7. On July 5 next, the Earth will reach aphelion, the point of its orbit where the distance from the Sun is maximum, approximately $1.52 \cdot 10 ^ { 11 } \mathrm {~m}$. Perihelion is instead the point at minimum distance from the Sun, approximately $1.47 \cdot 10 ^ { 11 } \mathrm {~m}$. Determine, in an appropriate coordinate system, the equation that represents the Earth's trajectory around the Sun.