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.
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.