C/2021 U4 (Leonard)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	1 Nov 2021	19.7	1.900 AU	0.959 AU	02h37m	-11°43'	153.7°	13.4°	353°
Perihelion	22 Dec 2021	21.0	1.786 AU	1.943 AU	23h01m	-28°42'	66.0°	30.2°	75°
Today	18 Dec 2025	33.4	12.126 AU	12.911 AU	15h21m	-11°50'	35.8°	2.7°	294°

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. (17.0 + 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/2021 U4 (Leonard) are:

    e (Eccentricity)                : 0.9607810
    q (Perihelion distance)         : 1.7860680
    i (Inclination)                 : 152.87610
    Ω (Longitude of ascending node) : 241.80210
    ω (Argument of perihelion)      : 237.30730
    L (Longitude of perihelion)     : 187.59804
    B (Latitude of perihelion)      : -22.56242
    T (Time of perihelion passage)  : 2459571.27730
    P (Orbital period in years)     : 307.33

    Epoch                           : 2025 Nov 12
    Reference                       : MPEC 2021-W31

    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
2025-12-17 00:00 UT   15 20 59.9   -11 50 39    12.916   12.118   34.5    2.6   294   33.4 
2025-12-18 00:00 UT   15 21 12.9   -11 50 50    12.912   12.124   35.4    2.7   294   33.4 
2025-12-18 08:07 UT   15 21 17.3   -11 50 54    12.911   12.126   35.8    2.7   294   33.4 
2025-12-19 00:00 UT   15 21 25.7   -11 51 01    12.908   12.130   36.4    2.8   294   33.4 
2025-12-20 00:00 UT   15 21 38.4   -11 51 10    12.903   12.136   37.3    2.8   293   33.4 
2025-12-21 00:00 UT   15 21 50.9   -11 51 19    12.899   12.142   38.3    2.9   293   33.4 
2025-12-22 00:00 UT   15 22 03.1   -11 51 26    12.894   12.148   39.2    2.9   293   33.4 
2025-12-23 00:00 UT   15 22 15.3   -11 51 31    12.888   12.154   40.2    3.0   292   33.4 
2025-12-24 00:00 UT   15 22 27.2   -11 51 36    12.883   12.160   41.2    3.1   292   33.4 
2025-12-25 00:00 UT   15 22 38.9   -11 51 39    12.877   12.166   42.1    3.1   292   33.4 
2025-12-26 00:00 UT   15 22 50.4   -11 51 41    12.871   12.172   43.1    3.2   292   33.4 
2025-12-27 00:00 UT   15 23 01.7   -11 51 42    12.865   12.178   44.0    3.2   291   33.4 
2025-12-28 00:00 UT   15 23 12.8   -11 51 41    12.859   12.184   45.0    3.3   291   33.4 

    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.