliptic plane, blue if it is below. (Left-click and drag to rotate the view; Right-click and drag to move the view; Use scroll wheel to zoom in our out.)

Light curve

The light curve chart below shows the estimated development of the comet's magnitude. Blue and black dots are visual and photometric CCD observations from COBS. The light curve is based on the absolute magnitude and slope parameter as calculated from an MPEC, or the latest values provided by the minor planet center. (11.0 + 5 log[∆] + 10.0 log[r]). An additional green curve is displayed when an increase in apparent brightness is expected due to forward scattering of sunlight, which occurs when a dust-rich comet is located between the Earth and the Sun. (See Marcus 2007)

Charts

The all-sky chart below shows the path of the comet over the same period as the light curve. The comet's current position is marked yellow.

The following chart shows the short-term path of the comet in a field of view that is optimized for (~10x50) binoculars and finderscopes.

Orbital elements

The orbital elements of C/2019 S4 (Lemmon) are:

    e (Eccentricity)                : 0.9863020
    q (Perihelion distance)         : 3.4381720
    i (Inclination)                 : 92.08380
    Ω (Longitude of ascending node) : 63.15290
    ω (Argument of perihelion)      : 71.36810
    L (Longitude of perihelion)     : 56.99755
    B (Latitude of perihelion)      : 71.25605
    T (Time of perihelion passage)  : 2458949.18690
    P (Orbital period in years)     : 3976.54

    Epoch                           : 2025 Nov 12
    Reference                       : MPEC 2021-S45

    Classification(s):              : Nearly isotropic; Returning (a < 10000 AU); External (P > 200 years)

Ephemerides

Date       Time       RA (2000)    DEC (2000)    delta   radius  elong  phase   PA    magn
2025-12-12 00:00 UT   15 53 49.8   -34 42 03    15.957   15.045   21.3    1.4   242   28.8 
2025-12-13 00:00 UT   15 54 05.8   -34 43 43    15.957   15.050   22.1    1.4   244   28.8 
2025-12-13 18:20 UT   15 54 17.9   -34 44 60    15.957   15.054   22.6    1.4   245   28.8 
2025-12-14 00:00 UT   15 54 21.6   -34 45 24    15.958   15.055   22.8    1.5   246   28.8 
2025-12-15 00:00 UT   15 54 37.3   -34 47 05    15.957   15.061   23.6    1.5   247   28.8 
2025-12-16 00:00 UT   15 54 53.0   -34 48 46    15.957   15.066   24.3    1.5   248   28.8 
2025-12-17 00:00 UT   15 55 08.5   -34 50 29    15.956   15.072   25.1    1.6   250   28.8 
2025-12-18 00:00 UT   15 55 23.9   -34 52 11    15.955   15.077   25.9    1.6   251   28.8 
2025-12-19 00:00 UT   15 55 39.2   -34 53 55    15.954   15.082   26.7    1.7   252   28.8 
2025-12-20 00:00 UT   15 55 54.3   -34 55 38    15.953   15.088   27.5    1.7   253   28.8 
2025-12-21 00:00 UT   15 56 09.4   -34 57 22    15.951   15.093   28.4    1.8   254   28.8 
2025-12-22 00:00 UT   15 56 24.3   -34 59 07    15.949   15.099   29.2    1.8   255   28.8 
2025-12-23 00:00 UT   15 56 39.0   -35 00 51    15.947   15.104   30.0    1.9   256   28.8 

    Terminology:
            
    delta:  distance between comet and earth in AU
    radius: distance between comet and sun in AU
    magn:   magnitude (brightness) estimate    
    ra:     right ascension in hours (24h = 360deg)
    dec:    declination in degrees
    elong:  elongation in degrees (angle sun-earth-comet)    
    phase:  phase angle in degrees (angle sun-comet-earth)        
    AU:     Astronomical Unit (mean distance between earth and sun: 149597870.7 km      
    

Orbital elements usually provided by the MPC (Minor Planet Center).
Observations contributed by observers worldwide, via COBS (Comet Observation Database) or the MPC (Minor Planet Center)
Calculations by a modified version of AAPlus, a C# implementation of the AA+ project by PJ Naughter from the algorithms presented in the book "Astronomical Algorithms" by Jean Meeus.
NGC2000 dso catalog and star labels from VizieR as provided by the Strasbourg astronomical Data Center.
Tycho2 catalog from the ESO archive.