Why Can’t I See Mercury at Midnight, December 4, 2012?


DSC08137

This image captured during twilight on December 1,
has three planets and a bright star visible in it.  When visible,
Mercury appears during morning twilight or
evening twilight  Click the image to see it larger.

Mercury never appears in the sky at midnight as seen from the skies of Earth. On December 4, this elusive planet appears at its greatest angular separation from the sun.

mercury_greatest_elongation

This chart shows its position at sunrise on this morning, along with Venus. The red arc indicates Mercury’s invisible orbital path; it indicates the position of the planet as it reaches the farthest extent of its orbit from the sun.  So Mercury appears as far from the sun as can as seen from our planet.

inner_solar_system_dec-4_2012

Both planets’ orbits are closer to the sun and are wholly within Earth’s orbit. As such they appear near the sun. Naturally, the time of the day when the sun appears in our sky is called daytime. Since Venus and Earth are always near the sun, they are mostly in the sky during the daytime. They can appear in the sky before or after the sun rises or sets. Mercury appears to swivel back and forth around the sun about every 116 days, jumping from morning sky to evening sky. Because it is so close to the sun, it rarely appears in a dark sky, as indicated by the image at the top of this article showing Mercury with Venus and Saturn during morning twilight in early December 2012. Unlike, Mars, Jupiter and the other outer planets, Mercury cannot appear on the side of earth opposite the sun, as indicated by the “midnight” line on the chart above. (Click the image to see it larger.)  So Mercury and Venus never appear in the skies of Earth at midnight.

Brilliant Venus can shine in a dark sky because it can rise into the sky before twilight begins or set well after evening twilight ends, but never at midnight from mid-latitudes. (It is visible at midnight from north of the Arctic Circle in certain circumstances.) Mercury rises during morning twilight or sets during evening twilight and is very difficult to see.

To locate Mercury during December 2012, our monthly sky watching summary has information about where to look. Binoculars will help locate this elusive planet.

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