When you look at the sun (do not look at the sun), isn't it kind of weird to think that *THAT* is the *CENTER* of Earth's orbit? I mean technically Earth's orbit is an ellipse, so it has "two centers / foci", but its eccentricity is close enough to 0 we can effectively call it a circle. Even if it weren't close to a circle, the sun is quite a few orders of magnitude massive than the Earth, so the barycenter of the Earth-Sun system, which both masses can be understood to revolve around, would effectively be at the Sun's position, making the Sun "one of the centers". 

Oh yeah, and the Earth also undergoes apsidal precession, but over incredible large time-spans (like 100K years for a full 360 degrees) so for our purposes, it's basically a circle. 

Anyways, when you're standing outside on a hot summer day, the mind is just so wired to think of "up" as, well, literally up and into the sky, perpendicular to the ground. But in terms of Earth's orbital plane, "up" really is perpendicular to that view direction. So when the sun is directly overhead, it's like your body is sticking out directly into the orbital plane of the Earth. 

It might seem obvious, but like, just go outside and stare into the sky and *really* just think about this for a moment. Wherever the Sun is located in your sky, that's the center of Earth's orbit, and true "UP" is at an angle.