Comet ISON at Perihelion

Astronomers all over the world are training their eyes and telescopes on Comet ISON as it approaches its closest distance to the sun, called perihelion.

At noon on Wednesday November 27, Comet ISON should be visible at the plotted location in the view of the SOHO satellite’s LASCO C3 camera. The next day it will pass perihelion at 1:44 p.m. EST. If it survives, it will be moving towards the top of the LASCO field. Credit: Starry Night Software

Perihelion will occur at 1:44 p.m. on Thursday November 28, Thanksgiving Day in the U.S.A.

Rather than the traditional football game, we’d suggest you watch ISON’s progress around the sun instead.

While some predictions suggest that the comet may be visible in daylight, we’d rather you didn’t risk your vision by staring at the sun. It might be possible to block the sun from view with a well placed chimney or lamp post, but there is a better way, and it’s 100 percent safe.

There are several satellites in orbit around the sun which are trained on the sun all the time. One the oldest and still one of the best is SOHO, the Solar and Heliospheric Observatory.

SOHO was launched in 1995, and carries an array of cameras pointed at the sun. For our purposes, the most interesting one is the Large Angle and Spectrometric Coronagraph #3, or LASCO C3 for short. This shows a field of view about 32 degrees wide. The sun itself is blocked by a disk at the end of a stalk, its diameter marked by a small white circle. Because SOHO is in space, there is no atmosphere to scatter the sun’s light, so we can see the stars surrounding the sun, just as if we were watching a total solar eclipse. Actually, the view is much better than at a total eclipse, with stars down to about 7th magnitude visible.

Very early in SOHO’s history, astronomers realized that they could observe comets passing very close to the sun, called “sungrazers.” To date, more than 2400 comets have been discovered by careful skywatchers scanning SOHO’s LASCO C3 images. The images from LASCO are refreshed regularly about an hour apart, and relayed immediately to this web page:

Whenever you check this page, you will see the most recent image from space, usually not more than an hour or two old. Go there right now, and you will see the sun’s current coronal activity and, in the background, the stars on the far side of the sun. Over the next few days, if all goes well, you will see Comet ISON’s progress around the sun. It should enter LASCO C3’s field of view at the right side on Tuesday November 26.

ISON will pass just below the sun on Thursday and then move upwards, leaving the field on Saturday afternoon. In our graphic, the yellow circle shows the LASCO C3 field of view, the gray parabola is ISON’s path, and the stars of Scorpius are in the background. ISON is shown in its position on Wednesday November 27.

It’s impossible to say in advance exactly what we will see in the next few days. By watching the comet’s progress in the LASCO camera, you will participate in a great astronomical adventure.

Will the comet break up? Will it continue on to be one of the brightest comets in history? As of this writing, no one knows, but by watching it on LASCO, you will know as soon as anyone.