NavList:

Yeah, no. 

You were right the first time: it's just a great circle. And there is no other meaning to a "straight line" from the Moon to the Sun. I find that when people draw or visualize what they think is a "straight line" from the Moon to the Sun, and it doesn't seem to line up perpendicular to the horns, it's because they're visualizing something closer to a circle of equal altitude. And of course that doesn't work. It helps to hold your arm straight out in front of you and sweep. That's often a more successful way to generate a great circle. If you sweep a great circle from the Sun to the Moon, you will find that it hits the Moon dead-center on the illuminated limb. 

Frank Reed