C/2021 C1 (Rankin)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Perihelion	5 Dec 2020	20.0	3.473 AU	4.190 AU	13h59m	-24°40'	38.5°	10.2°	276°
Nearest approach	23 Mar 2021	19.2	3.613 AU	2.621 AU	12h33m	-04°42'	173.6°	1.8°	302°
Today	19 Dec 2025	28.5	13.857 AU	13.081 AU	05h55m	+62°43'	140.7°	2.6°	182°

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. (11.5 + 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 C1 (Rankin) are:

    e (Eccentricity)                : 0.9978840
    q (Perihelion distance)         : 3.4731570
    i (Inclination)                 : 143.04400
    Ω (Longitude of ascending node) : 186.80640
    ω (Argument of perihelion)      : 336.03430
    L (Longitude of perihelion)     : 206.36185
    B (Latitude of perihelion)      : -14.13467
    T (Time of perihelion passage)  : 2459188.89240
    P (Orbital period in years)     : 66498.70

    Epoch                           : 2025 Nov 12
    Reference                       : MPEC 2023-KD6

    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
2025-12-18 00:00 UT   05 57 18.0   +62 43 53    13.072   13.847  140.6    2.6   185   28.5 
2025-12-19 00:00 UT   05 56 30.4   +62 43 51    13.077   13.853  140.6    2.6   184   28.5 
2025-12-19 16:39 UT   05 55 57.3   +62 43 47    13.081   13.857  140.7    2.6   182   28.5 
2025-12-20 00:00 UT   05 55 42.8   +62 43 45    13.083   13.858  140.7    2.6   182   28.5 
2025-12-21 00:00 UT   05 54 55.2   +62 43 36    13.088   13.864  140.7    2.6   180   28.5 
2025-12-22 00:00 UT   05 54 07.6   +62 43 22    13.094   13.870  140.7    2.6   178   28.5 
2025-12-23 00:00 UT   05 53 20.2   +62 43 04    13.100   13.875  140.7    2.6   176   28.5 
2025-12-24 00:00 UT   05 52 32.8   +62 42 42    13.106   13.881  140.6    2.6   174   28.5 
2025-12-25 00:00 UT   05 51 45.6   +62 42 16    13.113   13.887  140.5    2.6   172   28.5 
2025-12-26 00:00 UT   05 50 58.5   +62 41 46    13.120   13.892  140.4    2.6   170   28.5 
2025-12-27 00:00 UT   05 50 11.5   +62 41 13    13.127   13.898  140.3    2.6   168   28.5 
2025-12-28 00:00 UT   05 49 24.8   +62 40 35    13.134   13.903  140.1    2.6   167   28.5 
2025-12-29 00:00 UT   05 48 38.2   +62 39 53    13.142   13.909  139.9    2.6   165   28.5 

    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.