What is the ecliptic longitude of the Sun at the December solstice?

The Sun’s ecliptic longitude at the December solstice is 270 degrees.

Ecliptic longitude measures a position along the Sun’s apparent yearly path through the sky, starting at the March equinox. The four key seasonal points divide that circle into quarters: 0 degrees at the March equinox, 90 degrees at the June solstice, 180 degrees at the September equinox, and 270 degrees at the December solstice.

At 270 degrees, the Sun reaches its most southerly declination, about 23.4 degrees south of the celestial equator. This produces the shortest day and longest night of the year in the Northern Hemisphere, while the Southern Hemisphere experiences its summer solstice.

The longitude is a coordinate, not a date. The December solstice can occur on different calendar dates and at different times depending on the year and time zone. Astronomers use the apparent ecliptic longitude, including small effects such as aberration and nutation, when defining the exact instant of the solstice.