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Mercury: The Swift Planet

Our solar system's innermost planet reaches greatest elongation tonight. Here's some background before you step outside to view its best 2018 apparition.
spectra_mercury
This colorful version of Mercury was assembled from spectral data taken with the MESSENGER spacecraft, highlighting various minerals on the planet's surface.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Tonight, Mercury reaches greatest elongation, meaning it will stand apart from the Sun at the largest angle as seen from Earth. This leaves it readily visible in the sky after the Sun has set, and tonight marks its very best apparition of the year. Mercury will lie 18° east of the Sun and 12° high in the west 30 minutes after sundown, and sit 6° high 30 minutes later, in a darker sky. At magnitude –0.4, it will shine as the third-brightest object in the sky, after Venus and Sirius, leaving it easily identifiable to the naked eye. Pull out your telescope, and you'll see its 7.3"-diameter disk slightly less than half-lit.

After tonight, the tiny planet will appear lower in the sky each day. But stay tuned, because this Sunday, Mercury, Venus, and the Moon will paint a beautiful picture as they cluster together in the sky, 10° high 30 minutes after sunset.
mercury_1
Tiny Mercury is covered in craters, but hides a surprise in some of those craters at its poles, where sunlight never reaches.
NASA
Despite the fact that astronomers have been observing Mercury with telescopes for centuries, how much do we really know about this strange world? The planet Mercury lies closest to the Sun, orbiting every 88 days. This tiny, crater-ridden world has intrigued planetary scientists for decades, housing water ice at its poles and zipping around on an orbit with enough ellipticity that the apparent size of our star changes dramatically between its closest and farthest points.

Although secure today in its place as the innermost planet, astronomers once thought that Mercury might actually hold second place. Its unique orbit precesses, or shifts in orientation, around the Sun, which was believed due to an unseen planet orbiting closer than Mercury. Now, though, we know that this precession is actually due to the effects of general relativity, as the Sun's mass deforms space-time around it.

Mercury rotates on its axis with nearly no tilt whatsoever, meaning the planet experiences no seasons and there are places at the poles where sunlight never touches, blocked by steep crater walls. Solar radiation and micrometeorite impacts are two of the major forces that have shaped — and continue to influence — this world, and we are still digging for clues on its history and evolution. Only two spacecraft have ever visited, and the entire surface wasn't even imaged until earlier this century.

Below, we've compiled a few resources on Mercury, to help you learn more about our solar system's smallest, swiftest planet before you cast your eyes skyward tonight.

Mercury at its evening best
Our March issue's Sky this month section highlights Mercury's best apparition of the year; you'll also find a sky chart to help you spot the planet.

Tour the solar system: Mercury
Our Tour the solar system video series takes a closer look at the planet Mercury.

Web Extra: Mercury in the spotlight
Our February 2016 story, "Torrid Mercury," highlights one of MESSENGER's biggest discoveries — water ice at the planet's poles.

Weird Object: Mercury
Get the facts on Mercury and its place in the solar system in this overview by Astronomy contributor Bob Berman.

Astronomy magazine podcast: MESSENGER at Mercury
Step back in time to just before MESSENGER passed Mercury in early 2008 to hear Senior Editor Rich Talcott explain the spacecraft's journey.

Astronomy magazine podcast: Mercury revealed
Continue your time travel journey with this July 2008 podcast, which takes you through some of MESSENGER's earliest discoveries.
Would you like to learn more? Check out our free downloadable eBook: Venus and Mercury: Hot, volatile planets, which includes the original magazine spread of "Torrid Mercury’s icy poles."
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