November 30, 2018. If you could see the stars in the daytime, you’d see the sun shining in front of the border of the constellations Ophiuchus and Scorpius on this date. The sun crosses a constellation boundary, into Ophiuchus.
At about this time each year, the sun passes out of Scorpius to enter Ophiuchus. Like Scorpius, Ophiuchus is a constellation of the zodiac … but unlike Scorpius, Ophiuchus is not one of the traditional twelve zodiacal constellations.
The sun will remain in front of Ophiuchus until December 18.
The ecliptic – which translates on our sky’s dome as the sun’s annual path in front of the background stars – actually passes through 13 constellations, as defined by the International Astronomical Union (IAU), although this is not commonly known. After all, when you read the horoscope in the daily newspaper or a monthly magazine, you see only 12 constellations, or signs, mentioned.
There are the 12 traditional zodiacal constellations that have been with us since ancient times. The relatively recent addition of Ophiuchus as a member of the zodiac has increased the number to 13.
Today’s constellation boundaries were drawn out by the International Astronomical Union in the 1930s.
Look at the chart carefully, and you’ll see that the border between Ophiuchus and the constellation Scorpius for the most part lies just south of, or below, the ecliptic. In ancient times, the Ophuichus-Scorpius border was likely placed to the north of, or above, the ecliptic. Had the International Astronomical Union placed its constellation boundary where the ancients might have, the sun’s annual passing in front of Scorpius would be from about November 23 till December 18, not November 23 to November 30.
Bottom line: As seen from Earth, the sun passes in front of the constellation Ophiuchus each year from about November 30 to December 18.