C/2021 D1 (SWAN)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Perihelion	27 Feb 2021	10.8	0.910 AU	1.721 AU	23h24m	+12°47'	23.9°	26.2°	28°
Nearest approach	14 Mar 2021	10.9	0.950 AU	1.680 AU	00h39m	+22°45'	29.4°	31.0°	33°
Today	10 Dec 2025	27.6	14.851 AU	14.931 AU	11h08m	-17°50'	83.4°	3.8°	292°

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 10.0 + 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 D1 (SWAN) are:

    e (Eccentricity)                : 0.9910820
    q (Perihelion distance)         : 0.9096120
    i (Inclination)                 : 31.34370
    Ω (Longitude of ascending node) : 311.13950
    ω (Argument of perihelion)      : 72.50260
    L (Longitude of perihelion)     : 20.87954
    B (Latitude of perihelion)      : 29.74249
    T (Time of perihelion passage)  : 2459272.61070
    P (Orbital period in years)     : 1030.11

    Epoch                           : 2025 Nov 12
    Reference                       : MPEC 2022-F14

    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
2025-12-09 00:00 UT   11 08 43.4   -17 47 05    14.948   14.841   81.9    3.8   291   27.6 
2025-12-10 00:00 UT   11 08 43.6   -17 48 52    14.938   14.847   82.8    3.8   292   27.6 
2025-12-10 15:46 UT   11 08 43.6   -17 50 01    14.931   14.851   83.4    3.8   292   27.6 
2025-12-11 00:00 UT   11 08 43.6   -17 50 37    14.928   14.853   83.7    3.8   292   27.6 
2025-12-12 00:00 UT   11 08 43.3   -17 52 20    14.918   14.859   84.7    3.8   293   27.6 
2025-12-13 00:00 UT   11 08 42.7   -17 54 03    14.907   14.865   85.6    3.8   293   27.6 
2025-12-14 00:00 UT   11 08 41.9   -17 55 43    14.897   14.870   86.5    3.8   293   27.6 
2025-12-15 00:00 UT   11 08 40.8   -17 57 22    14.887   14.876   87.5    3.8   294   27.6 
2025-12-16 00:00 UT   11 08 39.4   -17 59 00    14.876   14.882   88.4    3.8   294   27.6 
2025-12-17 00:00 UT   11 08 37.7   -18 00 36    14.866   14.888   89.4    3.8   294   27.6 
2025-12-18 00:00 UT   11 08 35.8   -18 02 10    14.856   14.894   90.3    3.8   295   27.6 
2025-12-19 00:00 UT   11 08 33.6   -18 03 43    14.845   14.900   91.2    3.8   295   27.6 
2025-12-20 00:00 UT   11 08 31.1   -18 05 14    14.835   14.906   92.2    3.8   296   27.6 

    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.