In the SOHO images for today, there was a white dot at the bottom right of the image. There is also a strange white line parallel to the top and bottom of the image that crosses most of the image. From my amount of research, I've assumed that the dot is either Mercury or Venus, most likely the latter though. But what about the line across tbe image that crosses the planet? Did I even guess right that the dot is a planet

Here's the image: The image The line seems to get thicker around the dot, and seems to be coming from the planet, but thst doesn't make sense. Also, none of the other images have the dot or line, as they are zoomed in too far.

  • 1
    $\begingroup$ Double check other images to see if it seems to be one-of-a-kind, or rare. Here is what bright things can look like for example. The thin horizontal line might be just a typical CCD saturation artifact There are also plenty of artifacts when cosmic rays collide in or near the silicon CCD $\endgroup$
    – uhoh
    May 27, 2016 at 5:24
  • 1
    $\begingroup$ The line does appear to be more like a digital glitch, probably due to the other bright object. $\endgroup$ May 27, 2016 at 15:59

2 Answers 2


It might be Mercury - thanks for the tags.

It might be...

It is Venus - thanks for the tags. :)

In the mean time enjoy seeing the Pleiades pass closer to the sun than you'd normally see them!

These LASCO C3 images from SOHO were downloaded sohodata.nascom.nasa.gov/cgi-bin/data_query. The square frame is about 15.9 degrees wide.

enter image description here

I like to use the NIH free software ImageJ for image manipulation, including making GIFs.

note: SOHO is in orbit around Sun-Earth L1, which means it's within about a half-million km near the Sun-Earth line, but about 1.5 million km closer. While Mercury's apparent position relative to the sun will be slightly different as seen from SOHO compared to Earth, it's a small effect.

enter image description here

Apparent angular separation of Mercury and Venus from the Sun for the month of May 2016.

Coordinates for SOHO are from JPL's Horizons web interface at ssd.jpl.nasa.gov/horizons.cgi. I pasted the coordinates into this Python script and used Skyfield to get the positions of the sun and planets.

def angsep(a, b):

    top = (a*b).sum(axis=0)
    bot = np.sqrt( (a**2).sum(axis=0) * (b**2).sum(axis=0) )

    return np.arccos(top/bot)

import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import load
from skyfield.positionlib import ICRF

data = load('de421.bsp')

sun     = data['sun']
mercury = data['mercury']
venus   = data['venus']
earth   = data['earth']

days = range(1, 33)
ts   = load.timescale()
t    = ts.utc(2016, 5, days, 0, 0, 0)

# SOHO_pos downloaded from http://ssd.jpl.nasa.gov/horizons.cgi

sun_pos     = sun.at(t).ecliptic_position().km
mercury_pos = mercury.at(t).ecliptic_position().km
venus_pos   = venus.at(t).ecliptic_position().km
earth_pos   = earth.at(t).ecliptic_position().km

r_earth_sun     = sun_pos     - earth_pos
r_earth_mercury = mercury_pos - earth_pos
r_earth_venus   = venus_pos   - earth_pos

r_SOHO_sun      = sun_pos     - SOHO_pos
r_SOHO_mercury  = mercury_pos - SOHO_pos
r_SOHO_venus    = venus_pos   - SOHO_pos

mercury_sep_from_earth = angsep(r_earth_mercury, r_earth_sun)
venus_sep_from_earth   = angsep(r_earth_venus,   r_earth_sun)

mercury_sep_from_SOHO  = angsep(r_SOHO_mercury,  r_SOHO_sun)
venus_sep_from_SOHO    = angsep(r_SOHO_venus,    r_SOHO_sun)

degs = 180. / np.pi


plt.subplot(1, 2, 1)
plt.plot(days, degs*mercury_sep_from_earth, '--g')
plt.plot(days, degs*venus_sep_from_earth, '-b')
plt.plot(days[25:26], degs*venus_sep_from_earth[25:26], 'ok')
plt.text(22, 5, 'Venus', fontsize=16)
plt.text(22, 15, 'Mercury', fontsize=16)
plt.title('Seen from Earth')

plt.subplot(1, 2, 2)
plt.plot(days, degs*mercury_sep_from_SOHO, '--g')
plt.plot(days, degs*venus_sep_from_SOHO, '-b')
plt.plot(days[25:26], degs*venus_sep_from_SOHO[25:26], 'ok')
plt.text(22, 5, 'Venus', fontsize=16)
plt.text(22, 15, 'Mercury', fontsize=16)
plt.title('Seen from SOHO')


Here is another image from sungrazer.nrl.navy.mil/index.php?p=transits/transits that sheds some light on the subject. It was taken in May, 2000.

enter image description here

And this one is from here:

enter image description here

  • $\begingroup$ I've asked a related question - how to get SOHO's ICRF coordinates so we can do this correctly! $\endgroup$
    – uhoh
    May 27, 2016 at 5:48

The white line is known as a "saturation trail" and is definitely a CCD artifact.

Each pixel can store only a certain amount of electrons (of the order of 100,000). If a pixel is illuminated by a bright star and/or if the exposure time is long enough, that pixel will fill up, and the electrons will start to fill the neighboring pixels: the CCD is saturated. When the image is read, all the extra electrons will be spread over the column containing the saturated pixels, making a saturation trail

(Source: Oliver Hainaut at the European Southern Observatory)


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