C/2021 T4 (Lemmon)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	20 Jul 2023	8.6	1.489 AU	0.539 AU	19h58m	-56°03'	144.6°	23.3°	359°
Perihelion	31 Jul 2023	9.1	1.481 AU	0.675 AU	16h23m	-43°15'	121.1°	35.9°	86°
Today	24 Oct 2025	22.2	8.374 AU	8.970 AU	11h24m	+23°00'	50.6°	5.3°	309°

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. This lightcurve is being recalculated every 6 hours based on the available COBS/MPC observations (currently 8.2 + 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/2021 T4 (Lemmon) are:

    e (Eccentricity)                : 0.9995910
    q (Perihelion distance)         : 1.4809410
    i (Inclination)                 : 160.77850
    Ω (Longitude of ascending node) : 257.88800
    ω (Argument of perihelion)      : 329.75250
    L (Longitude of perihelion)     : 286.72592
    B (Latitude of perihelion)      : -9.54607
    T (Time of perihelion passage)  : 2460156.97680
    P (Orbital period in years)     : 217882.16

    Epoch                           : 2025 Oct 23
    Reference                       : MPC184409

    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
2025-10-23 00:00 UT   11 24 33.9   +22 59 47     8.977    8.363   49.4    5.2   310   22.2 
2025-10-24 00:00 UT   11 24 47.7   +23 00 01     8.972    8.371   50.2    5.2   310   22.2 
2025-10-24 09:01 UT   11 24 52.8   +23 00 07     8.970    8.374   50.6    5.3   309   22.2 
2025-10-25 00:00 UT   11 25 01.3   +23 00 18     8.966    8.378   51.1    5.3   309   22.2 
2025-10-26 00:00 UT   11 25 14.5   +23 00 38     8.961    8.386   52.0    5.4   309   22.2 
2025-10-27 00:00 UT   11 25 27.5   +23 01 02     8.955    8.394   52.9    5.4   308   22.2 
2025-10-28 00:00 UT   11 25 40.2   +23 01 30     8.950    8.401   53.8    5.5   308   22.2 
2025-10-29 00:00 UT   11 25 52.5   +23 02 00     8.944    8.409   54.7    5.5   307   22.2 
2025-10-30 00:00 UT   11 26 04.6   +23 02 35     8.938    8.416   55.6    5.6   307   22.2 
2025-10-31 00:00 UT   11 26 16.3   +23 03 12     8.931    8.424   56.5    5.6   307   22.2 
2025-11-01 00:00 UT   11 26 27.8   +23 03 53     8.925    8.432   57.4    5.7   306   22.2 
2025-11-02 00:00 UT   11 26 38.9   +23 04 38     8.918    8.439   58.3    5.7   306   22.2 
2025-11-03 00:00 UT   11 26 49.6   +23 05 26     8.912    8.447   59.2    5.8   306   22.2 

    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.