Empirical determination of the DIB's λ₀, using stars with 170° ⩽ l ⩽ 190° (i.e., within 10° of the Galactic anticenter) and |b| ⩽ 2°. On the x axis is the distance in longitude from l = 180°, and the y axis shows the measured central λ. The gray points are the individual sightline measurements and uncertainties, and the red points indicate the median value and absolute deviation in each Δl = 075 bin. The solid black line is the best-fitting linear relationship of the individual measurements, whose value at l = 180° is taken as λ₀.