C/2021 D1 (SWAN)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Perihelion	26 Feb 2021	10.8	0.912 AU	1.720 AU	23h24m	+12°44'	24.1°	26.4°	29°
Nearest approach	14 Mar 2021	10.9	0.952 AU	1.679 AU	00h38m	+22°44'	29.7°	31.1°	33°
Today	6 Sep 2026	28.4	16.398 AU	17.324 AU	10h58m	-16°08'	22.5°	1.3°	181°

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)                : 0.9910430
    q (Perihelion distance)         : 0.9115530
    i (Inclination)                 : 31.31780
    Ω (Longitude of ascending node) : 311.05200
    ω (Argument of perihelion)      : 72.61530
    L (Longitude of perihelion)     : 20.92484
    B (Latitude of perihelion)      : 29.73841
    T (Time of perihelion passage)  : 2459272.17050
    P (Orbital period in years)     : 1026.67

    Epoch                           : 2026 May 29
    Reference                       : MPC194173

    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
2026-09-05 00:00 UT   10 57 60.0   -16 05 35    17.309   16.387   23.1    1.4   177   28.3 
2026-09-06 00:00 UT   10 58 13.8   -16 07 01    17.317   16.393   22.8    1.4   180   28.3 
2026-09-06 21:07 UT   10 58 25.9   -16 08 18    17.324   16.398   22.5    1.3   181   28.4 
2026-09-07 00:00 UT   10 58 27.6   -16 08 28    17.325   16.399   22.4    1.3   182   28.4 
2026-09-08 00:00 UT   10 58 41.4   -16 09 57    17.333   16.404   22.1    1.3   184   28.4 
2026-09-09 00:00 UT   10 58 55.2   -16 11 27    17.340   16.410   21.8    1.3   186   28.4 
2026-09-10 00:00 UT   10 59 09.0   -16 12 57    17.347   16.415   21.6    1.3   189   28.4 
2026-09-11 00:00 UT   10 59 22.7   -16 14 29    17.354   16.421   21.4    1.3   191   28.4 
2026-09-12 00:00 UT   10 59 36.5   -16 16 03    17.360   16.426   21.2    1.3   194   28.4 
2026-09-13 00:00 UT   10 59 50.2   -16 17 37    17.366   16.432   21.1    1.3   196   28.4 
2026-09-14 00:00 UT   11 00 03.8   -16 19 12    17.372   16.438   21.0    1.3   199   28.4 
2026-09-15 00:00 UT   11 00 17.5   -16 20 48    17.378   16.443   20.9    1.3   201   28.4 
2026-09-16 00:00 UT   11 00 31.1   -16 22 26    17.384   16.449   20.9    1.2   204   28.4 

    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.