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When Mercury makes its closest approach to the Sun, astronomers call this perihelion. So, the perihelion of Mercury is 46 million kilometers (29 million miles), or 0.307 astronomical units (1 AU is the distance from the Sun to the Earth).
The furthest point of a planet’s orbit is called the aphelion. So in the case of Mercury, its furthest point stretches out to 70 million km (44 million miles), or 0.466 astronomical units.