Anti-Sun

The sun was shining on the sea,
Shining with all his might:
He did his very best to make
The billows smooth and bright–
And this was odd, because it was
The middle of the night.
The moon was shining sulkily,
Because she thought the sun
Had got no business to be there
After the day was done–
“It’s very rude of him,” she said,
“To come and spoil the fun.”

–Nonsense, of Lewis Carroll’s kind. As he and his Walrus and Carpenter pretended not to know, if you look out at the night you are looking at where the Sun isn’t. Still, it’s interesting to imagine a Counter-Sun; that is, to know where the exact anti-Sun position is, opposite to – 180 degrees around the celestial sphere from – the real Sun.

I decided to make a diagram of the travel of this dark ghost of the Sun. In December the Sun itself sags along the southernmost part of the ecliptic, through Ophiuchus into Sagittarius. So its opposite travels along the high-northern ecliptic, sliding among the stars of the winter constellations Taurus and Gemini.

antiSun1412Anti-Sun and Moon are drawn at twice size, and at their position for 0h Universal Time.

A planet when it passes by the anti-Sun is at opposition (the climax of its time for being viewed). The Moon when it passes by the same position is Full, as it is this December 6 at 12:26 Universal Time – half way between the anti-Sun’s Dec. 5 and 6 positions. The anti-Sun could be the shadow of the Earth thrown by the real Sun. So, if the Moon were traveling a few degree farther north, there would be a lunar eclipse.

If you look in the section about meteor showers in our Astronomical Calendar 2014 or 2015, you will find something about the Antihelion Source. This, as I learned from Alastair McBeath, is a meagre all-year drift of meteors inward across Earth’s orbit, so that they appear to come roughly from the anti-Sun’s direction. Presumably there are meteors coming from nearly all directions in approximately the ecliptic plane, but those coming from the night side – that is, inward – are visible, and those coming from somewhat forward – the morning part of the night side – appear more numerous, like flies hitting a car window. So the rough position of the antihelion source in mid December is near Mu Geminorum, which is about the anti-Sun’s Dec. 26 position.

As for the word “antihelion,” I’ll quote what Alastair wrote to me when I grumbled that better Greek would be “anthelion”:

“We [the community of meteor scholars] agonized over the name for months. Like you, I said it should be “Anthelion” not “Antihelion,” but when I investigated more closely, I found either prefix is acceptable in English usage, and both actually occur in the meteor literature for sources around this area. “Antihelion” has (at least) two advantages however. One is there’s no question about its pronunciation, while “Anthelion” could end up as An-th-elion (like the ludicrous discussions about “afelion” for aphelion a few years ago). The other is that “anthelion” is already used for the point in the sky opposite the Sun, but at the same elevation above the horizon as it, during daylight, specifically for the halo effects that occur around there. “Antihelion” does a somewhat better job of stressing that it’s more nearly opposite the Sun, thus not observable when the Sun is…”

Evidently I differ from Alastair about the pronunciation of “anthelion” and “aphelion.” But yes, besides the antihelion which is completely opposite to the Sun, there is the rare halo phenomenon called anthelion: a faint “Sun” that is reflected from airborne ice crystals and faces the Sun across the daytime sky.

6 thoughts on “Anti-Sun”

  1. A very interesting entry by Guy, with interesting responses. I too would love to see the ghostly anti-sun plotted on one chart or other in the next Astronomical Calendar. Those of us who have dark enough skies to look for the gegenschein know there are times of year when it is lost in the brighter Milky Way, so having the anti-sun’s positions on various dates would be helpful to such searches.
    I’ve seen the halo phenomenon know as an anthelion a few times, though perhaps only once did I see some of its attendant effects. I’ve also seen paranthelia at other locations on the “parhelic circle,” in one case a paranthelion that was amazingly bright.
    When I read Alastair’s comments about the variant ways that English can spell certain sounds, I flashed back to a linguistics class I had in grad school. The professor told us that there are 16 (I think 16) ways to spell the sound “sh” in English and asked us to name them all. We raised our hands and soon had identified all but one of the ways of spelling the sound. We gave up and he told us the final spelling: “cha” as in just one English word: fuchsia (which he claimed became pronounced as it is in English to avoid a most indelicate resemblance to another word.

      1. Still can’t seem to get the middle letters of fuchsia to appear by themselves because the site keeps spell-correcting ti to “cha.”

  2. Colleagues without English as their first language tend to have fewer pronunciation variants for words such as “ant(i)helion” than those who do, I’ve found. Doubtless this is because of the many variant ways in which some letters and combinations can be pronounced in English, which most other languages have the good sense to ignore!

    The Anti-Sun position is also a handy guide to the rough centre of the round patch called the Gegenschein or Counterglow, a localised brighter area of the set of weak glows across the near-ecliptic part of the night sky produced by the zodiacal light. The brighter element of these, and thus that more often seen (although you do need a good, very dark, transparent sky), is the cone of light visible for a while around the sunrise or sunset horizons when the Sun is 15°-20° or a little more below the horizon. From mid-northern latitudes, this cone is more easily seen in the evening sky in spring and the morning sky of autumn (as described in the February and August notes respectively in the “Astronomical Calendar”). The Gegenschein sits around the ecliptic directly opposite the Sun, so is highest each day at local midnight, and thus is at its most easily-seen during December to mid January.

    About a month ago, the European Southern Observatory reported having observed exozodiacal light in the habitable zones around nine of 92 sampled stars beyond our Solar System, the latest such stars to be added to a list which already included Vega, Fomalhaut and Tau Ceti. The results, including an artist’s impression of how the light might appear to a viewer on a world around such a star, were announced here: http://www.eso.org/public/news/eso1435/ .

  3. An interesting way to think about the night sky. Might we hope that the “anti-sun” location find its way into the star charts of the 2016 Astronomical Calendar?

    1. I’ll try to remember to do it! There are many kinds of chart it could fit into, so the question will be: Which?

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