  Long-period comet C/2020 S3 (Erasmus) reached perihelion at 0.398 au on UT
2020 December 12.67, making it a bright, near-Sun object. Images taken between
mid-November and December 2020 using the HI-1 camera and COR2 coronagraph
onboard STEREO-A, as well as the LASCO/C3 coronagraph onboard SoHO, show
significant variations in the plasma tail position angles. To analyze these
variations, a simple technique was developed to calculate the aberration
angles. These angles are defined as the angle between the sun-comet line and
the tail axis, measured in the orbital plane. The aberration angles were found
to range from $1.2^\circ$ to $46.8^\circ$, with an average (median) value of
approximately $20.3^\circ$ ($16.3^\circ$). By considering the aberration
angles, the solar wind radial velocities during the observations were inferred
to range from 73.9 km/s to 573.5 km/s, with mean (median) values of
approximately 205.5 km/s (182.3 km/s). Throughout the observations, two periods
were identified where the tails showed forward tilting, which cannot be
explained by aberration alone. In one case, this anomalous position angle was
sustained for at least 11 days and is possibly due to co-rotating interaction
regions. In the other case, the tail exhibited dramatic excursions from
180$^\circ$ to 150$^\circ$ back to 210$^\circ$ over a limited period of around
34 hours. This behavior is tentatively explained as a consequence of the
interaction with a halo Coronal Mass Ejection that was launched from NOAA 12786
and arrived at comet C/2020 S3 during the time when the tail displayed its
wagging behavior.
