Mercury‘s 88-day orbit around the Sun will carry it to its closest point to the Sun – its perihelion – at a distance of 0.31 AU from the Sun.
Unlike most of the planets, which follow almost exactly circular orbits around the Sun only varying in their distance from the Sun by a few percent, Mercury has a significantly elliptical orbit.
Its distance from the Sun varies between 0.307 AU at perihelion (closest approach to the Sun), and 0.467 AU at aphelion (furthest recess from the Sun). This variation, of over 50%, means that its surface receives over twice as much energy from the Sun at perihelion as compared to aphelion. – in-the-sky.org