Comet ISON Approach

Comet ISON has the potential to be one of the most amazing comets of a lifetime. It's orbital trajectory has it passing so close to the Sun that it may evaporate into a gigantic cloud of dust and debris that would be visible with a naked eye in daylight. Today the comet is 1/3 the distance from the Sun as the Earth and it is still descending toward perihelion.

There are multiple images being posted on the Internet and they frequently generate the question, "which way is up?" This posting might help to show which way things are moving, and which way is "up" from the perspective of a person standing on the ground, on the planet Earth.

This first excellent photo was taken from the ground, likely with telephoto magnification. Except for the magnification this is the way the comet would have appeared if you were to see it with the naked eye or with binoculars. The Sun is below the horizon to the left, and the comet's tail is extended up and to the right, away from the Sun.

Here is an image from my SkyMap software, showing the positioning of these objects before dawn this morning, looking southeast from my house. The horizon is the border between the blue and grey backgrounds. The white line is the ecliptic, the apparent path of the Sun caused by the orbit of the planet Earth around the Sun. The Sun is shown as a hollow yellow circle on the ecliptic below the horizon. Notice the angle of the ecliptic, up and to the right, generally in the same direction as the photo above.

There are two comets showing on the map, comet Encke and comet ISON. Planet Mercury (and Saturn very much farther away) is also lined up near the comets. Encke and Saturn are not very bright so I will not make much note of them.

Here is the really interesting photo, taken from a spacecraft designed to watch the Sun. The objects are labeled but their relative positions are confusing. Where is the spacecraft positioned so it could capture this image? How can the planet Earth be in the image along with the comets and Mercury all at the same time? Which way are things moving?

Professional astronomers have looked at so many images where orientation is rearranged that they don't even notice when it looks odd like this. So they didn't bother to explain this one.

The answer is easily demonstrated by flipping the image horizontally. Extend a line from the Sun (on the left) through the planet Earth to represent the ecliptic. Note that Mercury is above the line and the two comets are below the line. This matches the SkyMap image very well. Rotate the photograph by 47 degrees and it will look even more like the SkyMap image.

Now you can see that the north pole is up, Earth is in the foreground, moving toward Mercury and the comets, and rotating in that same direction. So when you are facing toward the comets, Mercury, and the rising Sun, the spacecraft that took this photo is behind you.

Here's a link to a NASA JPL YouTube video that was created on October 14, 2013. It makes several excellent points regarding the direction things are moving and it shows the alignment of the planets Earth and Mercury that match my "flipped" version of the spacecraft image.