C/2021 S1 (ATLAS)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	30 Dec 2021	18.5	6.141 AU	5.301 AU	06h23m	+57°00'	146.1°	5.1°	175°
Perihelion	2 Mar 2022	18.7	6.126 AU	5.774 AU	05h56m	+49°29'	106.3°	8.9°	98°
Today	18 Sep 2025	22.5	10.473 AU	11.251 AU	09h43m	-21°54'	37.6°	3.4°	235°

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.)

The orbital elements of C/2021 S1 (ATLAS) are:

            e (Eccentricity)                : 1.0029440
            q (Perihelion distance)         : 6.1262980
            i (Inclination)                 : 52.03560
            Ω (Longitude of ascending node) : 299.85270
            ω (Argument of perihelion)      : 148.32540
            L (Longitude of perihelion)     : 279.06788
            B (Latitude of perihelion)      : 24.45515
            T (Time of perihelion passage)  : 2459640.58560

            Epoch                           : 2025 Sep 17
            Reference                       : MPEC 2023-J29

            Classification(s):              : Nearly isotropic; New (a > 10000 AU)
        

The light curve chart below shows the estimated development of the comet's magnitude. Blue and black dots are visual and photometric CCD observations respectively from COBS or the MPC. 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. (7.00 + 5 log[∆] + 10.00 log[r]).

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.
A more printer-friendly version of the same chart can be found further down this page as well.

The following chart shows the current location of the comet in a smaller, upside-down telescopic field of view.

A printable version of the short-term path of the comet in a field of view that is optimized for (~10x50) binoculars and finderscopes.

Ephemerides:

Date       Time       RA (2000)    DEC (2000)    delta   radius  elong  phase   PA    magn
2025-09-18 00:00 UT   09 43 22.0   -21 52 56    11.251   10.471   37.5    3.3   234   22.5 
2025-09-18 06:42 UT   09 43 27.9   -21 54 10    11.251   10.473   37.6    3.4   235   22.5 
2025-09-19 00:00 UT   09 43 43.1   -21 57 22    11.251   10.476   37.9    3.4   235   22.5 
2025-09-20 00:00 UT   09 44 04.0   -22 01 50    11.251   10.481   38.3    3.4   237   22.5 
2025-09-21 00:00 UT   09 44 24.8   -22 06 20    11.250   10.486   38.7    3.4   238   22.5 
2025-09-22 00:00 UT   09 44 45.4   -22 10 51    11.250   10.491   39.2    3.5   239   22.5 
2025-09-23 00:00 UT   09 45 05.8   -22 15 23    11.249   10.496   39.6    3.5   240   22.5 
2025-09-24 00:00 UT   09 45 26.0   -22 19 58    11.248   10.501   40.1    3.5   241   22.5 
2025-09-25 00:00 UT   09 45 46.0   -22 24 34    11.247   10.505   40.6    3.6   243   22.5 
2025-09-26 00:00 UT   09 46 05.8   -22 29 11    11.246   10.510   41.0    3.6   244   22.5 
2025-09-27 00:00 UT   09 46 25.4   -22 33 50    11.244   10.515   41.5    3.6   245   22.5 
2025-09-28 00:00 UT   09 46 44.8   -22 38 30    11.242   10.520   42.0    3.7   246   22.5 

    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.