What it is. A planet does not travel a loop around the Sun. The planet is a fixed node, and its orbit is a perfect circle. The Sun sits on that circle, opposite the planet, and it is the Sun that slides in and out along the line joining them. In fact the Sun is the only body in the system that moves at all — and it moves on a wavelength; nothing else does. What we read as the planet swinging nearer and farther is the Sun's own movement — its wavelength — not the planet's. The orbital "ellipse," and the whole idea of eccentricity, is the shadow of the Sun's slide seen from the Sun's own side.
But the fixed node is not idle. Two streams of time pour into it from the Sun — the origin of time — and the planet reads them as its own measure: a linear wavelength running straight in along the radius, and an orbital wavelength running around it. These are the incoming waves, propagated from the Sun, that science mistakes for a planet in motion. The planet holds still; the wavelengths arrive.
And go deeper still: in truth neither body travels through space at all. What moves is the space-time dimension between the Sun and the planet — a dimensional sphere that swells and contracts in the gap between them. That moving sphere of space-time is the whole of what we call orbit. The "movement" of a planet, and the "slide" of the Sun, are both this one thing seen from two sides: the dimension itself in motion, carrying the appearance of travel, while the bodies that sit in it never leave their nodes. Orbit is not a journey through space — it is a sphere of space-time, breathing.
The figure. The slide is WAVE = a × e. For the Earth the Sun moves about 2.49 million km each way, so the gap runs from 147,098,655 km (Sun nearest = perihelion) to 152,074,916 km (Sun farthest = aphelion). For Mercury the Sun slides 11.9 million km, which is why its orbit looks the most eccentric of all. The planet's circle never bends by a metre.
Current science. Each planet moves along an elliptical orbit with the Sun fixed at one focus, held to its path by gravity; the variation in distance is the planet travelling the ellipse.