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When can Mercury be seen in the morning or evening sky?
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Mercury
Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
Mercury is challenging to observe because it is the planet closest to the Sun, meaning it is never visible when the sky is fully dark. It can only be seen in the morning or evening sky during dusk or twilight, as it never reaches an angular distance of more than about 28° from the Sun.123
Because its rising or setting always occurs within approximately two hours of the Sun’s, the planet is typically obscured by the Sun's glare or hidden near the horizon, where it must be viewed through the turbulence of Earth’s atmosphere. Mercury returns to the same point relative to the Sun in the morning or evening sky every 116 days, a duration known as its synodic period.12