C/2021 O1 (Nishimura)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	6 Aug 2021	9.4	0.799 AU	1.687 AU	08h11m	+30°57'	19.0°	24.4°	316°
Perihelion	14 Aug 2021	9.3	0.783 AU	1.701 AU	09h07m	+28°54'	16.4°	21.4°	333°
Today	24 Dec 2025	26.7	14.261 AU	15.182 AU	17h39m	-42°00'	19.8°	1.3°	203°

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 9.2 + 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 O1 (Nishimura) are:

    e (Eccentricity)                : 0.9998840
    q (Perihelion distance)         : 0.7827140
    i (Inclination)                 : 27.55740
    Ω (Longitude of ascending node) : 41.51200
    ω (Argument of perihelion)      : 74.45830
    L (Longitude of perihelion)     : 114.09545
    B (Latitude of perihelion)      : 26.46948
    T (Time of perihelion passage)  : 2459441.44670
    P (Orbital period in years)     : 554265.27

    Epoch                           : 2025 Nov 12
    Reference                       : MPEC 2022-A21

    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
2025-12-23 00:00 UT   17 38 37.6   -42 00 02    15.174   14.250   19.3    1.3   198   26.7 
2025-12-24 00:00 UT   17 39 00.4   -42 00 29    15.179   14.256   19.6    1.3   201   26.7 
2025-12-24 18:17 UT   17 39 17.8   -42 00 49    15.182   14.261   19.8    1.3   203   26.7 
2025-12-25 00:00 UT   17 39 23.2   -42 00 56    15.183   14.263   19.9    1.3   203   26.7 
2025-12-26 00:00 UT   17 39 45.9   -42 01 24    15.187   14.269   20.3    1.4   206   26.7 
2025-12-27 00:00 UT   17 40 08.5   -42 01 54    15.191   14.275   20.7    1.4   208   26.7 
2025-12-28 00:00 UT   17 40 31.1   -42 02 24    15.194   14.281   21.1    1.4   211   26.7 
2025-12-29 00:00 UT   17 40 53.6   -42 02 55    15.197   14.288   21.5    1.4   213   26.7 
2025-12-30 00:00 UT   17 41 16.0   -42 03 28    15.200   14.294   22.0    1.5   215   26.7 
2025-12-31 00:00 UT   17 41 38.4   -42 04 01    15.203   14.300   22.5    1.5   217   26.7 
2026-01-01 00:00 UT   17 42 00.6   -42 04 35    15.205   14.306   23.1    1.5   219   26.7 
2026-01-02 00:00 UT   17 42 22.8   -42 05 11    15.208   14.313   23.6    1.6   221   26.7 
2026-01-03 00:00 UT   17 42 44.8   -42 05 47    15.210   14.319   24.2    1.6   223   26.7 

    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.